Droevendaalsesteeg 10
6708 PB Wageningen
The Netherlands
Suzanne McGowan
Over
Understanding how and why aquatic ecosystems respond to environmental changes over a variety of timescales.
Biografie
I combine limnology and palaeolimnology (the use of lake sediment cores to investigate past environments) to understand aquatic ecosystems. I use long-term studies to investigate how environmental problems such as nutrient pollution, climate change and hydrological manipulations have changed lakes and aquatic biota. Combining long-term monitoring, palaeolimnology and experimental studies provides integrated insights into ecosystem functioning. I am an expert in the use of chlorophyll and carotenoid pigments as bioindicators of algae and phototrophic bacteria. Such approaches are particularly informative for tracking past dynamics of cyanobacterial blooms. I have developed and applied the use of pigment proxies in sediment cores to investigate long-term changes across a very broad range of systems spanning six continents and also frequently work with other sedimentary proxies such as diatoms and stable isotopes. I am interested in pushing the capabilities of palaeolimnology beyond descriptions and quantifications of environmental change towards its use as a tool for understanding mechanisms. Landscape-scale comparisons of multiple sediment cores can help to distinguish how local versus regional stressors interact to alter the trajectory of lake ecosystem development. Comparisons of socio-economic data and sedimentary archives can help to understand linkages in socioecological systems.
Onderzoeksgroepen
Dossier
Koolstofopslag in de natuurCV
Nevenfuncties
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Publicaties
Peer-reviewed publicaties
Sentinel responses of Arctic freshwater systems to climate
While the sentinel nature of freshwater systems is now well-recognized, widespread integration of freshwater processes and patterns into our understanding of broader climate-driven Arctic terrestrial ecosystem change has been slow. We review the current understanding across Arctic freshwater systems of key sentinel responses to climate, which are attributes of these systems with demonstrated and sensitive responses to climate forcing. These include ice regimes, temperature and thermal structure, river baseflow, lake area and water level, permafrost-derived dissolved ions and nutrients, carbon mobilization (dissolved organic carbon, greenhouse gases, and radiocarbon), dissolved oxygen concentrations, lake trophic state, various aquatic organisms and their traits, and invasive species. For each sentinel, our objectives are to clarify linkages to climate, describe key insights already gained, and provide suggestions for future research based on current knowledge gaps. We suggest that tracking key responses in Arctic freshwater systems will expand understanding of the breadth and depth of climate-driven Arctic ecosystem changes, provide early indicators of looming, broader changes across the landscape, and improve protection of freshwater biodiversity and resources.https://doi.org/10.1139/as-2022-0021Habitat heterogeneity enables spatial and temporal coexistence of native and invasive macrophytes in shallow lake landscapes
Macrophyte invasive alien species (IAS) fitness is often hypothesised to be associated with beneficial environmental conditions (environmental matching) or species-poor communities. However, positive correlations between macrophyte IAS abundance and native plant richness can also arise, due to habitat heterogeneity (defined here as variation in abiotic and native biotic conditions over space and time). We analysed survey and palaeoecological data for macrophytes in satellite lakes along the Upper Lough Erne (ULE) system (Northern Ireland, UK), covering a gradient of eutrophication and connectivity to partition how environmental conditions, macrophyte diversity and habitat heterogeneity explained the abundance of Elodea canadensis, a widely distributed non-native macrophyte in Europe. E. canadensis abundance positively correlated with macrophyte richness at both the within- and between-lake scales indicating coexistence of native and invasive species over time. E. canadensis was also more prolific in highly connected and macrophyte-rich lakes, but sparser in the more eutrophic-isolated ones. Partial boosted regression trees revealed that in eutrophic-isolated lakes, E. canadensis abundances correlated with water clarity (negatively), plant diversity (positively), and plant cover (negatively) whereas in diverse-connected lakes, beta diversity (both positively and negatively) related to most greatly E. canadensis abundance. Dense macrophyte cover and unfavourable environmental conditions thus appear to confer invasibility resistance and sufficient habitat heterogeneity to mask any single effect of native biodiversity or environmental matching in controlling E. canadensis abundance. Therefore, in shallow lake landscapes, habitat heterogeneity variously enables the coexistence of native macrophytes and E. canadensis, reducing the often-described homogenisation effects of invasive macrophytes. © 2021 The Authors. River Research and Applications published by John Wiley & Sons Ltd.https://doi.org/10.1002/rra.3839Changing water quality and thermocline depth along an aquaculture gradient in six tropical crater lakes
https://doi.org/10.1007/s10750-022-05065-7Understanding how lakes respond to changes in nutrient loading along a productivity gradient can help identify key drivers of aquatic change, thereby allowing appropriate mitigation strategies to be developed. Physical, chemical and biological water column measurements combined with long-term water monitoring data for six closely located crater lakes, in Southeast Asia, were compared to assess the response of lakes along a productivity gradient equating to a transect of increasing aquaculture intensity. Increasing chlorophyll a (phytoplankton biomass) in the upper waters appeared to modify the thermocline depth and light availability causing a shift from a deep chlorophyll maximum at low aquaculture intensity to the emergence of algal dead zones lower in the water column with high aquaculture intensity. High phosphorus loading and light limitation from enhanced algal biomass, associated with high aquaculture intensity, exacerbated nitrogen drawdown, leading to the prevalence of potentially nitrogen-fixing cyanobacteria. Seasonal overturn during the cooler season resulted in low dissolved oxygen concentrations in the epilimnion, potential harmful algal blooms, a reduction in the habitable depth for fish and ultimately increased mortality amongst farmed fish.
In flux: Annual transport and deposition of suspended heavy metals and trace elements in the urbanised, tropical Red River Delta, Vietnam
Due to the depositional environment, river deltas are said to act as filters and sinks for pollutants. However, many deltas are also densely populated and rapidly urbanizing, creating new and increased sources of pollutants. These sources pose the risk of tipping these environments from pollution sinks to sources, to the world's oceans. We provide detailed seasonal and annual assessments of metal contaminants in riverine suspended particulate matter (SPM) across the densely populated Red River Delta (RRD), Vietnam. The global contributions of elements from the RRD are all 40) and concentrations of As higher than national regulation limits (>17 mg/Kg) at all sites other than one upstream, agricultural-dominated tributary in the dry season. These ‘hotspots’ are characterised by high inputs of organic matter (e.g. manure fertiliser and urban wastewater), which influences elemental mobility in the particulate and dissolved phases, and are potentially significant sources of pollution downstream. In addition, in the marine and fresh water mixing zone, salinity effects metal complexation with organic matter increasing metals in the particulate phase. Our calculations indicate that the delta is currently acting as a pollutant sink (as determined by high levels of pollutant deposition ∼50%). However, increased in-washing of pollutants and future projected increases in monsoon intensity, saline intrusion, and human activity could shift the delta to become a source of toxic metals. We show the importance of monitoring environmental parameters (primarily dissolved organic matter and salinity) in the RRD to assess the risk of transport and accumulation of toxic metals in the delta sediments, which can lead to net-increases in anthropogenic pollution in the coastal zone and the incorporation of toxic elements in the food chain.https://doi.org/10.1016/j.watres.2022.119053Urbanization and seasonality strengthens the CO2 capacity of the Red River Delta, Vietnam
https://doi.org/10.1088/1748-9326/ac9705Tropical rivers are dynamic CO2 sources. Regional patterns in the partial pressure of CO2 (pCO2) and relationships with other a/biotic factors in densely populated and rapidly developing river delta regions of Southeast Asia are still poorly constrained. Over one year, at 21 sites across the river system in the Red River Delta (RRD), Vietnam, we calculated pCO2 levels from temperature, pH, and total alkalinity and inter-linkages between pCO2 and phytoplankton, water chemistry and seasonality were then assessed. The smaller, more urbanized, and polluted Day River had an annual median pCO2 of 5000 ± 3300 µatm and the larger Red River of 2675 ± 2271 µatm. pCO2 was 1.6 and 3.2 times higher during the dry season in the Day and Red rivers respectively than the rainy season. Elevated pCO2 levels in the Day River during the dry season were also 2.4-fold higher than the median value (2811 ± 3577 µatm) of calculated and direct pCO2 measurements in >20 sub/tropical rivers. By further categorizing the river data into Hanoi City vs. other less urban-populated provinces, we found significantly higher nutrients, organic matter content, and riverine cyanobacteria during the dry season in the Day River across Hanoi City. Forward selection also identified riverine cyanobacteria and river discharge as the main predictors explaining pCO2 variation in the RRD. After accounting for the shared effects (14%), river discharge alone significantly explained 12% of the pCO2 variation, cyanobacteria uniquely a further 21%, while 53% of the pCO2 variance was unexplained by either. We show that the urbanization of rivers deltas could result in increased sources of riverine pCO2, water pollution, and harmful cyanobacterial blooms. Such risks could be mitigated through water management to increase water flows in problem areas during the dry season.
Paleolimnological records for tracking dam-induced changes in the composition and supply of sediment to middle Yangtze floodplain lakes
A global boom in dam construction has reduced sediment loading of large rivers, as well as their connected floodplains. In order to explore potential effects of hydrological regulation on floodplain ecosystems, this study presents concentrations of inorganic elements, organic matter content and grain size spectra in 210Pb-dated sediment cores of five lakes in the middle Yangtze River floodplain. These lakes had free hydrological connectivity with the Yangtze River before the operation of local sluice gates, and hence K-rich but Al-poor particulates from the upper Yangtze reaches can be transported into lakes during periods of high river discharge. After hydrological regulation, sedimentary K/Al ratios registered substantial decreases in the study lakes, probably due to the declining supply of fine-grained and K-enriched riverine particulates from the Yangtze River. In East Dongting Lake, a hydrologically open lake proximal to the Three Gorges Dam (TGD), the sharp decrease in K/Al ratio after 2003 mainly responded to declining sediment supply from the Yangtze River after the TGD operation. In addition, prolonged water retention time probably promoted aquatic production and greater deposition of organic matter, as indicated by recent increases in organic matter content in the upper strata of the dammed lakes. Taken together, sedimentary K/Al ratios provide essential information on effects of past hydrological changes on sediment supply and aquatic productivity over multi-decadal timescales in these floodplain lakes with scarce monitoring data. The impacts of dam construction during recent decades on sediment composition in middle Yangtze floodplain lakes might be widespread in other similar floodplains worldwide. Thus sedimentary records can be utilized to inform and direct floodplain management.https://doi.org/10.1016/j.catena.2022.106643Eutrophication has a greater influence on floodplain lake carbon cycling than dam installation across the middle Yangtze region
https://doi.org/10.1016/j.jhydrol.2022.128510Carbon cycling in shallow floodplain lakes is complex due to variability in delivery of flood-derived allochthonous organic matter (OM). Human activities have potential to significantly modify the carbon balance of lakes by damming which restricts external OM inputs and via eutrophication which can increase the in-lake production of algae and/or aquatic plants. In order to understand how these human activities influence carbon cycling in shallow floodplain lakes over decadal-centennial timescales, we analysed C/N ratios and δ13C from terrestrial plants, catchment soils, aquatic plants and dated sediment cores from six heavily modified lakes in the middle Yangtze floodplain. Submerged macrophytes (−21.4 ± 4.6 ‰) had higher δ13C than C3 plants from the catchment (−26.6 ± 0.6 ‰) and emergent and floating plants (−26.6 ± 4.0 ‰). Increases in sedimentary chlorophyll a (from primary producers) were associated with a decline in sedimentary δ13C in the severely eutrophic Dongting, Luhu, Wanghu and Poyang Lakes after the 1980s. In contrast, sedimentary δ13C increased in Honghu and Futou Lakes which have abundant submerged macrophytes. The timing and scale of sedimentary δ13C changes indicated stronger responses to eutrophication than damming, with eutrophication responses ranging from a macrophyte proliferation to the dominance of phytoplankton.
Evidence for centennial-scale Mid-Holocene episodes of hypolimnetic anoxia in a high-altitude lake system from central Tian Shan (Kyrgyzstan)
Few sedimentary archives of lake meromixis are available in palaeolimnological records, because long-term observations are limited in time and indisputable sediment proxies of hypolimnetic anoxia are still scarce. Here we use visible and near infrared (VNIR), and short-wave infra-red (SWIR) hyperspectral imaging combined with geochemical analyses to reconstruct lake stratification history, redox status and mixing conditions in the water column of Lake Son Kol (Kyrgyzstan) in Central Tian Shan during the last 8500 years. In particular, the detection of Bacteriopheophytin a (Bphe a), a pigment produced by anoxygenic phototrophic bacteria at the chemocline of meromictic lakes, emphasizes episodes of multi-decadal to centennial hypolimnetic anoxia in Lake Son Kol. Phases of hypolimnetic anoxia are inferred from the deposition of dark organic sediments within a stratified lake system, which occurred during periods of increased snowmelt (and solid winter precipitation) and warmer spring/summer temperatures that promoted floods and the export of terrestrial material from the catchment. Prolonged euxinic conditions in bottom waters (involving the development of a stable chemocline) are reported around 8500, 8400, 8200–7800, 7700–7500, 7300–7000, 6500–6100, 6000–5700 and 5500–5250 cal yr BP. At ca. 5250 cal yr BP, the chemocline abruptly vanished as Lake Son Kol tipped into a regime with predominantly cooler and well-mixed conditions (predominance of oxygenic phototrophs), coeval with higher lake levels. The disappearance of hypolimnetic anoxia in Lake Son Kol coincides with strengthened wind conditions that imply enhanced lake overturning and upward mixing of nutrients in the water column. This study reveals the strong potential of hyperspectral imaging, in combination with more classical palaeolimnological approaches, to reconstruct the lake trophic and mixing history and explore the controlling mechanisms at work on decadal to centennial timescales. Our results outline how abrupt ecosystem changes may occur even in the absence of anthropogenic climate change.https://doi.org/10.1016/j.quascirev.2020.106748Anthropocene climate warming enhances autochthonous carbon cycling in an upland Arctic lake, Disko Island, West Greenland
The Arctic is rapidly changing, disrupting biogeochemical cycles and the processing, delivery and sedimentation of carbon (C), in linked terrestrial-aquatic systems. In this investigation, we coupled a hydrogeomorphic assessment of catchment soils, sediments and plants with a recent lake sediment sequence to understand the source and quality of organic carbon present in three Arctic upland lake catchments on Disko Island, located just south of the low- high Arctic transition zone. This varied permafrost landscape has exposed soils with less vegetation cover at higher altitudes, and lakes received varying amounts of glacial meltwater inputs. We provide improved isotope and biomarker source identifications for palaeolimnological studies in highlatitude regions, where terrestrial vegetation is at or close to its northerly and altitudinal range limit. The poorly developed catchment soils lead to lake waters with low dissolved organic carbon (DOC) concentrations (≤ 1.5 mgL-1). Sedimentary carbon=nitrogen (C/N) ratios, the C isotope composition of organic matter (δ13Corg) and biomarker ratios (n-alkanes, n-alkanols, n-alkanoic acids and sterols) showed that sedimentary organic matter (OM) in these lakes is mostly derived from aquatic sources (algae and macrophytes). We used a 210Pb-dated sediment core to determine how carbon cycling in a lake-catchment system (Disko 2) had changed over recent centuries. Recent warming since the end of the Little Ice Age (LIA ∼ 1860 CE), which accelerated after ca. 1950, led to melt of glacier ice and permafrost, releasing nutrients and DOC to the lake and stimulating pronounced aquatic algal production, as shown by a > 10-fold increase in β-carotene, indicative of a major regime shift. We also demonstrate that recent increases in catchment terrestrial vegetation cover contributed to the autochthonous response. Our findings highlight that in Arctic lakes with sparsely developed catchment vegetation and soils, recent Anthropocene warming results in pronounced changes to in-lake C processing and the deposition of more reactive, predominately autochthonous C, when compared with extensively vegetated low-Arctic systems. © Author(s) 2021.https://doi.org/10.5194/bg-18-2465-2021High rates of biodeposition and N-excretion indicate strong functional effects of mussels (Bivalvia: Unionida) in certain anthropogenic tropical freshwater habitats
The functional roles of freshwater mussels (Unionida) in tropical systems are poorly understood. We quantified the effects of mussel filtration, excretion and deposition in three anthropogenic tropical systems, i.e. a man-made lake, abandoned mining pool and rice paddy channel. Sinanodonta cf. woodiana (non-native) was present at all three sites, whilst Pilsbryoconcha compressa (native) was present in the channel only. Clearance rates, biodeposition rates and effects on suspended algal pigment and dissolved nutrient concentrations were quantified in controlled, replicated experiments in laboratory tanks with water from original habitats. Clearance rates were generally low and did not explain the high biodeposition rates observed. A considerable proportion of the natural diet of these populations may therefore consist of material that was not available in tanks, i.e. benthic or deposited algae. Deposition rates in lake and channel populations exceeded published rates from temperate and Mediterranean habitats, presumably due to prevalence of non-palatable material and/or higher metabolic rates in tropical systems. The presence of S. cf. woodiana but not P. compressa led to a strong increase in total ammonia nitrogen concentrations and N:P ratios, exceeding estimations from other systems. This study suggests that freshwater mussels play different functional roles in anthropogenic tropical habitats than in temperate systems.https://doi.org/10.1007/s10750-020-04464-ySynergistic impacts of nutrient enrichment and climate change on long-term water quality and ecological dynamics in contrasting shallow-lake zones
Anthropogenic and climatic stressors on freshwater ecosystems are of global concern. However, the interactions and effects of multiple stressors (e.g., nutrient enrichment, climate warming, altered wind and precipitation) acting over different spatial and temporal scales are often complex and remain controversial. Here, we reconstructed one-century dynamics of eutrophication and primary producer communities in algal-dominated and macrophyte-dominated zones of a large shallow lake (Taihu, China), by integrating sedimentary photosynthetic pigments and geochemical records with water monitoring and historical archives. We aimed to explore the long-term underlying mechanisms of the responses of water quality and lake biota to multiple environmental perturbations. We found that water quality degradation and algal community modification showed similar trends but distinct timings and trajectories in contrasting ecological zones. Onset and intensity of eutrophication in north Meiliang Bay (since the 1950s) exceeded far beyond that of macrophyte-dominated Eastern Taihu (~1990s). Anthropogenic nutrients overtook past climatic control on production and composition of phototrophic assemblages. More importantly, lake phytoplankton responded markedly to climate warming, decreasing wind speed, and extreme weathers after cultural eutrophication. Synergistic interactions of nutrients and climate on lake ecosystems became increasingly significant in promoting harmful algal blooms (HABs) dominated by Microcystis, close to the hyper-eutrophic north lake zones. The asynchronous limnological and ecological responses also indicated the modulating roles of lake ecological regime and catchment hydrogeomorphic characteristic. Collectively, our findings suggest that mitigation of eutrophication and HABs calls for a triple management strategy integrating anthropogenic nutrients, climate change, and lake-catchment setting. © 2021 Association for the Sciences of Limnology and Oceanography.https://doi.org/10.1002/lno.11878Paleoecological evidence for a multi-trophic regime shift in a perialpine lake (Lake Joux, Switzerland)
Freshwater ecosystems are under new and increasing threats from anthropogenic change. Ability to detect and predict consequences of environmental perturbations on ecosystem function and water quality is limited by the lack of empirical data over relevant time scales. Paleoecological records present a unique opportunity to broaden understanding of ecological transitions over decadal to millennial timescales. This study tested the occurrence of regime shifts to track changes throughout the lake food web beyond the typical instrumental era, using both “traditional” paleoecological proxies (e.g., cladoceran zooplankton, zoobenthos, and pigments) and more recently developed molecular genetic methods based on sedimentary DNA. We used sediment cores from the perialpine Lake Joux (Swiss Jura), where the history of human settlement and land-use practices in the catchment has been well documented since the Medieval period. Paleoecological evidence revealed an abrupt and unprecedented biological reorganization in the second half of the 20th century, following several centuries of relatively stable communities despite growing human pressure. Time-varying autoregression computed using dynamic linear modelling identified this transition, triggered by the onset of rapid cultural eutrophication in the 1950s, as a true regime shift. Since this time, despite decades of re-oligotrophication, biotic communities of Lake Joux have not returned to pre-disturbance composition, most likely due to other confounding factors, including climate warming, that may prevent the lake from returning to an earlier equilibrium state. Paleoecological reconstruction further suggested that cladocerans responded earlier to disturbance, which is highly relevant for lake monitoring and management strategies. © 2021 Elsevier Ltdhttps://doi.org/10.1016/j.ancene.2021.100301Holocene lake phosphorus species and primary producers reflect catchment processes in a small, temperate lake
This paleolimnological study aims to investigate how natural processes and anthropogenic land-use changes have affected sedimentary phosphorus (P) forms and primary producers in a small, temperate lake (Lake Fuglsø, Denmark) throughout the Holocene. Our multi-proxy approach uses pollen, X-ray fluorescence scanning, carbon (C) and nitrogen contents and stable isotopes, sequential P extraction, 31P nuclear magnetic resonance spectroscopy, pigments, diatoms, and plant macrofossils from a 14C-dated sediment record. We found three periods of human impact: (1) low disturbance from domestic grazing during the early/mid Neolithic (~3600 to ~2600 BC), (2) higher disturbance because of animal husbandry and some grain cultivation during the Late Bronze and Pre-Roman Iron Age (~800 BC to AD ~100), and (3) strong disturbance caused by domestic grazing, intensified crop cultivation and, in particular, by retting of fiber plants during the Middle Ages and Renaissance (AD ~1000 to ~1700). Cultural eutrophication during the latter phase caused unprecedented changes in the lake, including altered species composition, high production, and strongly accelerated sediment accumulation rates. Generally, catchment deforestation was related to elevated proportions of metal (iron, aluminum, calcium)-bound P forms in the sediment, while high tree cover correlated with elevated proportions of P forms associated with organic material (“organic” P, humic-bound P, refractory organic P) and loosely bound P. During phases with forest in the catchment, silicon (Si) inputs to the lake were insufficient and diatom frustules were mostly absent in the sediments. In contrast, diatoms thrived in the lake when the landscape was open and erosional Si influx was high. This study is the first to show long-term (~eight millennia) and recurring Si limitation of diatoms, a finding that may explain the absence of diatoms in sediment records of other sites too. In summary, human land-use with preceding deforestation accelerated the transport of nutrients and elements from the terrestrial to the aquatic environment, leading to substantial and irreversible changes in Lake Fuglsø. Our study is a good example of the tight links between catchment processes and lake status, indicating that catchment dynamics should be considered in lake restoration projects, particularly for lowland lakes with high catchment : lake area ratios.https://doi.org/10.1002/ecm.1455Tropical Asian mega-delta ponds: Important and threatened socio-ecological systems
This paper uses multimedia to showcase the narratives and lived experiences of those who live and work in tropical Asian mega-deltas, and as such is the first journal article of its kind in the field of Regional Geography. Using videos, photography and audio this paper describes the characteristics of ponds and their place in the intrinsically connected human-environmental fabric of these delta regions. The aim is to bring to life descriptive inventories and provide greater weight in support of our conclusion that tropical Asian mega-delta ponds are important and threatened systems. River deltas comprise just 1% of land cover worldwide but support the livelihoods of more than 500 million people. Delta research has historically focused on the major river channels and the socio-ecological role of ponds has been overlooked despite their large number and surface area. Ponds are intrinsically linked to daily life (potable water, sanitation, bathing, washing), industry (aquaculture, agriculture) and the natural-cultural heritage (religion, folklore) of deltas. In contrast to the larger river channels, ponds are likely to be significant stores and processors of nutrients, including carbon, and pollutants at annual to decadal scales, on account of their heavy anthropogenic use and smaller individual sizes. Consequently, they are severely polluted water sources and pose significant public health risks. In this review, we use case studies from three Asian mega-deltas (the Red River Delta and the Mekong River Delta, Vietnam, and the Ganges-Brahmaputra-Meghna Delta, India and Bangladesh) to highlight the importance of Asian mega-delta ponds as important socio-ecological systems in their own right. We discuss future environmental challenges, knowledge gaps on the ecological function and biodiversity of these habitats, management and policy practices, and the capacity of ponds to achieve the UN Sustainable Development Goals.https://doi.org/10.1002/geo2.103Lake ecosystem on the Qinghai–Tibetan Plateau severely altered by climatic warming and human activity
Modelling, monitoring, and experimental data have shown that global climate change can impact aquatic phytoplankton communities directly, through the effects of warming on primary producers, as well as indirectly through cascading effects from higher trophic levels. Although both concepts are common in modern limnological studies, it remains unclear whether the ‘top-down’ effects from higher trophic levels on phytoplankton exert strong effects in natural systems over long (centennial) timescales. Here, we use multiproxy data including pigments, zooplankton remains, nutrient concentrations, and paleoclimate indicators from a sediment core in Dagze Co, Central Tibet (a two-trophic level lake) to reconstruct algal production, zooplankton community, nutrient and salinity changes. Our results show that top-down effects of higher trophic levels offset effects from warming and nutrient addition on algal growth. Warming enhanced glacial meltwater inflow to the lake, and intensive human activities increased nutrient inputs. Changes in lake salinity and N:P ratios coincided with zooplankton community shifts during the past 600 years, and Daphnia tibetana replaced the brine shrimp, Artemia tibetiana, after the relocation of a town to upstream of the lake in the 1980s led to overharvesting of the brine shrimp. These shifts contributed strongly to changes in algal communities, with changes in zooplankton leading to strong top-down effects that decreased algal production through increasing grazing pressure despite increasing nutrient concentrations. Our results suggest that the typical external drivers (climate and nutrients) of lake ecosystems may be suppressed by internal shifts in plankton communities in lakes.https://doi.org/10.1016/j.palaeo.2021.110509Can δ18O help indicate the causes of recent lake area expansion on the western Tibetan Plateau? A case study from Aweng Co
Glacier-fed lakes on the Tibetan Plateau (TP) have undergone rapid expansions since the late 1990s, concurrent with the changing climate. However, the dominant cause(s) of lake area increases is still debated. To identify the drivers of lake expansion, we studied Aweng Co, a glacier-fed lake in the western TP, where surface area has increased (0.74 km2 year−1) since the late 1970s and most rapidly (0.998 km2 year−1) since the late 1990s. A water balance model was used to clarify the reasons for increased lake water volume, supported by stable isotope hydrology and the δ18O change recorded in recent sediments. Results showed that glacial meltwater probably had the biggest impact on changes in Aweng Co lake level in recent decades, but that precipitation was also an important contributor. Our study shows that δ18O of carbonate (δ18Ocarb) has great potential for indicating source changes of water supply in such lakes, but there is a need to be cautious when interpreting δ18Ocarb due to the influence of multiple hydrological factors, which can change in dominance over time.https://doi.org/10.1007/s10933-020-00158-6Local and Regional Drivers of Environmental Changes in Two Subtropical Montane Ponds (Central China) Over the Last Two Centuries
Central China, one of the Earth’s distinctive ecoregions due to its endemic subtropical biota, has been subjected to enhanced nitrogen deposition and climate warming during recent decades. However, the extent and timescale of ecological changes are largely unexplored. Multiproxy analyses (diatoms, photosynthetic pigments and geochemistry) of 210Pb-dated sediment cores from two shallow ponds within an alpine basin (central China) were used to investigate the response of primary producer communities to external stressors during the last two centuries. The study sites include one drainage pond and one seepage pond. Both ponds exhibited unambiguous changes in production and composition of photoautotrophs since the early twentieth century, which are linked to climate warming, nitrogen deposition and local factors (for example, lake morphometry, desiccation and macrophyte). Although primary producers responded to regional warming and nitrogen deposition, the ecological responses differed among ponds due to local factors. In the deeper seepage pond, light attenuation due to terrestrial organic matter input caused recent decreases in carotenoids and small fragilarioid taxa. In contrast, the co-occurrence of euterrestrial and tychoplanktonic diatoms in the shallower drainage pond was indicative of its hydrological instability. Our results indicate that subtropical montane ponds in the East Asian monsoon region appear to be strongly influenced by a combination of local (for example, catchment-lake connectivity) and regional driving forces (for example, warming and nitrogen deposition). © 2020, Springer Science+Business Media, LLC, part of Springer Nature.https://doi.org/10.1007/s10021-020-00535-2Climatic and environmental change in the western Tibetan Plateau during the Holocene, recorded by lake sediments from Aweng Co
Understanding the strength and extent of the Asian summer monsoon (including the East Asian summer monsoon and the Indian summer monsoon) in the Tibetan Plateau (TP) region is crucial for predicting possible changes in the regional eco-environment and water resources under global warming. Due to the lack of well-dated and high-resolution paleoclimate records, long-term monsoon dynamics are still not well understood in the western TP, which is currently influenced by both the Indian summer monsoon (ISM) and the westerlies. Here we present a multi-proxy lacustrine record covering the past 10,500 years from Aweng Co, an alpine lake at the northern limit of the modern ASM in western Tibet. Our results show that the western TP was mainly controlled by the ISM during the Holocene and the regional ecosystem/environment was sensitive to climate change. The climate was the wettest between 10.5 and 7.3 cal kyr BP, when terrestrial plants in the catchment were productive and the biomass of benthic algae was low possibly due to limited sunlight at the lake bottom due to high lake level. From 7.3 to 5.0 cal kyr BP the climate shifted towards drier conditions, resulting in a decline in terrestrial plant cover. Between 5.0 and 3.1 cal kyr BP, the climate became even drier, resulting in a further decline in vegetation cover in the catchment. Between 4.6 and 3.1 cal kyr BP, 100% endogenic dolomite precipitated from the lake water, possibly induced by high Mg/Ca ratios. After 3.1 cal kyr BP, the climate was the driest and frequent centennial-scale droughts occurred. The lake level was low and would have resulted in more light reaching the lake bottom, favoring the growth of benthic algae. The reconstructed lake level change of Aweng Co agrees well with the paleo-shoreline records in the southern TP, demonstrating that the ISM evolution played a key role in lake hydrological processes in this region. A comparison of paleoclimate records shows the ISM reached 34.5° N in the western TP during the Holocene.https://doi.org/10.1016/j.quascirev.2021.106889The Indian Sundarbans: Biogeochemical Dynamics and Anthropogenic Impacts
The Sundarbans region is one of the richest ecosystems in the world and is located on one of the world’s largest deltas – the Ganges–Brahmaputra–Meghna system. The Indian Sundarbans have exceptional biodiversity, including rare and globally threatened species, and is made up of a mangrove forest ecosystem with an interconnected network of rivers. The hydrology of the Sundarbans underpin ecosystem health and the potential impact of humans on the region, as the tidal cycle changes water salinity diurnally and freshwater supply changes seasonally with the monsoon. The Indian Sundarbans face multiple pressures with both a reduction in freshwater supply and rising relative sea-level, leading to increased salinization of the mangrove forest. Human-driven alteration of the Sundarbans river catchments is reducing sediment flow, and when coupled with land-use change, is leading to subsidence, deforestation, nutrient enrichment, and heavy metal pollutants impacting the health of the ecosystem. All of these impacts have important ramifications for carbon fluxes that could exacerbate climate change and ecosystem health. In this chapter, we present an overview of our current understanding of biogeochemical dynamics and anthropogenic impacts on the Indian Sundarbans, with a particular focus on water quality, aquatic ecology, and carbon dynamics.https://doi.org/10.1007/978-3-030-68980-3_15Healthy waterways and ecologically sustainable cities in Beijing-Tianjin-Hebei urban agglomeration (northern China): Challenges and future directions
The cities across the northern dry region of China are exposed to multiple sustainability challenges. Beijing-Hebei-Tianjin (BTH) urban agglomeration, for example, experiences severe water shortages due to rapidly expanding urban populations, industrial use, and irrigation-intensive agriculture. Climate change has further threatened water resources security. Overuse of water resources to meet the demand of various water sectors has far-reaching health and environmental implications including ecosystem sustainability. Surface water and groundwater pollution present public health risks. Despite the extraordinary policies and efforts being made and implemented by the Government of China, the BTH region currently lacks coordination among stakeholders leading to poor water governance. Consultation among scientists, engineers and stakeholders on regional water security issues is crucial and must be frequent and inclusive. An international symposium was held in Shijiazhuang in early November 2019 to identify some of the key water security challenges and scope of an idealized future eco-city in the region by developing a sustainability framework. This work drew on experiences from across China and beyond. Scientists agree that integration of science, technology, and governance within an appropriate policy framework was particularly significant for combating the issue of water insecurity, including in the region's newly developed city, Xiong'an New Area. An emerging concept, “Healthy Waterways and Ecologically Sustainable Cities” which integrates social, ecological and hydrological systems and acts as an important pathway for sustainability in the 21st century was proposed in the symposium to tackle the problems in the region. This high level biophysical and cultural concept empowers development goals and promotes human health and wellbeing. The framework on healthy waterways and ecologically sustainable cities can overcome sustainability challenges by resolving water resource management issues in BTH in a holistic way. To implement the concept, we strongly recommend the utilization of evidence-based scientific research and institutional cooperation including national and international collaborations to achieve the Healthy Waterways and Ecologically Sustainable Cities goal in the BTH in future. This article is categorized under: Water and Life > Conservation, Management, and Awareness.https://doi.org/10.1002/wat2.1500Early historical forest clearance caused major degradation of water quality at Lake Væng, Denmark
Although humans have impacted their environment over millennia, details of these impacts, especially on aquatic systems, is still surprisingly scarce despite potential disturbance by early land use. This study examined a high-resolution radiocarbon-dated Holocene record from the Danish Lake Væng, using geochemical and biological proxies, and related the observed impacts to other lake records with catchment disturbance. The results indicate a lengthy and varying history of aquatic eutrophication linked to human activity. Modest impacts on the lake coincided with the first signs of landscape disturbance during the Neolithic (c. 4500 cal. yrs BP). Observed impacts intensified in the Late Bronze and Pre-Roman Iron Age. Viking Age/Medieval deforestation and erosional inputs to the lake associated with new ploughing technology (1200 cal. yrs BP), however, led to a major reorganisation of the aquatic ecosystem. Filamentous bloom-forming cyanobacteria, common today in heavily culturally impacted lakes, reached a historical maxima. The lake ecosystem subsequently recovered somewhat but remains eutrophic to date. The erosion record from Lake Væng shows a striking similarity with other Danish lake records, especially the notable increase in Medieval Period catchment inputs, which are observed in other European lacustrine records. Numerous European lowland lakes may have shifted into a degraded ecological state millennia ago, but degradation intensified during the onset of the Medieval Period. Hence, assuming pre-industrial conditions as relatively pristine reference baselines for more recent cultural eutrophication could be flawed in landscapes intensively used by humans for millennia.https://doi.org/10.1016/j.ancene.2021.100302Effects of climate change on a subtropical montane peatland over the last two centuries: Evidence from diatom records
Climate variability can induce rapid changes in peatland ecosystems, affecting both carbon cycling and vegetation succession. Diatoms are an important group of ubiquitous and diverse algae in peatlands. Until now, the responses of diatom communities to climate variability have rarely been explored in peatlands, especially in subtropical regions. In this study, monitoring and paleolimnological datasets were combined to decipher environmental changes of a subtropical montane peatland (central China) over the last two centuries. Seasonal monitoring data revealed that diatom communities were closely correlated with precipitation, depth to the water table, conductivity, nitrate and temperature. Sedimentary records revealed that temporal changes in diatom assemblages and geochemical elements displayed similar trends in two peat cores after the 1950s. The first gradient in diatom composition represented a shift from Pinnularia species to taxa preferring less-acidic habitats, which was closely linked to climate warming and the enrichment of inorganic elements (e.g. sodium and calcium) since the early 20th century. Meanwhile, changes in diatom communities were further related to precipitation variability, atmospheric deposition and local hydrogeomophic setting. Taken together, the succession of diatom communities was closely linked to climate-regulated availability of nutrients and moisture in this subtropical peatland over the last two centuries. In order to achieve sustainable management of these scarce peatlands, further biological monitoring and paleoecological studies are needed to improve our knowledge of peatland ecosystem evolution in response to future climate change. © The Author(s) 2021.https://doi.org/10.1177/09596836211003220Changing nutrient cycling in Lake Baikal, the world’s oldest lake
Lake Baikal, lying in a rift zone in southeastern Siberia, is the world’s oldest, deepest, and most voluminous lake that began to form over 30 million years ago. Cited as the “most outstanding example of a freshwater ecosystem” and designated a World Heritage Site in 1996 due to its high level of endemicity, the lake and its ecosystem have become increasingly threatened by both climate change and anthropogenic disturbance. Here, we present a record of nutrient cycling in the lake, derived from the silicon isotope composition of diatoms, which dominate aquatic primary productivity. Using historical records from the region, we assess the extent to which natural and anthropogenic factors have altered biogeochemical cycling in the lake over the last 2,000 y. We show that rates of nutrient supply from deep waters to the photic zone have dramatically increased since the mid-19th century in response to changing wind dynamics, reduced ice cover, and their associated impact on limnological processes in the lake. With stressors linked to untreated sewage and catchment development also now impacting the near-shore region of Lake Baikal, the resilience of the lake’s highly endemic ecosystem to ongoing and future disturbance is increasingly uncertain.https://doi.org/10.1073/pnas.2013181117Mercury loading within the Selenga River basin and Lake Baikal, Siberia
Mercury (Hg) loading in Lake Baikal, a UNESCO world heritage site, is growing and poses a serious health concern to the lake's ecosystem due to the ability of Hg to transform into a toxic form, known as methylmercury (MeHg). Monitoring of Hg into Lake Baikal is spatially and temporally sparse, highlighting the need for insights into historic Hg loading. This study reports measurements of Hg concentrations from water collected in August 2013 and 2014 from across Lake Baikal and its main inflow, the Selenga River basin (Russia, Mongolia). We also report historic Hg contamination using sediment cores taken from the south and north basins of Lake Baikal, and a shallow lake in the Selenga Delta. Field measurements from August 2013 and 2014 show high Hg concentrations in the Selenga Delta and river waters, in comparison to pelagic lake waters. Sediment cores from Lake Baikal show that Hg enrichment commenced first in the south basin in the late-19th century, and then in the north basin in the mid-20th century. Hg flux was also 20-fold greater in the south basin compared to the north basin sediments. Hg enrichment was greatest in the Selenga Delta shallow lake (Enrichment Ratio (ER) = 2.3 in 1994 CE), with enrichment occurring in the mid-to late-20th century. Local sources of Hg are predominantly from gold mining along the Selenga River, which have been expanding over the last few decades. More recently, another source is atmospheric deposition from industrial activity in Asia, due to rapid economic growth across the region since the 1980s. As Hg can bioaccumulate and biomagnify through trophic levels to Baikal's top consumer, the world's only truly freshwater seal (Pusa sibirica), it is vital that Hg input at Lake Baikal and within its catchment is monitored and controlled.https://doi.org/10.1016/j.envpol.2019.113814Towards the conservation of Borneo’s freshwater mussels: rediscovery of the endemic Ctenodesma borneensis and first record of the non-native Sinanodonta lauta
The freshwater mussel fauna of Borneo is highly endemic, with at least 11 species being unique to that island. Most of these species have not been recorded for at least 50 years owing to a lack of sampling effort and large-scale habitat destruction and degradation. Surveys conducted in 2016 across much of Malaysian Borneo failed to locate four out of five native species historically recorded in the study area. The present study aimed to determine the diversity and distribution of freshwater mussels of Brunei and adjacent Limbang Division, Malaysia. In 2018, we conducted interviews with locals, recorded environmental data and surveyed mussels at 43 sites, and conducted interviews at a further 38 sites. Only one population of native mussels, i.e. Ctenodesma borneensis, was found in a small tributary of the Limbang River situated in a patch of intact rainforest, representing the first record of this Bornean endemic genus since 1962. In addition, Sinanodonta lauta was found in a pond in Lawas district, representing the first record of this species outside its native East Asian distribution. Our data suggest that C. borneensis can sustain populations in relatively undisturbed habitats and is likely to have suffered population losses across northern Borneo. The first molecular phylogenetic analysis (COI + 28S) including an endemic Bornean freshwater mussel genus revealed that Ctenodesma is phylogenetically divergent from all other previously sampled lineages, rendering it a particularly valuable conservation target.https://doi.org/10.1007/s10531-020-01971-1Rapidly rising transboundary atmospheric pollution from industrial and urban sources in Southeast Asia and its implications for regional sustainable development
Transboundary atmospheric pollution is a major concern throughout much of Southeast Asia (SEA), although most attention has, to date, focused on episodic haze events associated with biomass burning in the region. Here, we reconstruct long-term variations in transboundary inputs of chromium (Cr), an industrial pollutant, to Singapore over the period 1900-2017 by adopting a novel catchment-reservoir mass balance methodology that combines a national emissions inventory and a paleolimnological approach. Results show periods of low (before the 1950s) and relatively stable (the 1950s-1980s) levels of transboundary Cr deposition in Singapore followed by an unambiguous increase from ca. 1990 onwards, most likely linked to the onset of rapid industrialisation in neighbouring parts of Malaysia and Indonesia. Notably, from ca. 2000 onwards, fluxes of transboundary Cr deposition in Singapore further increased by 3% per year, almost doubling from 6 ± 3 tonne Cr yr-1 in 2000 to around 11 ± 3 tonne Cr yr-1 in 2017. This post-2000 rapid increase may reflect the effects of globalisation, pro-export driven economic growth policies and increasing capital inflows to the whole region, including from Singapore, all of which combined to drive industrialisation throughout much of SEA. The current trend of increasing transboundary pollution from anthropogenic activity highlights an urgent need for effective collaboration among countries in SEA in order to improve well-being and help guarantee sustainable development throughout the region.https://doi.org/10.1088/1748-9326/abb5ceDiatoms in a sediment core from a flood pulse wetland in Malaysia record strong responses to human impacts and hydro-climate over the past 150 years
Rapid development and climate change in southeast Asia is placing unprecedented pressures on freshwater ecosystems, but long term records of the ecological consequences are rare. Here we examine one basin of Tasik Chini (Malaysia), a UNESCO-designated flood pulse wetland, where human disturbances (dam installation, iron ore mining, oil palm and rubber cultivation) have escalated since the 1980s. Diatom analysis and organic matter geochemistry (δ13Corg and C/N ratios) were applied to a sediment sequence to infer ecological changes in the basin since c. 1900 CE. As the Tasik Chini wetland is a rare ecosystem with an unknown diatom ecology, contemporary diatom habitats (plant surfaces, mud surfaces, rocks, plankton) were sampled from across the lake to help interpret the sedimentary record. Habitat specificity of diatoms was not strongly defined and, although planktonic and benthic groupings were distinctive, there was no difference in assemblages among the benthic habitat surfaces. An increase in the proportion of benthic diatom taxa suggests that a substantial decrease in water level occurred between c. 1938 and 1995 CE, initiated by a decline in rainfall (supported by regional meteorological data), which increased the hydrological isolation of the sub-basin. Changes in the diatom assemblages were most marked after 1995 CE when the Chini dam was installed. After this time both δ13Corg and C/N decreased, suggesting an increase in autochthonous production relative to allochthonous river flood pulse inputs. Oil palm plantations and mining continued to expand after c. 1995 CE and we speculate that inputs of pollutants from these activities may have contributed to the marked ecological change. Together, our work shows that this sub-basin of Tasik Chini has been particularly sensitive to, and impacted by, a combination of human and climatically induced changes due to its hydrologically isolated position.https://doi.org/10.1002/geo2.90Diatom-based water-table reconstruction in Sphagnum peatlands of northeastern China
Peatlands are important ecosystems for biodiversity conservation, global carbon cycling and water storage. Hydrological changes due to climate variability have accelerated the degradation of global and regional ecosystem services of peatlands. Diatoms are important producers and bioindicators in wetlands, but comprehensive diatom-based inference models for palaeoenvironmental reconstruction in peatlands are scarce. To explore the use of diatoms for investigating peatland hydrological change, this study established a training set consisting of diatom composition and twelve environmental factors from 105 surface samples collected from five Sphagnum peatlands in northeastern China. Diatom communities were dominated by Eunotia species. Ordination analyses showed that depth to the water table (DWT) was the most important factor influencing diatom distribution, independently accounting for 4.99% of total variance in diatom data. Accordingly, a diatom-based DWT transfer function was developed and thoroughly tested. The results revealed that the best-performing model was based on weighted averaging with inverse deshrinking (R2 = 0.66, RMSEP = 8.8 cm with leave-one-out cross validation). Quantitative reconstruction of DWT on a short peat core collected from the Aershan Peatland (Inner Mongolia) recorded climate-mediated hydrological changes over the last two centuries. This study presents the first diatom-water table transfer function in Sphagnum peatlands, and highlights the potential of diatoms as a powerful tool to assess the magnitude of past hydrological changes in peatlands of northeastern China, as well as similar peaty environments worldwide. © 2020 Elsevier Ltdhttps://doi.org/10.1016/j.watres.2020.115648Using stable isotopes to estimate young water fractions in a heavily regulated, tropical lowland river basin
The young water fraction of streamflow (Fyw), an important hydrological variable, has been calculated for the first time, for a monsoon-fed coastal catchment in northern Vietnam. Oxygen stable isotopes (δ18O) from six river sites in the Day River Basin (DRB) were analysed monthly, between January 2015 and December 2018. River δ18O signatures showed sine wave variability, reflecting the amount effect and tropical (dry-rainy) seasonality of the region. The δ18O composition of precipitation ranged from −12.67 to +1.68‰, with a mean value of −5.14‰, and in-streamflow signatures ranged from −11.63 to −1.37‰ with a mean of −5.02‰. Fractions of young water (Fyw) were calculated from the unweighted and flow-weighted δ18O composition of samples. Unweighted Fyw ranged between 29 ± 8% and 82 ± 21% with a mean value of 51 ± 19%, and was not significantly different from flow-weighted Fyw (range between 33 ± 25% and 92 ± 73%, mean 52 ± 36%). Both unweighted and flow-weighted Fyw were highest in the middle of stream and lowest in downstream sites, capturing the impacts of landuse changes, hydrology and human activities in the catchment. Our calculations imply that more than a half of rainwater reaches the DRB river mainstream within the first 3 months. The Fyw is much higher than the global average (of one-third) and insensitive to discharge due to the combination of a humid catchment with high rainfall, low storage capacity, flat landscape and an intensive drainage system in the DRB. Also the low discharge sensitivity of Fyw in the DRB implies that the regional hydrology is severely altered by humans.https://doi.org/10.1002/hyp.13878Source and quantity of carbon influence its sequestration in Rostherne Mere (UK) sediment: a novel application of stepped combustion radiocarbon analysis
We explored the roles of phytoplankton production, carbon source, and human activity on carbon accumulation in a eutrophic lake (Rostherne Mere, UK) to understand how changes in nutrient loading, algal community structure and catchment management can influence carbon sequestration in lake sediments. Water samples (dissolved inorganic, organic and particulate carbon) were analysed to investigate contemporary carbon sources. Multiple variables in a 55-cm sediment core, which represents the last ~ 90 years of accumulation, were studied to determine historical production rates of algal communities and carbon sources. Fluctuations in net primary production, inferred from sedimentary diatom abundance and high-performance liquid chromatography (HPLC) pigment methods, were linked to nutrient input from sewage treatment works (STW) in the catchment. Stepped combustion radiocarbon (SCR) measurements established that lake sediment contains between 11% (~ 1929 CE) and 69% (~ 1978 CE) recalcitrant carbon, with changes in carbon character coinciding with peaks in accumulation rate and linked to STW inputs. Catchment disturbance was identified by radiocarbon analysis, and included STW construction in the 1930s, determined using SCR analysis, and recent nearby highway construction, determined by measurements on dissolved organic carbon from the lake and outflow river. The quantity of autochthonous carbon buried was related to diatom biovolume accumulation rate (DBAR) and decreased when diatom accumulation rate and valve size declined, despite an overall increase in net carbon production. HPLC pigment analysis indicated that changes in total C deposition and diatom accumulation were related to proliferation of non-siliceous algae. HPLC results also indicated that dominance of recalcitrant carbon in sediment organic carbon was likely caused by increased deposition rather than preservation factors. The total algal accumulation rate controlled the sediment organic carbon accumulation rate, whereas DBAR was correlated to the proportion of each carbon source buried.https://doi.org/10.1007/s10933-020-00141-1Shallow water phytoplankton responses to nitrate and salinity enrichment may be modified by benthic processes
The effects of salinity (as chloride [Cl] at 600, 1000, 1600, and 2500 mg L−1) and nitrate (as nitrogen [N] loading rates using concentrations of 1, 2, 5, and 10 mg L−1) additions on phytoplankton communities (as chlorophyll and carotenoid pigments) were determined using a fully factorial 3 m3 mesocosm pond experiment. Redundancy analysis followed by variance partitioning analysis (VPA) statistically compared phytoplankton with water chemistry, zooplankton, phytobenthos (aquatic plants and periphyton), and zoobenthos to understand relationships among benthic and pelagic components. Repeated measures analysis of variance (RM-ANOVA) indicated no interactive effects of the 2 treatments. In VPA, physicochemical variables explained the most variance (33.6%) in the phytoplankton pigment dataset relative to benthic primary producers (0.4%) and invertebrates (2.3%). Salinisation led to an increase in biomass of planktonic siliceous algae (Cl ≥1600 mg L−1) and chlorophytes and cyanobacteria (Cl ≥2500 mg L−1), which we infer was caused by increased phosphorus release from sediments while aquatic plants and periphyton declined. Nitrate additions increased the biomass of cryptophytes and chlorophytes at intermediate N loading rates using concentrations of 5 mg L−1 (associated with greater ammonium [NH4-N] availability and shifts in aquatic plant composition). These findings support the hypothesis that the relative availability of reduced versus oxidised N forms is an important driver of phytoplankton composition. Together, these results suggest that pelagic biota are highly sensitive to salinity and nitrate increases, and that the phytoplankton compositional shifts are driven by indirect effects on water chemistry (bioavailable P mobilisation, changes in N forms), which are mediated by benthic processes.https://doi.org/10.1080/20442041.2019.1634948Marine resource abundance drove pre-agricultural population increase in Stone Age Scandinavia
How climate and ecology affect key cultural transformations remains debated in the context of long-term socio-cultural development because of spatially and temporally disjunct climate and archaeological records. The introduction of agriculture triggered a major population increase across Europe. However, in Southern Scandinavia it was preceded by ~500 years of sustained population growth. Here we show that this growth was driven by long-term enhanced marine production conditioned by the Holocene Thermal Maximum, a time of elevated temperature, sea level and salinity across coastal waters. We identify two periods of increased marine production across trophic levels (P1 7600–7100 and P2 6400–5900 cal. yr BP) that coincide with markedly increased mollusc collection and accumulation of shell middens, indicating greater marine resource availability. Between ~7600–5900 BP, intense exploitation of a warmer, more productive marine environment by Mesolithic hunter-gatherers drove cultural development, including maritime technological innovation, and from ca. 6400–5900 BP, underpinned a ~four-fold human population growth.https://doi.org/10.1038/s41467-020-15621-1Admixture between Ancient Lineages, Selection, and the Formation of Sympatric Stickleback Species-Pairs
Ecological speciation has become a popular model for the development and maintenance of reproductive isolation in closely related sympatric pairs of species or ecotypes. An implicit assumption has been that such pairs originate (possibly with gene flow) from a recent, genetically homogeneous ancestor. However, recent genomic data have revealed that currently sympatric taxa are often a result of secondary contact between ancestrally allopatric lineages. This has sparked an interest in the importance of initial hybridization upon secondary contact, with genomic reanalysis of classic examples of ecological speciation often implicating admixture in speciation. We describe a novel occurrence of unusually well-developed reproductive isolation in a model system for ecological speciation: the three-spined stickleback (Gasterosteus aculeatus), breeding sympatrically in multiple lagoons on the Scottish island of North Uist. Using morphological data, targeted genotyping, and genome-wide single-nucleotide polymorphism data, we show that lagoon resident and anadromous ecotypes are strongly reproductively isolated with an estimated hybridization rate of only ∼1%. We use palaeoecological and genetic data to test three hypotheses to explain the existence of these species-pairs. Our results suggest that recent, purely ecological speciation from a genetically homogeneous ancestor is probably not solely responsible for the evolution of species-pairs. Instead, we reveal a complex colonization history with multiple ancestral lineages contributing to the genetic composition of species-pairs, alongside strong disruptive selection. Our results imply a role for admixture upon secondary contact and are consistent with the recent suggestion that the genomic underpinning of ecological speciation often has an older, allopatric origin.https://doi.org/10.1093/molbev/msz161Dissolved inorganic geogenic phosphorus load to a groundwater-fed lake: Implications of terrestrial phosphorus cycling by groundwater
The general perception has long been that lake eutrophication is driven by anthropogenic sources of phosphorus (P) and that P is immobile in the subsurface and in aquifers. Combined investigation of the current water and P budgets of a 70 ha lake (Nørresø, Fyn, Denmark) in a clayey till-dominated landscape and of the lake's Holocene trophic history demonstrates a potential significance of geogenic (natural) groundwater-borne P. Nørresø receives water from nine streams, a groundwater-fed spring located on a small island, and precipitation. The lake loses water by evaporation and via a single outlet. Monthly measurements of stream, spring, and outlet discharge, and of tracers in the form of temperature, δ18O and δ2H of water, and water chemistry were conducted. The tracers indicated that the lake receives groundwater from an underlying regional confined glaciofluvial sand aquifer via the spring and one of the streams. In addition, the lake receives a direct groundwater input (estimated as the water balance residual) via the lake bed, as supported by the artesian conditions of underlying strata observed in piezometers installed along the lake shore and in wells tapping the regional confined aquifer. The groundwater in the regional confined aquifer was anoxic, ferrous, and contained 4-5 μmol/L dissolved inorganic orthophosphate (DIP). Altogether, the data indicated that groundwater contributes from 64% of the water-borne external DIP loading to the lake, and up to 90% if the DIP concentration of the spring, as representative for the average DIP of the regional confined aquifer, is assigned to the estimated groundwater input. In support, paleolimnological data retrieved from sediment cores indicated that Nørresø was never P-poor, even before the introduction of agriculture at 6000 years before present. Accordingly, groundwater-borne geogenic phosphorus can have an important influence on the trophic state of recipient surface water ecosystems, and groundwater-borne P can be a potentially important component of the terrestrial P cycle.https://doi.org/10.3390/w11112213Response of boreal lakes to changing wind strength: Coherent physical changes across two large lakes but varying effects on primary producers over the 20th century
Near-surface wind speeds have changed over recent decades, raising questions about the extent to which these changes are altering the vertical thermal structure of lakes and affecting lake food webs. Neo- and paleolimnological techniques were used to assess wind-driven changes in lake thermal habitat and resulting effects on primary producers in two lakes in Isle Royale National Park, an island archipelago located in Lake Superior, where wind speed has increased in recent decades. Responses in Siskiwit Lake, a large (16 km2 surface area), deep (Zmax = 49 m), oligotrophic lake, were compared to those of Lake Desor, a moderately large (4.3 km2) but shallower (Zmax = 13 m), mesotrophic lake. High-frequency sensor data suggested that changes in wind speed affected epilimnion thickness in both lakes synchronously (ρ = 0.7, p < 0.001). Diatom-inferred mixing depths suggested a coherent shift in both lakes to deeper mixing (an increase of 3 and 6 m) since 1920 (ρ = 0.8), which was correlated with an increase in regional wind speed during the 20th century at the decadal-scale in Lake Desor and Siskiwit Lake (ρ = 0.6 and 0.4, respectively). In Lake Desor, algal biomass declined as mixing deepened from 1920 to 1980, and then cyanobacteria and cryptophyte pigments increased from 1980 to present, a period of inferred stable and deep mixing. Algal pigment concentrations in Siskiwit Lake were unchanged as mixing depth deepened. Although changes in wind speed altered lake physical structure similarly, the ecological consequences of these changes differed between lakes and were most likely influenced by lake-specific variability in nutrient and light availability.https://doi.org/10.1002/lno.11181Potential anthropogenic regime shifts in three freshwater lakes in Tropical East Asia
Regime shifts in ecology are characterised by major, often abrupt changes in ecosystem structure and functioning in response to one or more driving variables, or pressures. Changes in the provision of ecosystem services are a potential outcome. Despite the current combination of rapidly increasing pressures on what are often highly important socio-ecological systems, the resilience of lakes in the warm tropics to human perturbation is far less well understood than those at higher latitudes. This paper focuses on evidence of aquatic ecosystem change from a cluster of three deep, freshwater, volcanic crater lakes (Yambo, Mohicap, and Sampaloc) at low altitude on the island of Luzon, Philippines. The lakes support different intensities of aquaculture, an important livelihood but also a driver of poor water quality throughout tropical Asia. Measured and monitored climate and water quality data, in addition to sedimentary evidence from sediment cores collected from the three study lakes, were used to determine the magnitude and trajectory of changes in lake water quality. Sediment cores were radiometrically dated and analysed for organic matter, spheroidal carbonaceous particles, and diatom remains. Diatom data were zoned numerically using cluster analysis. Diatom remains were also used to infer past variations in pH and possible relationships between potential driving climatic variables (temperature and rainfall). Diatom data sets were explored using detrended component analysis and principle component analysis. Despite differences in intensity of aquaculture, a common trajectory and timing of a potential regime shift, characterised by a replacement of benthic with planktonic diatoms and an increase in diatom accumulation rates from the early to mid-1980s, is evident, and attests a low threshold for disturbance effects. A predominantly planktonic diatom flora has persisted even after recent improvements in environmental quality. The potential new regime may be less resilient and more susceptible to harmful algal blooms, abrupt expansions of anoxic conditions, and periodic mass fish kills when compared with its former state. The research further highlights the sensitivity of freshwater ecosystems in the warm tropics to disturbance pressures, and the risks to livelihoods, ecosystem services, and sustainable development.https://doi.org/10.1111/fwb.13256Arctic climate shifts drive rapid ecosystem responses across the West Greenland landscape
Prediction of high latitude response to climate change is hampered by poor understanding of the role of nonlinear changes in ecosystem forcing and response. While the effects of nonlinear climate change are often delayed or dampened by internal ecosystem dynamics, recent warming events in the Arctic have driven rapid environmental response, raising questions of how terrestrial and freshwater systems in this region may shift in response to abrupt climate change. We quantified environmental responses to recent abrupt climate change in West Greenland using long-term monitoring and paleoecological reconstructions. Using >40 years of weather data, we found that after 1994, mean June air temperatures shifted 2.2 °C higher and mean winter precipitation doubled from 21 to 40 mm; since 2006, mean July air temperatures shifted 1.1 °C higher. Nonlinear environmental responses occurred with or shortly after these abrupt climate shifts, including increasing ice sheet discharge, increasing dust, advancing plant phenology, and in lakes, earlier ice out and greater diversity of algal functional traits. Our analyses reveal rapid environmental responses to nonlinear climate shifts, underscoring the highly responsive nature of Arctic ecosystems to abrupt transitions.https://doi.org/10.1088/1748-9326/ab2928Determination of geochronology and sedimentation rates of shallow lakes in the middle Yangtze reaches using 210Pb, 137Cs and spheroidal carbonaceous particles
Accurate chronologies for recent sediments of shallow lakes in the Yangtze floodplain are critical to calibrate proxy records for reconstructing environmental changes during the past century. This study presents the results of detailed 210Pb analysis from eight lake sediment cores collected from the middle Yangtze reaches, southeast China. Unsupported 210Pb activities generally declined exponentially with mass depth in the eight cores. The chronologies and sedimentation rates for the sediment cores were calculated using different 210Pb-based mathematical models. The 137Cs chronomarker (i.e. the 1963 fallout peak) and the spheroidal carbonaceous particle (SCP) chronomarker (i.e. the start of the rapid increase in 1970 CE) were selected to validate the 210Pb dating. Sedimentation rates derived from different models were validated using historical data including lake area, arable land area, sediment discharge and reservoir volume in the Yangtze basin. The SCP-corrected CRS (constant rate of supply) model performs better than other models, based on validation using historical documents in the Yangtze basin. The 137Cs chronomarker might be erroneous due to catchment-driven 137Cs inputs from soil erosion and post-depositional diffusion. Both SCPs and 137Cs are susceptible to inputs from catchment soil erosion, but SCPs show no apparent degradation and post-depositional changes in lake sediments. The SCP profile provides a relatively reliable chronomarker, which can be used for validating 210Pb chronologies in these floodplain lakes. Generally, sedimentation rates in the eight lakes were <0.2 g cm−2 yr−1 before the 1930s, and then increased to a peak in the 1960s. Afterwards, sedimentation rates decreased and remained low after the 1980s.https://doi.org/10.1016/j.catena.2018.11.041Effects of mussels on nutrient cycling and bioseston in two contrasting tropical freshwater habitats
Freshwater mussels (Unionida) can strongly affect nutrient cycling in temperate ecosystems but data from the tropics is lacking. We quantified the effects of mussel filtration, excretion and biodeposition on nutrient and photosynthetic pigment concentrations in a tropical eutrophic lake and mesotrophic river, featuring one non-native and two native species, respectively. Changes in nutrient and pigment concentrations were measured over a 3 h period to assess effects on (1) the water column in field enclosures, and (2) water column and benthos combined in controlled laboratory experiments. In field enclosures in both systems, mussel density and biomass were significantly correlated with the magnitude of reduction in sestonic pigment concentrations. In laboratory experiments, presence of mussels led to reduced PO4 and increased TAN concentrations in both systems, lower combined sestonic and deposited pigment concentrations in the river but increases in the same in the lake. We conclude that excretion by mussels probably accelerated bioseston growth in both systems due to N-fertilisation, an effect that may be particularly common in tropical freshwaters, which are frequently N-limited. However, whilst river mussels reduced bioseston concentrations through rapid filtration, higher rates of N-excretion and/or deposition of undigested bioseston by lake mussels apparently resulted in a net increase of pigment concentrations.https://doi.org/10.1007/s10750-019-3937-4Transitions in diatom assemblages and pigments through dry and wet season conditions in the red river, hanoi (vietnam)
Background and aims – Biomonitoring is an important tool for assessing river water quality, but is not routinely applied in tropical rivers. Marked hydrological changes can occur between wet and dry season conditions in the tropics. Thus, a prerequisite for ecological assessment is that the influence of ‘natural’ hydrological change on biota can be distinguished from variability driven by water quality parameters of interest. Here we aimed to (a) assess seasonal changes in water quality, diatoms and algal assemblages from river phytoplankton and artificial substrates through the dry-wet season transition (February–July 2018) in the Red River close to Hanoi and (b) evaluate the potential for microscopic counts and high-performance liquid chromatography (HPLC) analysis of chlorophyll and carotenoid pigments for biomonitoring in large tropical rivers. Methods – River water (phytoplankton) and biofilms grown on artificial glass substrates were sampled monthly through the dry (February–April) to wet (May–August) season transition and analysed via microscopic and HPLC techniques. Key results – All phototrophic communities shifted markedly between the dry and wet seasons. Phytoplankton concentrations were low (c. thousands of cells/mL) and declined as the wet season progressed. The dominant phytoplankton taxa were centric diatoms (Aulacoseira granulata and Aulacoseira distans) and chlorophytes (Scenedesmus and Pediastrum spp.), with chlorophytes becoming more dominant in the wet season. Biofilm diatoms were dominated by Melosira varians, and areal densities declined in the wet season when fast-growing pioneer diatom taxa (e.g. Achnanthidium minutissimum, Planothidium lanceolatum) and non-degraded Chlorophyll a concentrations increased, suggesting active phytobenthos growth in response to scour damage. Otherwise, a-phorbins were very abundant in river seston and biofilms indicating in situ Chlorophyll a degradation which may be typical of tropical river environments. The very large range of total suspended solids (reaching > 120 mg/L) and turbidity appears to be a key driver of photoautotrophs through control of light availability.https://doi.org/10.5091/plecevo.2019.1627Recent ecological change in ancient lakes
Ancient lakes are among the best archivists of past environmental change, having experienced more than one full glacial cycle, a wide range of climatic conditions, tectonic events, and long association with human settlements. These lakes not only record long histories of environmental variation and human activity in their sediments, but also harbor very high levels of biodiversity and endemism. Yet, ancient lakes are faced with a familiar suite of anthropogenic threats, which may degrade the unusual properties that make them especially valuable to science and society. In all ancient lakes for which data exist, significant warming of surface waters has occurred, with a broad range of consequences. Eutrophication threatens both native species assemblages and regional economies reliant on clean surface water, fisheries, and tourism. Where sewage contributes nutrients and heavy metals, one can anticipate the occurrence of less understood emerging contaminants, such as pharmaceuticals, personal care products, and microplastics that negatively affect lake biota and water quality. Human populations continue to increase in most of the ancient lakes’ watersheds, which will exacerbate these concerns. Further, human alterations of hydrology, including those produced through climate change, have altered lake levels. Co-occurring with these impacts have been intentional and unintentional species introductions, altering biodiversity. Given that the distinctive character of each ancient lake is strongly linked to age, there may be few options to remediate losses of species or other ecosystem damage associated with modern ecological change, heightening the imperative for understanding these systems.https://doi.org/10.1002/lno.10938First human impacts and responses of aquatic systems: A review of palaeolimnological records from around the world
Lake sediments constitute natural archives of past environmental changes. Historically, research has focused mainly on generating regional climate records, but records of human impacts caused by land use and exploitation of freshwater resources are now attracting scientific and management interests. Long-term environmental records are useful to establish ecosystem reference conditions, enabling comparisons with current environments and potentially allowing future trajectories to be more tightly constrained. Here we review the timing and onset of human disturbance in and around inland water ecosystems as revealed through sedimentary archives from around the world. Palaeolimnology provides access to a wealth of information reflecting early human activities and their corresponding aquatic ecological shifts. First human impacts on aquatic systems and their watersheds are highly variable in time and space. Landscape disturbance often constitutes the first anthropogenic signal in palaeolimnological records. While the effects of humans at the landscape level are relatively easily demonstrated, the earliest signals of human-induced changes in the structure and functioning of aquatic ecosystems need very careful investigation using multiple proxies. Additional studies will improve our understanding of linkages between human settlements, their exploitation of land and water resources, and the downstream effects on continental waters.https://doi.org/10.1177/2053019617740365A landscape perspective of Holocene organic carbon cycling in coastal SW Greenland lake-catchments
Arctic organic carbon (OC) stores are substantial and have accumulated over millennia as a function of changes in climate and terrestrial vegetation. Arctic lakes are also important components of the regional C-cycle as they are sites of OC production and CO2 emissions but also store large amounts of OC in their sediments. This sediment OC pool is a mixture derived from terrestrial and aquatic sources, and sediment cores can therefore provide a long-term record of the changing interactions between lakes and their catchments in terms of nutrient and C transfer. Sediment carbon isotope composition (δ13C), C/N ratio and organic C accumulation rates (C AR) of 14C-dated cores covering the last ∼10,000 years from six lakes close to Sisimiut (SW Greenland) are used to determine the extent to which OC dynamics reflect climate relative to lake or catchment characteristics. Sediment δ13C ranges from −19 to −32‰ across all lakes, while C/N ratios are <8 to >20 (mean = 12), values that indicate a high proportion of the organic matter is from autochthonous production but with a variable terrestrial component. Temporal trends in δ13C are variable among lakes, with neighbouring lakes showing contrasting profiles, indicative of site-specific OC processing. The response of an individual lake reflects its morphometry (which influences benthic primary production), the catchment:lake ratio, and catchment relief, lakes with steeper catchments sequester more carbon. The multi-site, landscape approach used here highlights the complex response of individual lakes to climate and catchment disturbance, but broad generalisations are possible. Regional Neoglacial cooling (from ∼5000 cal yr BP) influenced the lateral transfer of terrestrial OC to lakes, with three lakes showing clear increases in OC accumulation rate. The lakes likely switched from being autotrophic (i.e. net ecosystem production > ecosystem respiration) in the early Holocene to being heterotrophic after 5000 cal yr BP as terrestrial OC transfer increased.https://doi.org/10.1016/j.quascirev.2018.09.006Late Quaternary climate change in the north-eastern highlands of Ethiopia: A high resolution 15,600 year diatom and pigment record from Lake Hayk
Multi-proxy analyses of an 8 m sediment core from Lake Hayk, a closed, freshwater lake in the north-central highlands of Ethiopia, provide a record of changing lake level and inferred regional climatic change for the last 15.6 cal ka years. Between ca. 15.6–15.2 cal ka BP, a lowstand was synchronous with Heinrich Event 1 and an intense drought across Eastern Africa. At ca. 15.2 cal ka BP a lake began to develop at the core site in response to wetter conditions, at the onset of the African Humid Period (AHP). However, in contrast to other lakes in eastern Africa, Hayk lake level fell around ca. 14.8 cal ka BP, indicating a climate shift towards aridity. The lake began filling again at ca. 12.3 cal ka BP and reached maximum water depth between ca. 12.0–10.0 cal ka BP. Lake level declined slowly during the Holocene, culminating in the termination of the AHP at Hayk between ca. 5.2–4.6 cal ka BP. In the late Holocene, ca. 2.2–1.3 cal ka BP, Lake Hayk was again deep and fresh with some evidence of short-term lake level variability. The palaeo-record from Lake Hayk indicates that while it experienced, to a broad degree, the same glacial-interglacial dynamics and sub-millennial shifts in climate found in other palaeolimnological records from eastern Africa, there are offsets in timing and rate of response. These differences reflect chronological discrepancies between records, as well as the varying climate sensitivities and site-specific factors of individual lake basins. This record highlights the different responses by lakes in a climatically vulnerable area of Ethiopia.https://doi.org/10.1016/j.quascirev.2018.09.005Spatial differences in dissolved silicon utilization in Lake Baikal, Siberia: Examining the impact of high diatom biomass events and eutrophication
Recent research has highlighted how Lake Baikal, Siberia, has responded to the direct and indirect effects of climate change (e.g., ice-cover duration), nutrient loading, and pollution, manifesting as changes in phytoplankton/zooplankton populations, community structure, and seasonal succession. Here, we combine and compare analyses of chlorophyll a (an estimate of total algal biomass), carotenoid pigments (biomarkers of algal groups), and lake water silicon isotope geochemistry (δ30SiDSi) to differentiate spatial patterns in dissolved silicon (DSi) uptake at Lake Baikal. A total of 15 sites across the three basins (south, central, and north) of Lake Baikal were sampled in August 2013 along a depth gradient of 0–180 m. Strong, significant correlations were found between vertical profiles of photic zone DSi concentrations and δ30SiDSi compositions (r = −0.81, p < 0.001), although these are strongest in the central basin aphotic zone (r = −0.98, p < 0.001). Data refute the hypothesis of DSi uptake by picocyanobacteria. Algal biomass profiles and high surface δ30SiDSi compositions suggest greater productivity in the south basin and more oligotrophic conditions in the north basin. δ30SiDSi signatures are highest at depth (20 m) in central basin sites, indicating greater (10–40%) DSi utilization at deep chlorophyll maxima. DSi limitation occurs in the pelagic central basin, probably reflecting a high diatom biomass bloom event (Aulacoseira baicalensis). Meanwhile in the more hydrologically restricted, shallow Maloe More region (central basin), both high δ30SiDSi compositions and picocyanobacteria (zeaxanthin) concentrations, respectively point to the legacy of an “Aulacoseira bloom year” and continuous nutrient supply in summer months (e.g., localized eutrophication).https://doi.org/10.1002/lno.10792Diatom evidence of 20th century ecosystem change in Lake Baikal, Siberia
Lake Baikal has been experiencing limnological changes from recent atmospheric warming since the 1950s, with rising lake water temperatures, reduced ice cover duration and reduced lake surface-water mixing due to stronger thermal stratification. This study uses lake sediment cores to reconstruct recent changes (c. past 20 years) in Lake Baikal’s pelagic diatom communities relative to previous 20th century diatom assemblage records collected in 1993 and 1994 at the same locations in the lake. Recent changes documented within the core-top diatom records agree with predictions of diatom responses to warming at Lake Baikal. Sediments in the south basin of the lake exhibit clear temporal changes, with the most rapid occurring in the 1990’s with shifts towards higher abundances of the cosmopolitan Synedra acus and a decline in endemic species, mainly Cyclotella minuta and Stephanodiscus meyerii and to a lesser extent Aulacoseira baicalensis and Aulacoseira skvortzowii. The north basin, in contrast, shows no evidence of recent diatom response to lake warming despite marked declines in north basin ice cover in recent decades. This study also shows no diatom-inferred evidence of eutrophication from deep water sediments. However, due to the localised impacts seen in areas of Lake Baikal’s shoreline from nutrient pollution derived from inadequate sewage treatment, urgent action is vital to prevent anthropogenic pollution extending into the open waters. © 2018 Roberts et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.https://doi.org/10.1371/journal.pone.0208765Changes and drivers of freshwater mussel diversity and distribution in northern Borneo
Human activities are threatening Borneo's unique biodiversity, but little is known on the status of freshwater invertebrates. We assessed changes in diversity and distribution of freshwater mussels (Bivalvia: Unionida) in northern Borneo, and identified drivers of present distribution and threats. Past distribution data were collected from literature and museum resources. Present distribution data were collected from 21 river basins, and 47 water quality, climatic, landscape and human variables explored as potential predictors of species presence/absence. Species delimitations were identified by morphology and COI barcoding, and haplotype networks generated. Our data indicate that over the past 50 years, four of originally five native species have become very rare or possibly locally extirpated. Since these four species are endemic to Borneo, other Bornean river basins should urgently be surveyed to identify any remaining populations. In the same time span, the non-native Sinanodonta woodiana has become the most widespread freshwater mussel in northern Borneo. The fifth native species was identified as Rectidens sumatrensis and found in four Sarawakian river basins, thus contradicting previous assumptions of an endemic Bornean Rectidens species. Although a number of stable R. sumatrensis populations are retained across Sarawak, the species' strong spatial contraction in mainland Sundaland and apparent low tolerance to eutrophication suggest that it is vulnerable to further habitat alteration. Our results indicate that Borneo's (endemic) freshwater invertebrate biodiversity is declining rapidly. Comprehensive surveys targeting an array of invertebrate and vertebrate taxa are needed to identify Borneo's freshwater biodiversity hotspots, where conservation efforts should be concentrated.https://doi.org/10.1016/j.biocon.2018.01.012Regional variability in the atmospheric nitrogen deposition signal and its transfer to the sediment record in Greenland lakes
Disruption of the nitrogen cycle is a major component of global environmental change. δ15N in lake sediments is increasingly used as a measure of reactive nitrogen input but problematically, the characteristic depleted δ15N signal is not recorded at all sites. We used a regionally replicated sampling strategy along a precipitation and N-deposition gradient in SW Greenland to assess the factors determining the strength of δ15N signal in lake sediment cores. Analyses of snowpack N and δ15N-NO3 and water chemistry were coupled with bulk sediment δ15N. Study sites cover a gradient of snowpack δ15N (ice sheet: −6‰; coast -10‰), atmospheric N deposition (ice sheet margin: ∼ 0.2 kg ha−1 yr−1; coast: 0.4 kg ha−1 yr−1) and limnology. Three 210Pb-dated sediment cores from coastal lakes showed a decline in δ15N of ca. -1‰ from ∼ 1860, reflecting the strongly depleted δ15N of snowpack N, lower in-lake total N (TN) concentration (∼ 300 μg N L−1) and a higher TN-load. Coastal lakes have 3.7–7.1× more snowpack input of nitrate than inland sites, while for total deposition the values are 1.7–3.6× greater for lake and whole catchment deposition. At inland sites and lakes close to the ice-sheet margin, a lower atmospheric N deposition rate and larger in-lake TN pool resulted in greater reliance on N-fixation and recycling (mean sediment δ15N is 0.5–2.5‰ in most inland lakes; n = 6). The primary control of the transfer of the atmospheric δ15N deposition signal to lake sediments is the magnitude of external N inputs relative to the in-lake N-pool.https://doi.org/10.1002/lno.10936Direct and indirect effects of Holocene climate variations on catchment and lake processes of a treeline lake, SW China
Sedimentary records of inorganic elements and pigments over the last 12,000 years are used to assess major changes in limnological conditions of Tiancai Lake (a small treeline lake, SW China), in response to Holocene climate variations. Algal communities shifted from the dominance of cyanobacteria and cryptophytes in the early Holocene, towards siliceous algae in the mid-Holocene and chlorophytes in the late Holocene. Algae responded to a combination of climate-mediated vegetation and soil development associated with allochthonous inputs of dissolved nutrients and organic matter, and sediment infilling. Decreases in Al, Pb, Cu and Zn from the early Holocene probably resulted from soil podsolization and the sequestration of these elements within soils. Changes in Mn and Fe were likely linked to redox condition in catchment soils and water column. Synchronous peaks in Ti, Ba, Ca, Sr, Na, K and Mg, median grain size and magnetic susceptibility coincided with the troughs in the chemical index of alteration, indicating that episodic cold events enhanced upland bedrock erosion and transported unleached and coarse detritus into the lake. These cold events broadly correlate with Holocene ice-rafting events in the North Atlantic. Although the cold events altered the influx of minerogenic elements by regulating upland bedrock erosion, climate-mediated vegetation and soil development led to a muted impact on primary producers. Holocene algal community shifts were subtle, reflecting the relative abundance of P (derived from weathering) and N (derived from soils) throughout the record, with the most pronounced effects on the lake biota being benthic expansion which occurred in response to sediment infilling.https://doi.org/10.1016/j.palaeo.2018.04.027Effects of dam construction and increasing pollutants on the ecohydrological evolution of a shallow freshwater lake in the Yangtze floodplain
Large river-floodplain systems which provide a variety of societal, economic and biological benefits are undergoing extensive and intensive human disturbance. However, floodplain lakes responses to multiple stressors are poorly understood. The Yangtze River and its floodplain which provide water and food resources for more than 300 million people are an important region in China. Hydrological regulation as well as socio-economic development have brought profound negative influence on this ecologically important area. To improve understanding of decadal-scale responses of floodplain lakes to multiple stressors, lake sediment proxies including particle size, geochemical elements, diatoms and chironomids were analysed in a lead-210 dated core from Futou Lake. The analyses show that dams constructed in 1935 and the early 1970s stabilized hydrological conditions in Futou Lake and impeded the interaction with the Yangtze River, resulting in a decrease in major elements (e.g., Mg, Al, Fe) transported into the lake and an increase of macrophyte-related chironomids (C. sylvestris-type, P. penicillatus-type and Paratanytarsus sp.). After the late 1990s, further decreases in major elements and increases in median grain size are attributed to the erosion of the Yangtze riverbed and declining supply of major elements-enriched sediments from the upper Yangtze caused by the impoundment of the Three Gorges Dam. Chironomid and diatom assemblages indicate that hydrological stabilization caused by dam constructions stimulated the growth of macrophytes, which may be important in buffering against an ecosystem state change towards a phytoplankton-dominated and turbid state with ongoing eutrophication. However, a recent increase in Zn, TP and the emergence of eutrophic diatom and chironomid species indicate initial signs of water quality deterioration which may be related to the combined effects of hydrological stabilization and aquaculture. Over all, the sediment record from Futou Lake emphasizes the importance of interactions between hydrological change and pollutant loads in determining floodplain lake ecosystem state.https://doi.org/10.1016/j.scitotenv.2017.11.181Characterisation of a major phytoplankton bloom in the River Thames (UK) using flow cytometry and high performance liquid chromatography
Recent river studies have observed rapid phytoplankton dynamics, driven by diurnal cycling and short-term responses to storm events, highlighting the need to adopt new high-frequency characterisation methods to understand these complex ecological systems. This study utilised two such analytical methods; pigment analysis by high performance liquid chromatography (HPLC) and cell counting by flow cytometry (FCM), alongside traditional chlorophyll spectrophotometry and light microscopy screening, to characterise the major phytoplankton bloom of 2015 in the River Thames, UK. All analytical techniques observed a rapid increase in chlorophyll a concentration and cell abundances from March to early June, caused primarily by a diatom bloom. Light microscopy identified a shift from pennate to centric diatoms during this period. The initial diatom bloom coincided with increased HPLC peridinin concentrations, indicating the presence of dinoflagellates which were likely to be consuming the diatom population. The diatom bloom declined rapidly in early June, coinciding with a storm event. There were low chlorophyll a concentrations (by both HPLC and spectrophotometric methods) throughout July and August, implying low biomass and phytoplankton activity. However, FCM revealed high abundances of pico-chlorophytes and cyanobacteria through July and August, showing that phytoplankton communities remain active and abundant throughout the summer period. In combination, these techniques are able to simultaneously characterise a wider range of phytoplankton groups, with greater certainty, and provide improved understanding of phytoplankton functioning (e.g. production of UV inhibiting pigments by cyanobacteria in response to high light levels) and ecological status (through examination of pigment degradation products). Combined HPLC and FCM analyses offer rapid and cost-effective characterisation of phytoplankton communities at appropriate timescales. This will allow a more-targeted use of light microscopy to capture phytoplankton peaks or to investigate periods of rapid community succession. This will lead to greater system understanding of phytoplankton succession in response to biogeochemical drivers.https://doi.org/10.1016/j.scitotenv.2017.12.128Historical atmospheric pollution trends in Southeast Asia inferred from lake sediment records
Fossil fuel combustion leads to increased levels of air pollution, which negatively affects human health as well as the environment. Documented data for Southeast Asia (SEA) show a strong increase in fossil fuel consumption since 1980, but information on coal and oil combustion before 1980 is not widely available. Spheroidal carbonaceous particles (SCPs) and heavy metals, such as mercury (Hg), are emitted as by-products of fossil fuel combustion and may accumulate in sediments following atmospheric fallout. Here we use sediment SCP and Hg records from several freshwater lentic ecosystems in SEA (Malaysia, Philippines, Singapore) to reconstruct long-term, region-wide variations in levels of these two key atmospheric pollution indicators. The age-depth models of Philippine sediment cores do not reach back far enough to date first SCP presence, but single SCP occurrences are first observed between 1925 and 1950 for a Malaysian site. Increasing SCP flux is observed at our sites from 1960 onward, although individual sites show minor differences in trends. SCP fluxes show a general decline after 2000 at each of our study sites. While the records show broadly similar temporal trends across SEA, absolute SCP fluxes differ between sites, with a record from Malaysia showing SCP fluxes that are two orders of magnitude lower than records from the Philippines. Similar trends in records from China and Japan represent the emergence of atmospheric pollution as a broadly-based inter-region environmental problem during the 20th century. Hg fluxes were relatively stable from the second half of the 20th century onward. As catchment soils are also contaminated with atmospheric Hg, future soil erosion can be expected to lead to enhanced Hg flux into surface waters. Lake sediment records from Southeast Asia provide first data on historical trends in fossil-fuel derived atmospheric pollution.https://doi.org/10.1016/j.envpol.2018.01.007Functional attributes of epilithic diatoms for palaeoenvironmental interpretations in South-West Greenland lakes
Benthic diatoms are commonly used for palaeoenvironmental reconstruction in Arctic regions, but interpretation of their ecology remains challenging. We studied epilithic diatom assemblages from the shallow margins of 19 lakes from three areas (coast-inland-ice sheet margin) along a climate gradient in Kangerlussuaq, West Greenland during two periods; shortly after ice-off (spring) and in the middle of the growth season (summer). We aimed to understand the distribution of Arctic epilithic diatoms in relation to water chemistry gradients during the two seasons, to investigate their incorporation into lake sediments and to assess their applicability as palaeoenvironmental indicators. Diatoms were correlated with nutrients in the spring and alkalinity/major ions in the summer, when nutrients were depleted; approximately half of the variance explained was independent of spatial factors. When categorised by functional attributes, diatom seasonal succession differed among regions with the most obvious changes in inland lakes where summer temperatures are warmer, organic nutrient processing is prevalent and silicate is limiting. These conditions led to small, motile and adnate diatoms being abundant in inland lakes during the summer (Nitzschia spp., Encyonopsis microcephala), as these functional attributes are suited to living within complex mats of non-siliceous microbial biofilms. Seasonal succession in silica-rich lakes at the coast was less pronounced and assemblages included Tabellaria flocculosa (indicating more acidic conditions) and Hannaea arcus (indicating input from inflowing rivers). The nitrogen-fixing diatom Epithemia sorex increased from the coast to the ice sheet, negatively correlating with a gradient of reactive nitrogen. The presence of this diatom in Holocene sediment records alongside cyanobacterial carotenoids during arid periods of low nitrogen delivery, suggests that it is a useful indicator of nitrogen limitation. Nitzschia species appear to be associated with high concentrations of organic carbon and heterotrophy, but their poor representation in West Greenland lake sediments due to taphonomic processes limits their palaeoenvironmental application in this region. Proportions of epilithic taxa in lake sediment records of coastal lakes increased during some wetter periods of the Holocene, suggesting that snowpack-derived nutrient delivery may offer diatom taxa living at lake margins a competitive advantage over planktonic diatoms during the “moating” ice melt period. Thus, further research investigating linkages between epilithic diatoms, snowpack and nutrient delivery in seasonally frozen lakes is recommended as these taxa live on the ‘front-line’ during the spring and may be especially sensitive to changes in snowmelt conditions.https://doi.org/10.1007/s10933-017-9968-9Vegetation transitions drive the autotrophy–heterotrophy balance in Arctic lakes
“Arctic greening” will alter vegetation quantity and quality in northern watersheds, with possible consequences for lake metabolic balance. We used paleolimnology from six Arctic lakes in Greenland, Norway, and Alaska to develop a conceptual model describing how climate-driven shifts in terrestrial vegetation (spanning herb to boreal forest) influence lake autotrophic biomass (as chlorophyll and carotenoid pigments). Major autotrophic transitions occurred, including (1) optimal production of siliceous algae and cyanobacteria/chlorophytes at intermediate vegetation cover (dwarf shrub and Betula; dissolved organic carbon (DOC) range of 2–4 mg L−1), below and above which UVR exposure (DOC; < 2 mgL−1) and light extinction (DOC; > 4 mgL−1), respectively limit algal biomass, (2) an increase in potentially mixotrophic cryptophytes with higher forest cover and allochthonous carbon supply. Vegetation cover appears to influence lake autotrophs by changing influx of (colored) dissolved organic matter which has multiple interacting roles—as a photoprotectant—in light attenuation and in macronutrient (carbon, nitrogen) supply.https://doi.org/10.1002/lol2.10086Regional versus local drivers of water quality in the Windermere catchment, Lake District, United Kingdom: The dominant influence of wastewater pollution over the past 200 years
Freshwater ecosystems are threatened by multiple anthropogenic stressors acting over different spatial and temporal scales, resulting in toxic algal blooms, reduced water quality and hypoxia. However, while catchment characteristics act as a ‘filter’ modifying lake response to disturbance, little is known of the relative importance of different drivers and possible differentiation in the response of upland remote lakes in comparison to lowland, impacted lakes. Moreover, many studies have focussed on single lakes rather than looking at responses across a set of individual, yet connected lake basins. Here we used sedimentary algal pigments as an index of changes in primary producer assemblages over the last ~200 years in a northern temperate watershed consisting of 11 upland and lowland lakes within the Lake District, United Kingdom, to test our hypotheses about landscape drivers. Specifically, we expected that the magnitude of change in phototrophic assemblages would be greatest in lowland rather than upland lakes due to more intensive human activities in the watersheds of the former (agriculture, urbanization). Regional parameters, such as climate dynamics, would be the predominant factors regulating lake primary producers in remote upland lakes and thus, synchronize the dynamic of primary producer assemblages in these basins. We found broad support for the hypotheses pertaining to lowland sites as wastewater treatment was the main predictor of changes to primary producer assemblages in lowland lakes. In contrast, upland headwaters responded weakly to variation in atmospheric temperature, and dynamics in primary producers across upland lakes were asynchronous. Collectively, these findings show that nutrient inputs from point sources overwhelm climatic controls of algae and nuisance cyanobacteria, but highlights that large-scale stressors do not always initiate coherent regional lake response. Furthermore, a lake's position in its landscape, its connectivity and proximity to point nutrients are important determinants of changes in production and composition of phototrophic assemblages.https://doi.org/10.1111/gcb.14299Changes in glacial meltwater alter algal communities in lakes of Scoresby Sund, Renland, East Greenland throughout the Holocene: Abrupt reorganizations began 1000 years before present
We investigated the response of lake algal communities to changes in glacial meltwater from the Renland Ice Cap (Greenland) through the Holocene to assess whether influxes always elicit consistent responses or novel responses. We measured sedimentary algal pigments in two proximal lakes, snow-fed Raven and glacier- and snow-fed Bunny Lake, and diatom community structure and turnover in Bunny Lake. Diatom data were not available in Raven Lake. We also modeled lake-level change in Bunny Lake to identify how glacial meltwater may have altered diatom habitat availability through time. Through a series of glacier advances and retreats over the Holocene, the algal response in Bunny Lake was relatively constant until approximately 1015 yr BP, after which there were major changes in sedimentary algal remains. Algal pigment concentrations sharply declined, and diatom species richness increased. Diatom community structure underwent three reorganizations. Until 1015 yr BP, assemblages were dominated by Pinnularia braunii and Aulacoseira pffaffiana. However, approximately 1015–480 yr BP, these species declined and Tabellaria flocculosa and Hannaea arcus became a significant component of the assemblage. Approximately 440 yr BP, A. pfaffiana increased along with species indicating elevated nitrogen. In contrast, the algal pigment records from nearby snow-fed Raven Lake showed different and minimal change through time. Our results suggest that changes in the magnitude and composition of meltwater in our two study lakes were unique over the last 1000 yr BP and elicited a non-linear threshold response absent during other periods of glacier advance and retreat. Deciphering the degree to which glaciers structure algal communities over time has strong implications for lakes as glaciers continue to recede.https://doi.org/10.1177/0959683616678468Changes in carbon and nitrogen cycling in a floodplain lake over recent decades linked to littoral expansion, declining riverine influx, and eutrophication
This study aimed to understand changes in the biogeochemical processing of organic matter (OM) in response to multiple stressors (e.g., littoral area expansion, wastewater input, and hydrological regulation) in East Dongting Lake (Central China) over the past 60 years, using analyses of total organic carbon (TOC), total nitrogen (TN), C/N ratios, δ13C, δ15N, and diatoms from 2 sediment cores collected from the littoral and central parts of the lake. OM mainly originated from phytoplankton and C3 plant-derived soil OM based on the ranges of C/N ratios (from 7 to 11) and δ13C (between −27‰ and −23‰). Littoral area expansion due to siltation caused an increasing influx of terrestrial soil OM in the 1980s and the 1990s, subsequently lowering δ13C values and rising C/N ratios in both sediment cores. Meanwhile, higher δ15N was linked to a high influx of isotopically heavy nitrate from urban and agricultural wastewaters. After 2000, slight decreases in TOC and TN in the littoral area were attributable to reducing inputs of external OM, likely linked to declining sediment influx from the upper reaches resulting from the Three Gorges Dam impoundment. Contrasting increases in TOC, TN, and C/N ratios in the central part indicated a high influx of terrestrial soil OM due to the declining distance from the shoreline with littoral area expansion. Declining δ15N values after 2000 indicated an increase in N2-fixing cyanobacteria with eutrophication. Changes in diatom assemblages in both the littoral and central zones reflected nutrient enrichment and hydrological alterations. These results indicate that littoral expansion, declining riverine influx, and anthropogenic nutrient inputs are potential driving forces for the biogeochemical processing of OM in floodplain lakes. This study provides sedimentary biogeochemical clues for tracking past limnological conditions of floodplain lakes that are subjected to increasing disturbances from hydrological regulation and eutrophication.https://doi.org/10.1002/hyp.11254Deciphering long-term records of natural variability and human impact as recorded in lake sediments: a palaeolimnological puzzle
Global aquatic ecosystems are under increasing threat from anthropogenic activity, as well as being exposed to past (and projected) climate change, however, the nature of how climate and human impacts are recorded in lake sediments is often ambiguous. Natural and anthropogenic drivers can force a similar response in lake systems, yet the ability to attribute what change recorded in lake sediments is natural, from that which is anthropogenic, is increasingly important for understanding how lake systems have, and will continue to function when subjected to multiple stressors; an issue that is particularly acute when considering management options for aquatic ecosystems. The duration and timing of human impacts on lake systems varies geographically, with some regions of the world (such as Africa and South America) having a longer legacy of human impact than others (e.g., New Zealand). A wide array of techniques (biological, chemical, physical and statistical) is available to palaeolimnologists to allow the deciphering of complex sedimentary records. Lake sediments are an important archive of how drivers have changed through time, and how these impacts manifest in lake systems. With a paucity of ?real-time? data pre-dating human impact, palaeolimnological archives offer the only insight into both natural variability (i.e., that driven by climate and intrinsic lake processes) and the impact of people. While there is a need to acknowledge complexity, and temporal and spatial variability when deciphering change from sediment archives, a palaeolimnological approach is a powerful tool for better understanding and managing global aquatic resources. WIREs Water 2017, 4:e1195. doi: 10.1002/wat2.1195 This article is categorized under: Water and Life > Stresses and Pressures on Ecosystems Science of Water > Water and Environmental Change Water and Life > Methodshttps://doi.org/10.1002/wat2.1195The arctic in the twenty-first century: Changing biogeochemical linkages across a paraglacial landscape of Greenland
The Kangerlussuaq area of southwest Greenland encompasses diverse ecological, geomorphic, and climate gradients that function over a range of spatial and temporal scales. Ecosystems range from the microbial communities on the ice sheet and moisture-stressed terrestrial vegetation (and their associated herbivores) to freshwater and oligosaline lakes. These ecosystems are linked by a dynamic glacio-fluvial-Aeolian geomorphic system that transports water, geological material, organic carbon and nutrients from the glacier surface to adjacent terrestrial and aquatic systems. This paraglacial system is now subject to substantial change because of rapid regional warming since 2000. Here, we describe changes in the eco-and geomorphic systems at a range of timescales and explore rapid future change in the links that integrate these systems. We highlight the importance of cross-system subsidies at the landscape scale and, importantly, how these might change in the near future as the Arctic is expected to continue to warm.https://doi.org/10.1093/biosci/biw158Can fish introductions alter nutrient cycles in previously fishless high-latitude lakes?
The additional input and enhanced cycling of nutrients derived from introduced fish can be a significant factor altering nutrient dynamics in oligotrophic lakes. To test this, we used a bioenergetic model to estimate the fish-derived nutrient load in Lake Kuutsjärvi, a historically fishless boreal lake of northern Fennoscandia. The lake was selected because of the absence of other anthropogenic stressors, a known stocking history and the possibility of quantitatively estimating the size-structure and biomass of the fish population through a mass removal. Subsequently, we used a mass balance model to compare fish-derived nutrients with other nutrient load pathways. For comparison over longer timescales, we used lake sediment records of diatoms, chlorophyll and carotenoid pigments, C:N ratios and stable isotopes to infer whether fish introduction produced detectable changes in the lake trophic state, primary productivity and terrestrial nutrient input. Based on the nutrient mass balance model, we found that phosphorus and nitrogen derived from fish were 0.46% and 2.2%, respectively, of the total load to the lake, suggesting that fish introduction could not markedly increase the nutrient load. Accordingly, the palaeolimnological record indicated little increase in primary production but instead a shift from pelagic to benthic production after fish introduction.https://doi.org/10.4081/jlimnol.2016.1364Modification of littoral algal assemblages by gardening caddisfly larvae
Sedentary herbivores may improve the food resources available to them by ‘gardening’, and most obviously by fertilising primary producers with excreted nutrients such as nitrogen. In five English lakes, spanning a gradient of nutrient availability, we predicted that fertilisation of the larval retreat by the littoral, gallery-building caddisfly Tinodes waeneri would result in: (a) a distinct algal assemblage from that in the background epilithon, and that (b) the difference would be greatest in the least productive lakes (where the importance of the nutrient subsidy from larvae should be greatest). Classes of algae present in samples of galleries and epilithon were investigated using chlorophyll and carotenoid pigment analysis and diatoms were identified. Galleries were characterised by a greater content of pigments indicative of diatoms, including fucoxanthin, than the background epilithon (which contained a higher proportion of chlorophyte algae). Redundancy analysis (RDA) of diatom counts indicated a clear separation between gallery and epilithic assemblages in all lakes, supporting hypothesis (a). Furthermore, in agreement with hypothesis (b), the assemblage of diatoms on galleries was most similar to that of the epilithon in the more productive lakes, with the greatest divergence in Windermere and Coniston, mainly due to a much greater relative proportion of Gomphonema in the epilithon than in the galleries, suggesting that fertilisation had a greater impact where background nutrient concentrations were low. Lakes in the RDA triplot were arranged in order of productivity along axis 1, with gallery assemblages in each case located towards the more productive end relative to the epilithon. This sedentary, retreat-building grazer, probably in common with many other taxa with such traits in aquatic ecosystems, improves its own food resources by gardening. Larvae can modify algal assemblages in their feeding patches by weeding and fertilisation. They create new surfaces for growth of the biofilm via continually adding to the front of their silken galleries and harvesting the biofilm (dominated by diatoms) that has developed on older parts of their galleries to the rear. It is also feasible that the physical structure of the gallery (separately from the effects of nutrients) plays some role in increasing the availability of algae within the territory. This behaviour is likely to extend the range of conditions under which such species can persist, raise the carrying capacity of their habitat, and has possible wider ecosystem consequences for productivity and nutrient cycling.https://doi.org/10.1111/fwb.12881Long-term perspectives on terrestrial and aquatic carbon cycling from palaeolimnology
Lakes are active processors and collectors of carbon (C) and thus recognized as quantitatively important within the terrestrial C cycle. Better integration of palaeolimnology (lake sediment core analyses) with limnological C budgeting approaches has the potential to enhance understanding of lacustrine C processing and sequestration. Palaeolimnology simultaneously assimilates materials from across lake habitats, terrestrial watersheds, and airsheds to provide a uniquely broad overview of the terrestrial-atmospheric-aquatic linkages across different spatial scales. The examination of past changes over decadal–millennial timescales via palaeolimnology can inform understanding and prediction of future changes in C cycling. With a particular, but not exclusive, focus on northern latitudes we examine the methodological approaches of palaeolimnology, focusing on how relatively standard and well-tested techniques might be applied to address questions of relevance to the C cycle. We consider how palaeolimnology, limnology, and sedimentation studies might be linked to provide more quantitative and holistic estimates of lake C cycling and budgets. Finally, we use palaeolimnological examples to consider how changes such as terrestrial vegetation shifts, permafrost thaw, the formation of new lakes and reservoirs, hydrological modification of inorganic C processing, land use change, soil erosion and disruption to global nitrogen and phosphorus cycles might influence lake C cycling. WIREs Water 2016, 3:211–234. doi: 10.1002/wat2.1130. This article is categorized under: Water and Life > Nature of Freshwater Ecosystems Science of Water > Water and Environmental Change Science of Water > Water Quality.https://doi.org/10.1002/wat2.1130Algal Blooms
Harmful algal blooms (HABs) in marine, brackish, and freshwater environments are caused by a broad range of microscopic algae and cyanobacteria. HABs are hazardous and sometimes fatal to human and animal populations, either through toxicity, or by creating ecological conditions, such as oxygen depletion, which can kill fish and other economically or ecologically important organisms. HAB hazards have increased globally over the past 40. years, because of eutrophication, translocation of exotic species via global shipping routes, climate-driven range expansions, and altered physical oceanographic conditions. Human vulnerability to HABs is greatest in communities that are nutritionally and economically reliant on fishery resources, but locally HABs also cause damage to tourist industries and have health-associated costs. Major research advances have been made in the monitoring, detection, modeling, forecast, prevention, and treatment of HABs, which have helped to mitigate health and economic risks. However, reducing HAB incidents in the future will be challenging, particularly in areas where food production and human populations (and therefore nutrient fluxes) are projected to increase. A further challenge lies in adequate communication of HAB risks and providing effective institutional structures to prepare for and respond to HAB incidents.https://doi.org/10.1016/B978-0-12-394847-2.00002-4A multi-stakeholder strategy to identify conservation priorities in Peninsular Malaysia
Malaysia, with its rapidly growing economy, exemplifies the tensions between conservation and development faced by many tropical nations. Here we present the results of a multi-stakeholder engagement exercise conducted to (1) define conservation priorities in Peninsular Malaysia and (2) explore differences in perceptions among and within stakeholder groups (i.e. government, academia, NGOs and the private sector). Our data collection involved two workshops and two online surveys where participants identified seven general conservation themes and ranked the top five priority issues within each theme. The themes were: (1) policy and management, (2) legislation and enforcement, (3) finance and resource allocation, (4) knowledge, research and development, (5) socio-economic issues, (6) public awareness and participation and (7) rights of nature. In spite of their very different backgrounds and agendas, the four stakeholder groups showed general agreement in their priority preferences except for two issues. Respondents from government and private sector differed the most from each other in their priority choices while academia and NGO showed the highest degree of similarity. This ranked list of 35 conservation priorities is expected to influence the work of policy-makers and others in Peninsular Malaysia and can be used as a model to identify conservation priorities elsewhere.https://doi.org/10.1080/23311843.2016.1254078Impacts of forestry planting on primary production in upland lakes from north-west Ireland
Planted forests are increasing in many upland regions worldwide, but knowledge about their potential effects on algal communities of catchment lakes is relatively unknown. Here, the effects of afforestation were investigated using palaeolimnology at six upland lake sites in the north-west of Ireland subject to different extents of forest plantation cover (4-64% of catchment area). 210Pb-dated sediment cores were analysed for carotenoid pigments from algae, stable isotopes of bulk carbon (δ13C) and nitrogen (δ15N), and C/N ratios. In lakes with >50% of their catchment area covered by plantations, there were two- to sixfold increases in pigments from cryptophytes (alloxanthin) and significant but lower increases (39-116%) in those from colonial cyanobacteria (canthaxanthin), but no response from biomarkers of total algal abundance (β-carotene). In contrast, lakes in catchments with <20% afforestation exhibited no consistent response to forestry practices, although all lakes exhibited fluctuations in pigments and geochemical variables due to peat cutting and upland grazing prior to forest plantation. Taken together, patterns suggest that increases in cyanobacteria and cryptophyte abundance reflect a combination of mineral and nutrient enrichment associated with forest fertilization and organic matter influx which may have facilitated growth of mixotrophic taxa. This study demonstrates that planted forests can alter the abundance and community structure of algae in upland humic lakes of Ireland and Northern Ireland, despite long histories of prior catchment disturbance.https://doi.org/10.1111/gcb.13194Effects of hydrological regulation and anthropogenic pollutants on Dongting Lake in the Yangtze floodplain
Dongting Lake, a wetland of international importance under the Ramsar Convention, is the hydrological lynchpin of the Yangtze floodplain. It is subjected to eutrophication and heavy metal pollution, but little is known of recent rapid degradation of its aquatic environment and potential causes. Here, we use sedimentary records of particle size, elements and pigments, combined with historical hydrological and limnological data, to assess aquatic environmental changes since ca 1960. Coarse particles in the sediments increased after 2003, while concentrations of K, Li, Al and Mg declined. These changes are probably linked to strong erosion of the downstream riverbed and declining supply of ion-rich material from the upper Yangtze reaches after the Three Gorges Dam (TGD) began to operate. Furthermore, enrichment of sedimentary heavy metals and total phosphorus since the 1990s can be attributed to the influx of anthropogenic pollutants. Biotic responses are manifested in higher concentrations of fossil algal pigments, especially those from chlorophytes and siliceous algae after the TGD impoundment. Multivariate analysis revealed that changes in fossil pigments were significantly correlated with total phosphorus and Pb and three indicators for hydrological conditions (i.e. coarse particles, water exchange ratio and sediment discharge), indicating that influx of anthropogenic pollutants and hydrological regulation by the TGD are main drivers of algal community change. Our results provide reference conditions for restoration of Dongting Lake and highlight the importance of hydrological connection with the mainstream and pollution control for continuing efforts to restore Dongting Lake, as well as other similar floodplain lakes worldwide.https://doi.org/10.1002/eco.1637Factors driving changes in freshwater mussel (Bivalvia, Unionida) diversity and distribution in Peninsular Malaysia
Freshwater mussels (Bivalvia, Unionida) fulfil important ecosystem functions and are one of the most threatened freshwater taxa globally. Knowledge of freshwater mussel diversity, distribution and ecology in Peninsular Malaysia is extremely poor, and the conservation status of half of the species presumed to occur in the region has yet to be assessed. We conducted the first comprehensive assessment of Peninsular Malaysia's freshwater mussels based on species presence/absence and environmental data collected from 155 sites spanning all major river catchments and diverse habitat types. Through an integrative morphological-molecular approach we recognised nine native and one widespread non-native species, i.e. Sinanodonta woodiana. Two species, i.e. Pilsbryoconcha compressa and Pseudodon cambodjensis, had not been previously recorded from Malaysia, which is likely a result of morphological misidentifications of historical records. Due to their restriction to single river catchments and declining distributions, Hyriopsis bialata, possibly endemic to Peninsular Malaysia, Ensidens ingallsianus, possibly already extinct in the peninsula, and Rectidens sumatrensis, particularly require conservation attention. Equally, the Pahang, the Perak and the north-western river catchments are of particular conservation value due to the presence of a globally unique freshwater mussel fauna. Statistical relationships of 15 water quality parameters and mussel presence/absence identified acidification and nutrient pollution (eutrophication) as the most important anthropogenic factors threatening freshwater mussel diversity in Peninsular Malaysia. These factors can be linked to atmospheric pollution, deforestation, oil-palm plantations and a lack of functioning waste water treatment, and could be mitigated by establishing riparian buffers and improving waste water treatment for rivers running through agricultural and residential land.https://doi.org/10.1016/j.scitotenv.2016.07.098Disentangling natural and anthropogenic drivers of changes in a shallow lake using palaeolimnology and historical archives
Shallow lakes are susceptible to catastrophic regime shifts characterised by the presence or absence or macrophytes. However, the long-term controls on macrophyte succession in shallow lakes are incompletely understood. To investigate this, we analysed multiple sediment proxies in Lake Rotcze (Eastern Poland), a small, shallow and densely macrophyte-covered lake to (1) reconstruct the ‘reference conditions’ (sensu WFD) and development of the lake in recent centuries, (2) compare historical evidence with the sedimentary record, and (3) identify the natural and anthropogenic drivers of macrophyte succession. Before the twentieth century, conditions in the lake may be referred to as ‘reference conditions’. Subsequently forest clearance in the catchment resulted in lower water transparency, but concurrent catchment drainage lowered water levels and increased macrophyte development. Since 1950 elevated nutrient supply and climatically driven increases in water levels led to the deterioration of water transparency and partial macrophyte withdrawal. At the end of the twentieth century lake-level drawdown led to low phytoplankton biomass and clear water creating a novel ecosystem where macrophytes invade the whole lake. These patterns suggest that both natural and anthropogenically induced water level fluctuations have been critical drivers of macrophyte development.https://doi.org/10.1007/s10750-015-2510-zComparisons of observed and modelled lake δ18O variability
With the substantial number of lake sediment δ18O records published in recent decades, a quantitative, process-based understanding of these systems can increase our understanding of past climate change. We test mass balance models of lake water δ18O variability against five years of monthly monitoring data from lakes with different hydrological characteristics, in the East-Midlands region of the UK, and the local isotope composition of precipitation. These mass balance models can explain up to 74% of the measured lake water isotope variability. We investigate the sensitivity of the model to differing calculations of evaporation amount, the amount of groundwater, and to different climatic variables. We show there is only a small range of values for groundwater exchange flux that can produce suitable lake water isotope compositions and that variations in evaporation and precipitation are both required to produce recorded isotope variability in lakes with substantial evaporative water losses. We then discuss the potential for this model to be used in a long-term, palaeo-scenario. This study demonstrates how long term monitoring of a lake system can lead to the development of robust models of lake water isotope compositions. Such systematics-based explanations allow us to move from conceptual, to more quantified reconstructions of past climates and environments.https://doi.org/10.1016/j.quascirev.2015.09.012Seasonal and Regional Controls of Phytoplankton Production along a Climate Gradient in South-West Greenland during Ice-Cover and Ice-Free Conditions
Across a small geographic area (<180 km), the region of South-West Greenland covers a natural climate gradient. Variation in temperature and precipitation result in marked differences in limnology at three discrete locations: ice sheet margin, inland, and the coast. Replicate lakes from each location were sampled for physical (temperature, light), chemical (dissolved oxygen, pH, conductivity, nutrients), and biological (chlorophyll a [Chl a], photosynthetic pigments) variables on three occasions within a 12-month period: July–August 2010, April–May 2011, and June–July 2011 spanning ice cover. Variation in ice phenology was linked to the climate gradient; however, phytoplankton production and community composition did not differ regionally. Large-scale seasonal fluctuations in temperature and nutrient availability were the strongest predictors of phytoplankton production, with a shift from nitrate to phosphorus controlled production between ice-cover and ice-free conditions. Underlying seasonal drivers, variables predicting production were unique to each location—ice sheet margin (soluble reactive phosphorus), inland (temperature), and coast (silicate)—and reflect local differences in nutrient availability. Results from the current study have important consequences when controls over phytoplankton production in Arctic lakes are inferred from a limited number of sites, but up-scaled to represent pan-Arctic trends.https://doi.org/10.1657/AAAR0015-003Establishing the impacts of freshwater aquaculture in tropical Asia: the potential role of palaeolimnology
Freshwater aquaculture is an important source of protein worldwide. Over-exploitation of fisheries can, however, add severely to pressures on ecosystem functioning and services. In Southeast Asia, aquaculture in freshwater lakes contributes significantly to the economy and to reductions in poverty and nutritional insecurity. However, overstocking and excessive feeding of fish can lead to a degradation of affected water bodies, manifest as eutrophication, toxic algal blooms, losses of biodiversity and amenity, anoxia and, in extreme cases, collapse of fisheries. Projected increased warming and storminess associated with global climate change are likely to magnify existing problems. Matching levels of aquaculture production with ecological carrying capacity is therefore likely to become increasingly challenging, requiring levels of data and understanding that are rarely available, a problem that is impossible to rectify in the short term using standard limnological approaches. This paper reviews the development of freshwater aquaculture in the Philippines, associated environmental impacts, and relevant environmental regulations and regulatory bodies. The potential role of palaeolimnology, a science that is relatively under-utilised in the tropics generally and in tropical Asia in particular, in complementing extant datasets, including monitoring records, is highlighted through reference to a preliminary study at Lake Mohicap. Lake Mohicap currently supports aquaculture and is one of a cluster of seven volcanic crater lakes on Luzon, the largest of the archipelago of islands forming the Philippines.https://doi.org/10.1002/geo2.13Effects of glacier meltwater on the algal sedimentary record of an alpine lake in the central US Rocky Mountains throughout the late Holocene
The effects of alpine glaciers on the hydrology, physical features, and biogeochemistry of lakes have been investigated over contemporary time scales. However, the influence of these factors on algal communities over longer time scales remains unclear, yet is critical to paleolimnological interpretation of environmental change in alpine regions. We examined sedimentary algal pigments and fossil diatom assemblages in two proximal lakes with equivalent local climates, one glacier-fed and one snow-fed, in the central Rocky Mountains (USA) to determine how glacier meltwater has altered algal records over the last 3,000 years. Differences between the records of the two lakes intensified during the Medieval Climate Anomaly and the Little Ice Age, with the glacier-fed lake exhibiting an overall increase in fossil algal pigment concentrations and greater diatom assemblage turnover. Starting 1,000 years ago, the glacier-fed lake in this study showed evidence of nitrogen enrichment from glacier meltwater, as indicated by increasing relative abundances of Asterionella formosa and, to a lesser extent, Fragilaria crotonensis. Since the Little Ice Age, diatom species richness declined in the glacier-fed lake, and further decreased following the 1950s, while assemblage turnover increased. These results demonstrate that glaciers can strongly alter the algal sedimentary record and should be considered when interpreting high-elevation lake records.https://doi.org/10.1007/s10933-015-9829-3The coming and going of a marl lake: Multi-indicator palaeolimnology reveals abrupt ecological change and alternative views of reference conditions
Eutrophication is the most pressing threat to highly calcareous (marl) lakes in Europe. Despite their unique chemistry and biology, comprehensive studies into their unimpacted conditions and eutrophication responses are underrepresented in conservation literature. A multi-indicator palaeolimnological study spanning ca. 1260-2009 was undertaken at Cunswick Tarn (UK), a small, presently eutrophic marl lake, in order to capture centennial timescales of impact. Specific aims were to (1) establish temporal patterns of change (gradual/abrupt) across biological groups, thereby testing theories of resistance of marl lake benthic communities to enrichment, and (2) compare the core record of reference condition with prevailing descriptions of high ecological status. Analyses of sediment calcium (Ca), phosphorus (P), pigments, diatoms, testate amoebae, cladocerans, and macrofossils, revealed three abrupt changes in ecosystem structure. The first (1900s), with biomass increases in charophytes and other benthic nutrient-poor indicators, supported ideas of resistance to eutrophication in Chara lakes. The second transition (1930s), from charophyte to angiosperm dominance, occurred alongside reductions in macrophyte cover, increases in eutrophic indicators, and a breakdown in marling, in support of ideas of threshold responses to enrichment. Core P increased consistently into the 1990s when rapid transitions into pelagic shallow lake ecology occurred and Cunswick Tarn became biologically unidentifiable as a marl lake. The moderate total P at which these changes occurred suggests high sensitivity of marl lakes to eutrophication. Further, the early record challenges ideas of correlation between ecological condition, charophyte biomass and sediment Ca. Instead, low benthic production, macrophyte cover, and Ca sedimentation, was inferred. Management measures must focus on reducing external nutrient and sediment loads at early stages of impact in order to preserve marl lakes.https://doi.org/10.3389/fevo.2015.00082Acceleration of cyanobacterial dominance in north temperate-subarctic lakes during the Anthropocene
Increases in atmospheric temperature and nutrients from land are thought to be promoting the expansion of harmful cyanobacteria in lakes worldwide, yet to date there has been no quantitative synthesis of long-term trends. To test whether cyanobacteria have increased in abundance over the past ~ 200 years and evaluate the relative influence of potential causal mechanisms, we synthesised 108 highly resolved sedimentary time series and 18 decadal-scale monitoring records from north temperate-subarctic lakes. We demonstrate that: (1) cyanobacteria have increased significantly since c. 1800 ce, (2) they have increased disproportionately relative to other phytoplankton, and (3) cyanobacteria increased more rapidly post c. 1945 ce. Variation among lakes in the rates of increase was explained best by nutrient concentration (phosphorus and nitrogen), and temperature was of secondary importance. Although cyanobacterial biomass has declined in some managed lakes with reduced nutrient influx, the larger spatio-temporal scale of sedimentary records show continued increases in cyanobacteria throughout the north temperate-subarctic regions.https://doi.org/10.1111/ele.12420Ecological sensitivity of marl lakes to nutrient enrichment: Evidence from Hawes Water, UK
Highly calcareous (marl) lakes are infrequent but important freshwater ecosystems, protected under the EU Habitats and Species Directive. Chara lakes have been considered resistant to eutrophication owing to the self-stabilising properties of charophyte meadows. However, the opposite is suggested by the large-scale biodiversity declines in marl lake taxa in Europe, and evidence of charophyte sensitivity to eutrophication. We combined contemporary, palaeolimnological and archival methods to investigate the eutrophication of Hawes Water, a shallow marl lake in north-west England (U.K.). Changes in aquatic macrophyte and invertebrate communities were reconstructed through the analysis of historical macrophyte surveys and sedimentary plant and animal macrofossils in two dated sediment cores from the littoral and deep zones of the lake. In addition, chlorophyll and carotenoid pigments were analysed to track changes in primary production from benthic and pelagic areas. Substantial changes in macrophyte communities were detected over centennial timescales, suggesting high ecosystem sensitivity considering the presently moderate phosphorus concentrations in Hawes Water (mean annual total phosphorus 20 μg L-1). Two apparent periods of threshold-like change were identified from the sediment record: (i) changes in cyanobacteria (aphanizophyll + myxoxanthophyll to canthaxanthin + zeaxanthin) and potentially in nutrient stoichiometry, reductions in the maximum macrophyte colonisation depth and water clarity, reduced charophyte and Potamogeton diversity, and increases in Nymphaeaceae; and (ii) severe reductions in light availability inferred from subdecadal doubling in phytoplankton abundance, substantial increases in Daphnia abundance and the extinction of charophytes from higher water depths. Further, change in both the littoral and deeper water has confined key marl lake taxa to smaller niches. In the littoral, increasing siltation and reed and Nymphaeaceae densities caused extinction of Littorella uniflora in the early 1900s and have reduced the evenness of Characeae with suspected imminent extinction of two highly localised Chara spp. In the deeper water, upslope creep of maximum colonisation depth has reduced habitat for intermediate-depth marl lake taxa leading to the loss of four Potamogeton and one Chara species, and replacement of these taxa by Nuphar lutea. The large changes in macrophyte community composition and increased incidences of turbid water have reduced the distinctive and valued marl lake features of Hawes Water, indicating that marl lakes can, as a habitat type, be highly sensitive to eutrophication. The persistence of abundant generalist macrophyte species at considerable water depth may be a feature of high-alkalinity lakes in clearwater, macrophyte-dominated states, but is a distinct eutrophication response in marl lakes rather than an indication of resistance to eutrophication.https://doi.org/10.1111/fwb.12650Spatial and temporal variability of lake ontogeny in south-western Greenland
Holocene palaeolimnological records of diatoms and β carotene (a proxy for aquatic production) from four lakes in the low Arctic region of south-western Greenland were used to investigate the role of climate on lake ontogeny. Two of the lakes are located in the maritime, coastal region near Sisimiut and two inland close to the head of Kangerlussuaq fjord, where there is a more continental climate. Diatom records from the four lakes (AT1, AT4, SS1381, SS8) had similar long-term ontogeny trends, independent of climatic setting and the changes are interpreted as responses to first order weathering controls on catchment/lake chemistry. Short-term excursions from these broad trends occurred in one coastal site (AT4) caused by intense erosion of the steep catchment, and at inland sites where temporary hydrological closure and lake level decline occurred during the mid-Holocene (~8000 - 5000 cal a BP). Algal production (as β carotene) was more closely and consistently correlated with climatic changes; it peaked during the mid-Holocene, the warmest period of the Holocene, at all sites and there were transient increases in production in inland lakes during the Medieval Climate Anomaly and Little Ice Age because of fertilization through increased aeolian dust deposition. A synthesis of seven palaeolimnological records from this region identified that only the mid-Holocene was correlated with diatom stratigraphic zones and there was considerable among-site variability in later Holocene lake response to climate forcing in this area. Comparable long-term trends in species assemblage turnover (DCA/CA axis 1 scores) clearly demonstrate that lakes have predictable ontogeny trends in this region, characterised by maximum alkalinity and nutrient availability in the first few millennia followed by progressive oligotrophication and alkalinity loss. However, individual lake and catchment characteristics (lake morphology, catchment geomorphology), when modified by climatic change (vegetation cover, erosion, weathering rates, aeolian dust deposition, lake level) can diverge from this ontogeny template leading to complex ecological transitions in lakes from this region.https://doi.org/10.1016/j.quascirev.2015.08.005Looking forward through the past: Identification of 50 priority research questions in palaeoecology
Priority question exercises are becoming an increasingly common tool to frame future agendas in conservation and ecological science. They are an effective way to identify research foci that advance the field and that also have high policy and conservation relevance. To date, there has been no coherent synthesis of key questions and priority research areas for palaeoecology, which combines biological, geochemical and molecular techniques in order to reconstruct past ecological and environmental systems on time-scales from decades to millions of years. We adapted a well-established methodology to identify 50 priority research questions in palaeoecology. Using a set of criteria designed to identify realistic and achievable research goals, we selected questions from a pool submitted by the international palaeoecology research community and relevant policy practitioners. The integration of online participation, both before and during the workshop, increased international engagement in question selection. The questions selected are structured around six themes: human-environment interactions in the Anthropocene; biodiversity, conservation and novel ecosystems; biodiversity over long time-scales; ecosystem processes and biogeochemical cycling; comparing, combining and synthesizing information from multiple records; and new developments in palaeoecology. Future opportunities in palaeoecology are related to improved incorporation of uncertainty into reconstructions, an enhanced understanding of ecological and evolutionary dynamics and processes and the continued application of long-term data for better-informed landscape management.https://doi.org/10.1111/1365-2745.12195Diatom communities along pH and hydrological gradients in three montane mires, central China
The distribution patterns of epiphytic diatom assemblages in three montane mires in central China were investigated to examine their relationships with selected environmental variables (pH and depth to water table, DWT). Two of the mires are considered to be in good ecological condition (Dajiuhu and Qizimeishan Mires) while Erxianyan Mire is extensively affected by acid deposition and human activities. A total of 206 taxa belonging to 56 genera were found in 44 Sphagnum samples. Multivariate analysis revealed that pH and DWT were significantly correlated with diatom distribution. In Erxianyan Mire, the characteristic taxa (Eunotia minor and Eunotia intermedia) had lower pH optima and may therefore be useful indicators of highly-acidic conditions. In Dajiuhu Mire, the dominant species had higher pH optima, and abundant xerotolerant taxa (Hantzschia amphioxys, Pinnularia borealis, Luticola mutica and Diadesmis contenta) were observed. In the partial canonical correspondence analyses with mire location as a covariable, the correlation between diatom data and pH was insignificant, likely because pH differences between mires were greater than those within mires. In contrast, diatom data were significantly correlated with DWT, suggesting that diatoms are good sensors of hydrological variability along the hollow to hummock gradient. Together, these data can expand current autecological information for these potential diatom indicator species, which is critical for refining our interpretations of bio-monitoring and palaeolimnological studies in montane mires.https://doi.org/10.1016/j.ecolind.2014.04.016Diatom response to heavy metal pollution and nutrient enrichment in an urban lake: Evidence from paleolimnology
Diatoms and geochemical stratigraphy were studied in sediment core samples collected from a heavily polluted urban lake (SE China) in order to track the history of eutrophication and heavy metal contamination. The sediment profile covered ca. 60 years (from ca. 1951 to 2011) based on 137Cs and Spheroidal carbonaceous particles (SCP) dating, and encompassed a period of rapid industrial development in this region. Diatoms experienced two visible shifts, including the replacement of benthic and epiphytic taxa by planktonic species (e.g., Cyclotella meneghiniana Kützing) in 1972, and the dominance of Cyclotella atomus Hustedt and Nitzschia palea (Kützing) W. Smith after 1999. Metals (i.e., Cd, Pb and Zn), total phosphorus, total nitrogen and total organic carbon all increased in the past 60 years. Redundancy analysis was used to correlate diatom with chemical change and explained 50.3-60% of total variation in diatom data for three periods (from 1951 to 1999, between 1951 and 2011 and from 1972 to 2011). The combined effects of nutrients and metals were the predominant factor, capturing 29.6-42.8% of the total variance. Nutrients alone accounted for little more variance than did metals alone for the first flora shift about 1972. The further shift after 1999 was more influenced by the sole effect of metals than that of nutrients. Increases in species (e.g., N. Palea) able to tolerate both nutrient-related and metal-related stressors were related to persistent nutrient and metal inputs. In addition, climate warming might exacerbate eutrophication and metal contamination in this lake.https://doi.org/10.1051/limn/2014004Nutrient limitation of periphyton growth in arctic lakes in south-west Greenland
Many arctic lakes are oligotrophic systems where phototrophic growth is controlled by nutrient supply. Recent anthropogenic nutrient loading is associated with biological and/or physico-chemical change in several lakes across the arctic. Shifts in nutrient limitation (nitrogen (N), phosphorus (P), or N + P) and associated effects on the growth and composition of algal communities are commonly reported. The Kangerlussuaq region of south-west Greenland forms a major lake district which is considered to receive little direct anthropogenic disturbance. However, long-range transport of pollutant N is now reaching Greenland, and it was hypothesised that a precipitation gradient from the inland ice sheet margin to the coast might also deliver increased N deposition. In situ nutrient bioassays were deployed in three lakes across the region: ice sheet margin, inland (close to Kangerlussuaq) and the coast (near Sisimiut), to determine nutrient limitation of lakes and investigate any effects of nutrients on periphyton growth and community composition. Nutrient limitation differed amongst lakes: N limitation (ice sheet margin), N and P limitation (inland) and N + P co-limitation (coast). Factors including variation in N supply, ice phenology, seasonal algal succession, community structure and physical limnology are explored as mechanisms to explain differences amongst lakes. Nutrient limitation of arctic lakes and associated ecological impacts are highly variable, even across small geographic areas. In this highly sensitive region, future environmental change scenarios carry a strong risk of significantly altering nutrient limitation; in turn, potentially severely impacting lake structure and function.https://doi.org/10.1007/s00300-014-1524-8Contrasting effects of nutrients and climate on algal communities in two lakes in the windermere catchment since the late 19th century
Disentangling the role of nutrient pollution and climate change on lake ecosystem functioning is paramount to protect water quality in lake catchments worldwide. For more effective management, however, we need to determine whether these two forcing factors interact at different spatial and temporal scales. This study compares centennial-scale archival data and lake sediment records of eutrophication from Blelham Tarn and previously published data from Lake Windermere's North Basin in the English Lake District. We aimed to quantify how lake morphometry, catchment characteristics and landscape position influence the relationship between climate, local land use and algal community change. Redundancy analysis revealed that increases in cyanobacterial pigments and stable isotopes of nitrogen and carbon in sediments of Blelham Tarn from the 1970s onwards correlate strongly with rising densities of sheep and cattle in the catchment. Concomitant installation of piped water and sewage processing facilities appeared to lead to the expansion of filamentous cyanobacteria. In contrast, elevated fossil pigments from siliceous algae after 1990 were related inversely to winter precipitation, suggesting seasonal changes in hydraulic flushing also influenced the algal community response to centennial-scale fertilisation. Abundance of vernal algae increased synchronously in Blelham Tarn and Lake Windermere's North Basin after regional agricultural intensification in the mid-nineteenth century. In contrast, differences in timing of wastewater disposal and treatment at each site led to asynchronous changes in summer taxa such as filamentous cyanobacteria. This study highlights that lake catchments can act as local filters to regional climate change, both due to differences in localised land-use and intrinsic hydrological features (e.g. catchment:lake area, flushing rate). Further, this paper highlights the ability of palaeolimnology to aid identification of significant nutrient sources over different spatial scales for effective catchment water management.https://doi.org/10.1111/fwb.12457The effects of hydrological extremes on former gravel pit lake ecology: Management implications
Urban gravel pit lake ecosystems are particularly susceptible to flooding and anthropogenic nutrient loading, and are therefore likely to be significantly affected by the future projections of precipitation extremes. Six shallow ex-gravel pit lakes at Attenborough Nature Reserve, (Nottinghamshire, U.K.), three connected to the nutrient-rich River Erewash and three isolated from it, were monitored from 2005-2008 (including the highest and 6th lowest rainfall years of a 40-year record). We aimed to compare lake ecological response to hydrological extremes and to see whether river-connected gravel pits responded differently to those fed mainly by groundwater. Our results showed that flood conditions in the river-connected lakes reduced the maximum phytoplankton biomass achieved and favoured smaller taxa (cryptophytes, Scenedesmus spp. and small centric diatoms). Consequently dissolved nitrate (NO3-N) and silica (SiO2) maxima were higher during flood years because of the reduced capacity for algal uptake, but dissolved and particulate phosphorus (P) concentrations were lower because increased flushing rates led to more effective removal of sediment-released P from the lake systems. Lakes isolated from the River Erewash responded less clearly to increased rainfall, except for a delay in the timing of the phytoplankton maximum, and a pronounced rise in SiO2 concentrations after drought conditions. Spatial comparisons also showed that nutrient pollution in the River Erewash and in storm drains feeding into one of the 'isolated' lakes (Beeston Pond) led to the existence of a turbid lake ecosystem state through the increased supply of dissolved NO3-N. The other isolated lakes had lower phytoplankton biomass and extensive aquatic plant coverage, but significantly greater proportions of bloom-forming cyanobacteria (Anabaena, Aphanizomenon), because of greater water retention times. Our results show that ex-gravel pit lakes connected to rivers may be more susceptible to ecological disruption from future flooding events and are heavily influenced by river water quality, but may be less susceptible to potentially toxic cyanobacteria blooms which may be favoured in lakes isolated from rivers with long water retention times. Artificial manipulation of urban lake hydrology to increase flushing with water of a higher quality or isolation of lakes from nutrient-rich inflows may be a useful management strategy, but the effectiveness of either approach depends on the ecological quality of the lake and inflowing river. Therefore, managing urban lakes in the face of climate change and increased urbanisation requires a detailed understanding of lake ecology and hydrology.https://doi.org/10.1127/fal/2014/0573Catchment-mediated atmospheric nitrogen deposition drives ecological change in two alpine lakes in SE Tibet
The south-east margin of Tibet is highly sensitive to global environmental change pressures, in particular, high contemporary reactive nitrogen (Nr) deposition rates (ca. 40 kg ha-1 yr-1), but the extent and timescale of recent ecological change is not well prescribed. Multiproxy analyses (diatoms, pigments and geochemistry) of 210Pb-dated sediment cores from two alpine lakes in Sichuan were used to assess whether they have undergone ecological change comparable to those in Europe and North America over the last two centuries. The study lakes have contrasting catchment-to-lake ratios and vegetation cover: Shade Co has a relatively larger catchment and denser alpine shrub than Moon Lake. Both lakes exhibited unambiguous increasing production since the late 19th to early 20th. Principle component analysis was used to summarize the trends of diatom and pigment data after the little ice age (LIA). There was strong linear change in biological proxies at both lakes, which were not consistent with regional temperature, suggesting that climate is not the primary driver of ecological change. The multiproxy analysis indicated an indirect ecological response to Nr deposition at Shade Co mediated through catchment processes since ca. 1930, while ecological change at Moon Lake started earlier (ca. 1880) and was more directly related to Nr deposition (depleted δ15N). The only pronounced climate effect was evidenced by changes during the LIA when photoautotrophic groups shifted dramatically at Shade Co (a 4-fold increase in lutein concentration) and planktonic diatom abundance declined at both sites because of longer ice cover. The substantial increases in aquatic production over the last ca. 100 years required a substantial nutrient subsidy and the geochemical data point to a major role for Nr deposition although dust cannot be excluded. The study also highlights the importance of lake and catchment morphology for determining the response of alpine lakes to recent global environmental forcing.https://doi.org/10.1111/gcb.12435Pigment Studies
Pigments, including chlorophylls, carotenoids, photoprotective compounds, and their derivatives produced by algae, phototrophic bacteria, and aquatic plants often preserve well in the sediments of aquatic environments. In sediment cores, they can yield estimates of past primary production, information on aquatic phototroph community composition, and indicate depositional and preservation conditions. This article describes the biochemical nature of pigments, including their preservation in sedimentary environments, techniques for pigment analysis, and a range of paleolimnological applications, including the investigation of eutrophication and aquatic food web changes, atmospheric deposition of contaminants, and hydrological change. Finally, examples of the use of pigments in inferring past climates over Holocene and Plio-Pleistocene time scales are given.https://doi.org/10.1016/B978-0-444-53643-3.00235-1Cascading effects of generalist fish introduction in oligotrophic lakes
Introduction of strictly planktivorous fish to lakes can alter plankton communities via cascading interactions in food webs. Less is known about the large-scale and long-term effects resulting from the introduction of fish with more generalist feeding habits, and the extent to which these effects depend on lake trophic status. Paleolimnological records of three oligotrophic lakes in Maine, USA were used to analyze the response of plankton communities to the introduction of white perch (Morone americana), a fish that often numerically dominates fish assemblages and switches from strict planktivory to a more generalist diet during ontogeny. After white perch introduction, cladoceran ephippia size increased up to 50 %, suggesting that the most important role of this generalist fish, with respect to water quality, is as a piscivorous trophic link. Algal standing crop declined by a quarter to over a half of pre-introduction levels, suggesting that top-down effects of white perch reduced phytoplankton biomass. In comparing these oligotrophic lakes to prior work in a eutrophic system, white perch introduction had similar effects on zooplankton body size; however, cascading effects to phytoplankton were only observed in low productivity lakes.https://doi.org/10.1007/s10750-013-1469-xClimate-driven changes in water level: A decadal scale multi-proxy study recording the 8.2-ka event and ecosystem responses in Lake Sarup (Denmark)
A two-stage change in lake level during the 8. 2-ka event was identified in Lake Sarup, Denmark (55°N), using a multiproxy approach on precise radiocarbon wiggle-matched annually laminated sediments deposited 8740-8060 cal. yr BP. Changes in δ13C and δ18O indicated closed lake hydrology driven by precipitation. The isotopic, sedimentary and plant macrofossil records suggested that the lake level started to decrease around 8400 cal. yr BP, the decrease accelerating during 8350-8260 before an abrupt increase during 8260-8210. This pattern shows that the climate anomaly started ~150 years before the onset of the 8. 2-ka cooling event registered in Greenland ice cores, but was synchronous with hydrologic change in the North American Lake Agassiz drainage. The lake level decrease was accompanied by a higher accumulation rate of inorganic matter and lower accumulation rates of cladoceran subfossils and algal pigments, possibly due to increased turbidity and reduced nutrient input during this drier period. Pigment analysis also showed added importance of diatoms and cryptophytes during this climate anomaly, while cyanobacteria became more important when the water level rose. Moreover, Nymphaeaceae trichosclereids were abundant during the period of algal enrichment. Cladoceran taxa associated with floating leaved plants or benthic habitats responded in a complex way to changes in water level, but the cladoceran assemblages generally reflected deep lake conditions throughout the period. The lake did not return to its pre-8. 2-ka event status during the period of analysis, but remained more productive for centuries after the climatic anomaly as judged from the pigment accumulation and assemblage composition. The change to more eutrophic conditions may have been triggered by erosion of marginal deposits. Together, these data confirm the chronology of hydrologic changes and suggest, for the first time, that lake levels exhibited both a decline and an increase in rapid succession in response to the 8. 2-ka event in southern Scandinavia.https://doi.org/10.1007/s10933-012-9673-7Persistence of protected, vulnerable macrophyte species in a small, shallow eutrophic lake (eastern Poland) over the past two centuries: Implications for lake management and conservation
The developmental history of shallow, eutrophic Lake Głębokie Uścimowskie, eastern Poland, spanning the last 180 years, was studied using analyses of pollen, plant macrofossils, diatoms, chlorophyll and carotenoid pigments, Cladocera, Chironomidae and geochemistry from lead-210 dated sediment cores. The lake was colonised by macrophytes and eutrophic through the whole investigation period, but from 1950 AD the trophic status increased slowly and eutrophication accelerated after 1980 AD, leading to hypertrophy. Despite this eutrophication history, macro- and microfossils indicated the continuing presence of Isoëtes lacustris spores and Elatine hydropiper seeds in the sediments of the lake, even though neither species were encountered in the recent and present-day lake macrophyte surveys. Both the species are classified as vulnerable (IUNC List) in Poland and are presently deemed extinct due to eutrophication in the Łęczna-Włodawa Lake District. We conclude the silica-rich substrates (51–77%), low in calcium (<1%) and organic matter (7–21%) in the lake were crucial for the survival of soft-water species like I. lacustris and species that prefer sandy littoral habitats like E. hydropiper. We also propose that water level fluctuations were important in modifying the extent of the littoral zone and sorting the substrate and thus in maintenance of appropriate habitat for these rare species. Therefore, substrate type and hydrological factors appear to be important to conservation of rare species in this lake.https://doi.org/10.1016/j.aquabot.2012.12.004Environmental change in the limfjord, Denmark (ca 7500-1500cal yrsBP): A multiproxy study
The Limfjord region of northern Jutland, Denmark, supports a rich archaeological record dating back to the Mesolithic, which documents long-term change in human practices and utilisation of marine resources since approximately 7500BP. The presence and availability of marine resources in the Limfjord is sensitively regulated by environmental parameters such as salinity, sedimentary regime, nutrient status and primary productivity, but long-term changes in these parameters are currently poorly understood. In this study a multiproxy approach (including sedimentary parameters, diatoms, molluscs, foraminifera, sedimentary pigments, C and O stable isotopes and plant macrofossils) has been adopted to assess environmental change over the period ca 7500-1500calyrsBP at Kilen, a coastal fjord (before AD 1856) situated in the Western Limfjord. A diatom-based salinity transfer function based on a pan-Baltic training set has been applied to the fossil diatom dataset for quantitative assessment of salinity change over the study period. This study demonstrates that large-scale shifts in salinity are a common feature of the Limfjord's long-term history and are driven by the level of connection with the North Sea and the Skagerrak respectively, which in turn is likely driven by the complex interplay between climate, sea-level change, current velocity and rates of erosion/sedimentary accretion. Three shifts in state at Kilen are identified over the study period: a deep, periodically stratified fjord with medium-high salinity (and high productivity) between ca 7500-5000BP, followed by a gradual transition to a shallow benthic system with more oceanic conditions (i.e. higher salinity, lower productivity, slower sedimentary accumulation rate and poorer fossil preservation) after ca 5000BP and no stratification after ca 4400BP, and lastly, within this shallow phase, an abrupt shift to brackish conditions around 2000BP. Environmental-societal interactions are discussed on the basis of the data presented in this study and current environmental hypotheses for cultural change are challenged.https://doi.org/10.1016/j.quascirev.2013.05.020Humans and climate as drivers of algal community change in Windermere since 1850
1.Historical archives, published data sets and lake sediments from the North and South Basins of Windermere were analysed to reconstruct changes in the lake and catchment since 1850 and determine the drivers of limnological variability. 2.Pastoral sheep farming has remained the dominant form of agriculture since 1850, but there was a decline in horse numbers (because of agricultural mechanisation), an increase in nitrogen fertiliser use and a doubling in sheep numbers since the 1950s. The human population in the Windermere catchment almost quadrupled between 1801 and 1921 coincident with the arrival of the railway in 1847, which led to urban expansion, sewer and piped water installation (since ~1860s) and the development of sewage treatment systems (after 1886). 3.C:N ratios and stable isotopes of carbon in 210Pb-dated sediment cores suggested that land disturbance increased the transport of terrestrial carbon to sediments of both basins after 1870, but algae increasingly contributed to the sediment matrix after ~1890. Nitrogen-stable isotope ratios generally declined in both basins consistent with an increase in an isotopically light source such as atmospheric N deposition or synthetic fertilisers. 4.Chlorophyll and carotenoid pigments in sediment cores showed that algal production increased gradually in the North Basin from ~1890 to 1940 to be maintained at higher abundance thereafter. South Basin pigments changed most markedly ~1860 (increase in cyanobacterial pigments), ~1945 (increase in siliceous algal and chlorophyte pigments) and ~1987 (increase in cyanobacterial pigments). 5.Redundancy and variance partitioning analyses suggested that sewage influx and the modernisation of agriculture were strongly and positively correlated with algal abundance. However, climate variables were a secondary driver of algal change in both lake basins. 6.Enrichment of the lake was caused by a combination of increases in population (after 1847), more efficient sewage disposal, intensification of agriculture and atmospheric deposition of N, which led to a greater than fivefold increase in primary production in the North Basin and marked shifts in algal communities of the South Basin. 7.After enrichment, lake phytoplankton responded markedly to fluctuations in climate. High precipitation in January-March led to lower abundance of siliceous algae (mostly Asterionella formosa) in both lake basins, possibly because A. formosa inocula were diluted, thus limiting the spring diatom bloom. 8.This combined palaeolimnological and archival study has shown that eutrophication of Windermere occurred before the start of the lake monitoring programme in 1945, but that nutrient enrichment enhanced the lake response to meteorological change. Therefore, future management of Windermere should aim for baseline conditions similar to those existing c. 1850 and consider that the control of eutrophication is essential in building resilience to future climate change. © 2011 Blackwell Publishing Ltd.https://doi.org/10.1111/j.1365-2427.2011.02689.xPopulation trends in the Slavonian grebe Podiceps auritus (L.) and Chironomidae (Diptera) at a Scottish loch
Loch Ruthven holds the largest British population of the rare water-bird Podiceps auritus, the Slavonian or horned grebe. The breeding success of this bird has fluctuated annually since records began in 1970. To investigate whether these trends are linked to the abundance of chironomid midges, which are an important food-source for the grebe chicks, we analysed a sediment core from the lake, which was sliced at 2. 5-mm intervals and provided near-annual sampling resolution. We also analysed diatoms and algal pigments in the lake sediments and inferred changes in total phosphorus from the diatom assemblage to determine whether changes in lake productivity have influenced the abundance of chironomids. Trends in grebe productivity, chironomid abundance and algal assemblages were compared against climate data to determine whether climate, specifically, the North Atlantic Oscillation, was the ultimate driver of the trends we recorded. Our results show that grebe breeding success is positively correlated with chironomid abundance and chironomid abundance is positively correlated with diatom-inferred total phosphorus. Lake productivity and chironomid abundance began to rise early in the twentieth century and continued to rise on a steeper trajectory from the mid-twentieth century to the present. Since the mid-1960s, chironomid abundance began to fluctuate erratically and since 1970 was in phase with grebe productivity, with the grebe trends most plausibly lagging by 1 year. These trends appear to correlate with inter-annual fluctuations in diatom-inferred total phosphorus. No correlation was found between grebe productivity or chironomid abundance and climate variables, suggesting that the size of the chironomid population and breeding success of Podiceps auritus at Loch Ruthven is resource-linked.https://doi.org/10.1007/s10933-012-9587-4Lake and catchment response to Holocene environmental change: Spatial variability along a climate gradient in southwest Greenland
The Kangerlussuaq area of southwest Greenland is a lake-rich landscape that covers a climate gradient: a more maritime, cooler and wetter coastal zone contrasts with a dry, continental interior. Radiocarbon-dated sediment sequences (covering ~11,200-8,300 cal year) from paired lakes at the coast and the head of the fjord were analysed for lithostratigraphic variables (organic-matter content, bulk density, Ti, Ca). Minerogenic and carbon accumulation rates from the four lakes were compared to determine catchment and lake response to Holocene climatic variability. Catchment erosion at the coast was dominated by cryonival processes, with considerable sediment production due to the limited vegetation cover and exposed rock faces. Input of minerogenic sediment at one site (AT4) was high (>1 gDW cm -2 year -1) during the period 5,800-4,000 cal year BP, perhaps reflecting intensification of cryogenic processes on northeast-facing slopes and rapid delivery to the lake. This period of erosional activity was not observed at the nearby, higher elevation site (AT1) due to the lower catchment relief; instead, there was an abrupt decline in carbon and minerogenic accumulation rates at ~5,800 cal year BP. Sediment accumulation rates at the inland sites were much lower (<0. 005 gDW cm -2 year -1) reflecting greater catchment stability (more extensive vegetation cover), lower relief and substantially lower precipitation, but synchronous increases in mineral accumulation rates from ~1,200 to 1,000 cal year BP may reflect wind erosion associated with regional cooling and local aridity. Carbon-accumulation-rate profiles were similar at the two inland sites, with higher-than-average accumulation (~6-8 g C m -2 year -1) during the early Holocene and a subsequent decline after ~6,000 cal year BP. At the inland lakes, both mineral and carbon accumulation rates exhibited a stronger link to climate, driven by trends in effective precipitation and regional aeolian activity. Catchment differences (relief, altitude) lead to more individualistic records in both erosion history and lake productivity at the coast.https://doi.org/10.1007/s10933-012-9616-3Interdecadal declines in flood frequency increase primary production in lakes of a northern river delta
Human activities and climate change have greatly altered flooding regimes in many of the world's river deltas, but the impact of such changes remains poorly quantified on decadal to multidecadal timescales. This study identified the response of delta lake primary production (measured as the concentration of sedimentary pigments) to variations in flood frequency using spatial surveys and paleolimnological analyses of lakes in the Peace-Athabasca Delta (PAD), Canada. Surveys of 61 lakes spanning a range of hydrological conditions showed that those lakes that received flood waters less frequently were associated with elevated algal production (surface sedimentary pigments) and, in some lakes, increased growth of emergent macrophytes and epiphytic diatoms. Paleolimnological analyses of five lakes corroborated the contemporary spatial survey results by showing that production of pigments from most algal groups increased during recent periods of lower flood frequency in the 20th century as determined from increases in cellulose-inferred lake-water oxygen isotope composition and plant macrofossils, but remained stable in a 'reference' basin. In general, past periods of elevated algal production coincided with the increased abundance of submerged macrophytes or emergent vegetation that provide habitat for attached algae. These results suggest that interdecadal declines in river discharge arising from increased aridity, hydrologic regulation or consumptive water use will cause long-term increases in primary production and alter ecosystem processes (carbon sequestration, biological diversity) in aquatic delta ecosystems similar to the PAD where lakes become nutrient-rich in the absence of flooding.https://doi.org/10.1111/j.1365-2486.2010.02304.xThe influence of Holocene tree-line advance and retreat on an arctic lake ecosystem: A multi-proxy study from Kharinei Lake, North Eastern European Russia
A consequence of predicted climate warming will be tree-line advance over large areas of the Russian tundra. Palaeolimnological techniques can be used to provide analogues of how such changes in tree-line advance and subsequent retreat affected lake ecosystems in the past. A Holocene sediment core taken from Kharinei Lake (Russia) was dated radiometrically and used for multi-proxy analyses with the aim of determining how climate and tree-line dynamics affected the productivity, community structure, carbon cycling and light regime in the lake. Pollen and macrofossil analyses were used to determine the dates of the arrival and retreat of birch and spruce forest. C:N ratios and percent loss-on-ignition were used to infer past changes in sediment organic matter. Visible-near-infrared spectroscopy and diatom analysis were used to infer past changes in lake-water carbon. Algal pigments and aquatic macrophytes were used to determine changes in lake productivity and light. Chironomids together with remains of the aquatic flora and fauna were used to provide information on past July temperature and continentality. Lake sedimentation was initiated shortly before 11,000 cal. years BP, when both chironomid- and pollen-inferred temperature reconstructions suggest higher summer temperatures than present, between 1 and 2°C warmer, and lake productivity was relatively high. A few trees were already present at this time. The spruce forest expanded at 8,000 cal. year BP remaining in the vicinity of the lake until 3,500 cal. year BP. This period coincided with a high concentration of organic material in the water column, and relatively high benthic productivity, as indicated by a high benthic: planktonic diatom ratio. After tree-line retreat, the optical transparency of the lake increased, and it became more open and exposed, and was thus subject to greater water-column mixing resulting in a higher abundance of diatom phytoplankton, especially heavily silicified Aulocoseira species. The colder climate resulted in a shorter ice-free period, the lake was less productive and there was a loss of aquatic macrophytes. Increased wind-induced mixing following forest retreat had a greater influence on the lake ecosystem than the effects of decreasing organic matter concentration and increased light penetration.https://doi.org/10.1007/s10933-011-9528-7Holocene palaeoecology of southwest Greenland inferred from macrofossils in sediments of an oligosaline lake
We retrieved a lake sediment record from an oligosaline, meromictic lake in southwest Greenland. The record spans the last 8,200 cal. years and was radiocarbon dated and analysed for macroscopic remains of plants and animals. The record extends the known history of several invertebrate species back in time, and provides minimum ages for their immigration to Greenland after the last deglaciation. Shells of the ostracod Ilyocypris bradyi were found in sediments dated to the time interval c. 7,000-6,500 cal. years BP. Shells of this species were found previously in a nearby oligosaline lake, where its occurrence was dated to about the same short time interval. The species is a thermophilous, non-arctic taxon that is absent from the present day Greenland fauna, and we suggest that its former occurrence in west Greenland marks the peak of the Holocene thermal maximum. This is in agreement with other records from Greenland.https://doi.org/10.1007/s10933-009-9368-xSedimentary records of sewage pollution using faecal markers in contrasting peri-urban shallow lakes
Sewage contamination in shallow lake sediments is of concern because the pathogens, organic matter and nutrients contribute to the deterioration of the water-bodies' health and ecology. Sediment cores from three shallow lakes (Coneries, Church and Clifton Ponds) within Attenborough nature reserve located downstream of sewage treatment works were analysed for TOC, C/N, δ13C, δ15N, bacterial coliforms and faecal sterols. 210Pb and 137Cs activities were used to date the sediments. Elemental analysis suggests that the source of organic matter was algal and down profile changes in δ13C indicate a possible decrease in productivity with time which could be due to improvements in sewage treatment. δ15N for Coneries Pond are slightly higher than those observed in Church or Clifton and are consistent with a sewage-derived nitrate source which has been diluted by non-sewage sources of N. The similarity in δ15N values (+12% to +10%) indicates that the three ponds were not entirely hydrologically isolated. Analysis by gas chromatography/mass spectrometry (GC/MS) reveals that Coneries Pond had sterol concentrations in the range 20 to 30μg/g (dry wt.), whereas, those from Clifton and Church Ponds were lower. The highest concentrations of the human-sourced sewage marker 5α-coprostanol were observed in the top 40cm of Coneries Pond with values up to 2.2μg/g. In contrast, Church and Clifton Pond sediments contain only trace amounts throughout. Down-profile comparison of 5β-coprostanol/cholesterol, 5β-coprostanol/(5β-coprostanol+5α-cholestanol) and 5β-epicoprostanol/coprostanol as well as 5α-cholestanol/cholesterol suggests that Coneries Pond has received appreciable amounts of faecal contamination. Examination of 5β-stigmastanol (marker for herbivorous/ruminant animals) down core concentrations suggests a recent decrease in manure slurry input to Coneries Pond. The greater concentration of β-sitosterol in sediments from Church and Clifton Ponds as compared to Coneries is attributed in part to their greater diversity and extent of aquatic plants and avian faeces.https://doi.org/10.1016/j.scitotenv.2010.09.033Spatial and temporal variability of prairie lake hydrology as revealed using stable isotopes of hydrogen and oxygen
Evaporation and groundwater fluxes are thought to regulate hydrologic variability in lakes of the northern Great Plains, but little is known of how the relative importance of these processes may vary in time or space. To address this issue, we measured the isotopic composition of water (δ 18O, δ2H) from 70 closed-basin lakes in southern Saskatchewan, Canada. All lakes occupied endorheic basins along a long gradient of salinity (0.2-115 g total dissolved solids L-1). Lakes exhibited synchronous seasonal changes in salinity (synchrony, S = 0.78) and δ18O (S = 0.84) during the dry summer of 2003 (∼ 195 mm rain), whereas coherence was reduced to 0.56 and 0.22, respectively, during the wet summer of 2004 (∼295 mm rain). However, despite evaporative enrichment of isotopic ratios during dry summers, hydrologic balances were regulated mainly by changes in water inflow (I) rather than evaporation (E) in both wet and dry years, with particularly strong influence of inflow (lowest E : I ratio) in dry southwestern regions. Analysis of isotopic composition also identified winter precipitation or groundwater as the most influential source of water to most lakes, despite only ∼30% of annual precipitation being delivered during winter. Therefore, although seasonal variability in lake chemistry was influenced by evaporation during summer, long-term mean chemical characteristics of prairie lakes were regulated mainly by changes in winter precipitation or groundwater influx.https://doi.org/10.4319/lo.2009.54.1.0101Paleolimnological evidence of the effects on lakes of energy and mass transfer from climate and humans
The premise of this article is that climate effects on lakes can be quantified most effectively by the integration of process-oriented limnological studies with paleolimnological research, particularly when both disciplines operate within a common conceptual framework. To this end, the energy (E)-mass (m) flux framework (Em flux) is developed and applied to selected retrospective studies to demonstrate that climate variability regulates lake structure and function over diverse temporal and spatial scales through four main pathways: rapid direct transfer of E to the lake surface by irradiance, heat, and wind; slow indirect effects of E via changes in terrestrial development and subsequent m subsidies to lakes; direct influx of m as precipitation, particles, and solutes from the atmosphere; and indirect influx of water, suspended particles, and dissolved substances from the catchment. Sedimentary analyses are used to illustrate the unique effects of each pathway on lakes but suggest that interactions among mechanisms are complex and depend on the landscape position of lakes, catchment characteristics, the range of temporal variation of individual pathways, ontogenetic changes in lake basins, and the selective effects of humans on m transfers. In particular, preliminary synthesis suggests that m influx can overwhelm the direct effects of E transfer to lakes, especially when anthropogenic activities alter m subsidies from catchments.https://doi.org/10.4319/lo.2009.54.6_part_2.2330The role of paleoecology in whole-ecosystem science
Paleoecological analyses provide exceptionally long time series of population abundance, community composition, environmental variability, ecosystem subsidies, and temporal variability that can uniquely inform and guide ecosystem managers and scientists conducting whole-ecosystem experiments. This chapter illustrates the potential of retrospective studies to inform, guide, and refine ecosystem experimentation and management using case studies derived from paleoecological studies of freshwater lakes. First, we illustrate how synchrony (temporal correlation) and variance partitioning analysis may be used to quantify the natural mechanisms underlying lake ontogeny in the absence of human effects in lakes from Southwest Greenland. Second, we show how a combination of biogeochemical (15N isotopes, pigments), time series and correlation analyses of sediment cores can be used to both reconstruct and characterise historical variance in sockeye salmon fisheries, and to elucidate the relationship between sockeye salmon and lake primary production in the Bristol Bay region of Alaska. Finally, we show how a combination of paleoecological analysis and long-term ecological monitoring programs can both identify the main causes of water quality change during the twentieth century and quantify the role of N in regulating primary production in lakes of the Canadian Prairies.https://doi.org/10.1007/978-0-387-77942-3_7A Late Holocene record of landscape degradation from Heygsvatn, the Faroe Islands
Diatom, magnetic susceptibility and loss-on-ignition (LOI) analyses of a 9-metre sediment core from Heygsvatn (Su{eth}uroy, the Faroe Islands) were used to investigate the response of lake and catchment to mid-Late Holocene environmental change. A 30-lake training set from Faroese lakes was also used to explore the relationships between diatoms and environmental variables and assist in the interpretation of the diatom stratigraphy. Canonical Correspondence Analysis showed that lake depth, light penetration, total phosphorus (TP), pH and dissolved organic carbon (DOC) were the most significant variables affecting diatom distribution in Faroese lakes (p < 0.06). Nine 14C AMS dates indicated that the sediment record covered the last ~ 5700 years. Exceptionally high rates of sediment accumulation, an increase in LOI and magnetic susceptibility and inversion of radiocarbon dates suggested that a period of elevated soil erosion occurred at Heygsvatn ca. 1700-1200cal. yr. BP. Because this erosion pre-dates the arrival of humans the most likely cause is climate deterioration (cooler, wetter conditions). The diatom stratigraphy supports this hypothesis by showing that during the period of erosion the lake was deeper or/or more depauperate in TP, consistent with the climate becoming wetter and leading to increased lake flushing. Subsequently, with sustained sediment erosion, the lake became more shallow and pH increased as the lake infilled. In combination, the proxies support the idea that the climate became wetter ca. 1700-1200cal. yr. BP in the Faroe Islands, but that the response of the landscape was partly dependent on the increased development of peat initiated during the onset of Neoglacial cooling after 5000cal. years BP.https://doi.org/10.1016/j.palaeo.2008.03.003Hydroecological responses of the Athabasca Delta, Canada, to changes in river flow and climate during the 20th century
We employ water-isotope tracers and multi-proxy paleolimnological records to characterize contemporary controls on water balances of floodplain lakes in the Athabasca Delta, Canada, within the context of its hydroecological evolution over the 20th century. The insight gained from these approaches is necessary to gauge the hydroecological resiliency of the Athabasca Delta to past and future changes in Athabasca River flow regime. Results obtained from three lakes located in different regions of the Athabasca Delta indicate that hydroecological conditions were strongly affected by an engineered meander cut-off on the Athabasca River in 1972, intended to maintain flow in the river main stem, and a natural bifurcation of one of the major distributaries (Embarras River) in 1982, in response to progressive overextension of the delta to the east. Climate warming and naturally declining river discharge have also contributed to directional change. Recent drying trends reconstructed from sediment cores at two of the three lakes are likely representative of rapidly evolving hydroecological conditions in the south-eastern sector, based on mapping of a recent high-magnitude ice-jam flood that failed to recharge this portion of the delta, while wetting in the region of the third lake due to increased frequency of river flooding reflects increasing diversion of Athabasca River flow northward. Our findings highlight the hydroecological sensitivity of the Athabasca Delta to changes in the magnitude and timing of discharge in the Athabasca River and heighten the need for informed management strategies to safeguard the integrity of this unique wetland ecosystem.https://doi.org/10.1002/eco.13Autotrophic response to lake age, conductivity and temperature in two West Greenland lakes
Predicted changes in future climate necessitate a better understanding of climate impacts on lake biota, and the role of within-lake processes in modifying biotic response. Therefore we examined two climate-related variables (lake-water conductivity and GRIP temperatures) and lake ontogeny (lake age), to determine their influence on lake autotrophic communities in two neighbouring closed-basin lakes from West Greenland spanning the past 8,000 years. Using sedimentary pigments as proxies for lake autotrophic communities, we used synchrony and variance partitioning analyses (VPA) to test three specific hypotheses (a) that lake primary production would increase with lake age, (b) that climate would be the dominant process controlling autotrophic communities in these pristine lakes and (c) that the response of autotrophs to conductivity and temperature would vary depending upon the age of the lake. The results supported our first hypothesis, showing that lakes changed significantly with age, exhibiting an increase and decline in production in the first millennium of their existence, followed by a steady increase in production and increasingly frequent abrupt switches between mixed and meromictic states. The highly synchronous detrended response (r = 0.769) of lake autotrophs in the two study lakes, supported our hypothesis that climate was the dominant factor controlling lake autotrophs. However, VPA revealed that our climate-related variables (temperature and conductivity) explained only small amounts of variance alone (50% variance explained) and with each other (>28% variance explained), such that autotrophic response changed as lakes aged. In spite of this, lakes sometimes responded independently, as a result of differences in the relative proportion of benthic to pelagic production and because of differences in lake morphometry. Together these results show that long-term control of lake autotrophs by climate and lake age is modified on shorter timescales by non-linear responses related to within-lake processes, and by the interaction of different climate variables with each other and with lake age.https://doi.org/10.1007/s10933-007-9105-2Spatial variability of climate and land-use effects on lakes of the northern Great Plains
Evaluation of the effects of climate change and human activities on lakes requires improved understanding of how stressors interact and the degree to which individual sentinel lakes represent broad spatial patterns of ecosystem response to disturbance. We surveyed modern water chemistry (major ions, conductivity, salinity, lake volume) and sediments (algal pigments, stable isotopes) in 21 lakes that surround Humboldt Lake, Saskatchewan, site of a 2,000-yr climate reconstruction, to quantify spatial synchrony (S, the mean among-lake correlation coefficient) of prairie lake response to climate variability, land use, and their interactions. Whole-lake mass balances of total dissolved substances constructed at each site revealed that evaporation of water controlled seasonal changes in salt content only in years with dry summers (2003), leading to widespread spatial coherence of ecosystems (S = 0.78). In contrast, variations in hydrologic inputs (precipitation, groundwater) and solute fluxes regulated salt balances of lakes during years with wet summers (2004, 2005) and substantially reduced lake synchrony (S = 0.13-0.58). Furthermore, >25% of sites exhibited increased nitrogen influx (as δ15N) and cyanobacterial production (as fossil pigments) between ca. 1920 and 2003, with particularly strong effects of land use recorded for northeastern sites, where evaporative forcing was greatest. Finally, principal component and canonical ordinations with redundancy analysis both explained ∼50% of the variance in lake sensitivity to climate and land use and revealed that the effects of climate and land use interacted strongly, but that the unique effects of each factor remained identifiable in modern lake surveys.https://doi.org/10.4319/lo.2008.53.2.0728Climate versus in-lake processes as controls on the development of community structure in a low-arctic lake (South-West Greenland)
https://doi.org/10.1007/s10021-007-9123-yThe dominant processes determining biological structure in lakes at millennial timescales are complex. In this study, we used a multi-proxy approach to determine the relative importance of in-lake versus indirect processes on the Holocene development of an oligotrophic lake in SW Greenland (66.99°N, 50.97°W). A 14C and 210Pb-dated sediment core covering approximately 8500 years BP was analyzed for organic-inorganic carbon content, pigments, diatoms, chironomids, cladocerans, and stable isotopes (δ13C, δ18O). Relationships among the different proxies and a number of independent controlling variables (Holocene temperature, an isotope-inferred cooling period, and immigration of Betula nana into the catchment) were explored using redundancy analysis (RDA) independent of time. The main ecological trajectories in the lake biota were captured by ordination first axis sample scores (18-32% variance explained). The importance of the arrival of Betula (ca. 6500 years BP) into the catchment was indicated by a series of partial-constrained ordinations, uniquely explaining 12-17% of the variance in chironomids and up to 9% in pigments. Climate influences on lake biota were strongest during a short-lived cooling period (identified by altered stable isotopes) early in the development of the lake when all proxies changed rapidly, although only chironomids had a unique component (8% in a partial-RDA) explained by the cooling event. Holocene climate explained less variance than either catchment changes or biotic relationships. The sediment record at this site indicates the importance of catchment factors for lake development, the complexity of community trends even in relatively simple systems (invertebrates are the top predators in the lake) and the challenges of deriving palaeoclimate inferences from sediment records in low-Arctic freshwater lakes.
Classification of hydrological regimes of northern floodplain basins (Peace-Athabasca Delta, Canada) from analysis of stable isotopes (δ18O, δ2H) and water chemistry
https://doi.org/10.1002/hyp.6229We used stable isotopes (δ18O and δ2H) and water chemistry to characterize the water balance and hydrolimnological relationships of 57 shallow aquatic basins in the Peace-Athabasca Delta (PAD), northern Alberta, Canada, based on sampling at the end of the 2000 thaw season. Evaporation-to-inflow ratios (E/I) were estimated using an isotope mass-balance model tailored to accommodate basin-specific input water compositions, which provided an effective, first-order, quantitative framework for identifying water balances and associated limnological characteristics spanning three main, previously identified drainage types. Open-drainage basins (E/I < 0.4; n = 5), characterized by low alkalinity, low concentrations of nitrogen, dissolved organic carbon (DOC) and ions, and high minerogenic turbidity, include large, shallow basins that dominate the interior of the PAD and experience frequent or continuous river channel connection. Closed-drainage basins (E/I ≥ 1.0; n = 16), in contrast, possess high alkalinity and high concentrations of nitrogen, DOC, and ions, and low minerogenic turbidity, and are located primarily in the relict and infrequently flooded landscape of the northern Peace sector of the delta. Several basins fall into the restricted-drainage category (0-4 # E/I <1.0; n = 26) with intermediate water chemistries and are predominant in the southern Athabasca sector, which is subject to active fluviodeltaic processes, including intermittent flooding from riverbank overflow. Integration of isotopic and limnological data also revealed evidence for a new fourth drainage type, mainly located near the large open-drainage lakes that occupy the central portion of the delta but within the Athabasca sector (n = 10). These basins were very shallow (<50 cm deep) at the time of sampling and isotopically depleted, corresponding to E/I characteristic of restricted- and open-drainage conditions. However, they are limnologically similar to closed-drainage basins except for higher conductivity and higher concentrations of Ca2+ and Na+, and lower concentrations of SiO2 and chlorophyll c. These distinct features are due to the overriding influence of recent summer rainfall on the basin water balance and chemistry. The close relationships evident between water balances and limnological conditions suggest that past and future changes in hydrology are likely to be coupled with marked alterations in water chemistry and, hence, the ecology of aquatic environments in the PAD.
PALEOLIMNOLOGY | Pigment Studies
Pigments of photosynthetic organisms including chlorophylls, carotenoids, photoprotective compounds and their derivatives produced by algas, phototrophic bacteria, and aquatic plants often preserve well in the sediments of aquatic environments. In sediment cores, they can yield an estimate past primary production in aquatic systems, and provide information about past communities of algas or photosynthetic bacteria. This chapter describes the biochemical nature of pigments including their preservation in sedimentary environments, techniques for pigment analysis and a range of paleolimnological applications, including the determination of eutrophication, changes in aquatic food-web structure, lake acidification, and climate change.https://doi.org/10.1016/B0-44-452747-8/00247-7Controls of algal abundance and community composition during ecosystem state change
https://doi.org/10.1890/04-1029Shallow lake ecosystems can shift between clear-water, macrophyte-rich conditions and turbid states with abundant phytoplankton. However, little is known about the controls of algal community composition and primary production before, during, and after ecosystem state change, because long time series that monitor biological change through the transition are scarce. Using proxy data sets derived from sediment cores from two shallow hypertrophic lakes in Denmark, variance-partitioning analysis (VPA) was used to determine the relative importance of changes in total phosphorus (diatom inferred), planktivorous fish density (zooplankton inferred), and submerged macrophyte communities (as macrofossil abundance) as determinants of algal abundance and community composition (as sedimentary pigments) over ecosystem state transitions since 1750 (CE) for Lake Lading and 1900 for Lake Søbygaard. Past variation in densities of planktivorous fish explained 12.3% and 18.2% of historical algal community change in lakes Lading and Søbygaard, respectively, while a further 22.3% and 6% of algal variability was explained by variation in macrophyte abundance. Total phosphorus (TP) alone explained nonsignificant amounts of variance (1.5%, 3.6%) but had a significant effect in combination with macrophytes and fish (27%, 13.4%). State transitions occurred ca. 1940 but were preceded by increases in benthic diatoms and macrophytes, suggesting that transitions were gradual rather than instantaneous. In contrast, green, colonial blue-green, and cryptophyte algae were abundant only during turbid states after ca. 1960 and were correlated to changes in planktivorous fish or fish-TP interactions. Contrary to expectations, the shift from predominantly benthic to pelagic algal production represented only a change in habitat and did not result in an increase in total abundance of primary producers.
A whole-lake experiment to determine the effects of winter droughts on shallow lakes
Lake-level fluctuations are common in the North American Great Plains region, where large-scale climate systems (El Niño, the Pacific Decadal Oscillation) and periodic droughts cause substantial hydrologic variability in both summer and winter. To date, most such research has focused on the effects of summer droughts on prairie lake ecosystems; therefore, we studied the impact of water-level decline during winter on ecosystem structure and function. Specifically, we hypothesized that lower lake levels during winter would increase anoxia, freezing and scouring of benthos, fish kills, herbivory by zooplankton, and nutrient release from sediments. In addition, we tested the hypothesis that winter droughts may initiate a switch between alternative stable states (turbid, clear). Physical, chemical, and biological variables were monitored from 1996 to 2001 in both Wascana Lake, which experienced a 50% decline in lake level, and Buffalo Pound Lake, where water levels were constant. A combination of before-after-control-impact (BACI) and multivariate analyses showed that drawdown resulted in elevated NH4-N concentrations following reinundation; otherwise there were few detectable effects on lake water chemistry (PO4-P, NO3-N, total dissolved nitrogen, total dissolved carbon) or pelagic food web structure (phytoplankton, zooplankton), and the experimental lake remained in a macrophyte-rich state. There was, however, a 2.5-fold increase in macrophyte abundance and a shift from a community dominated by Ceratophyllum demersum before drawdown to one composed of Potamogeton pectinatus after manipulation. Overall, the lack of substantial dewatering effects suggests that lakes of the northern Great Plains may be resilient to severe winter conditions, possibly because of the recruitment of fish from regional metapopulations during summer. Further, our results indicate that lower water levels during winter likely promote the buffer mechanisms that reinforce a macrophyte-rich, clear-water state in shallow prairie lakes.https://doi.org/10.1007/s10021-003-0152-xMillennial-scale relationships of diatom species richness and production in two prairie lakes
https://doi.org/10.4319/lo.2004.49.4_part_2.1290Insight into the causes and consequences of changes in aquatic biodiversity requires an improved understanding of the nature of the relationships between species richness and ecosystem function over a much longer temporal perspective than we currently possess. We used high-resolution paleoecological records from two prairie lakes to show that diatom species richness (as fossil frustules) was negatively correlated (r2 = 0.09-0.24, p < 0.001) with diatom production (as fossil pigments) during the past 2,000 yr. By comparing analyses from intervals of fresh and saline waters, we demonstrate that these significant richness-production relationships arose during freshwater periods (r 2 = 0.13-0.45, p < 0.001) and could be eliminated (r2 < 0.02, p > 0.1) by abiotic disturbances such as droughts. Procrustes analyses of the concordance of species change within freshwater communities and the change in richness-production relationships through time revealed that shifts in diatom community composition could have a large influence in determining the negative relationship between richness and production. Finally, significant correlations (r2 = 0.09-0.24, p < 0.0001) between past diatom species richness and ratios of stable isotopes (primarily δ15N) suggested that C and N biogeochemical cycles are also linked to changes in algal biodiversity. Taken together, these analyses suggest that the ongoing disruption of climate and biogeochemical systems by humans may obscure the relationship between aquatic biodiversity and ecosystem function in the future.
Holocene records of effective precipitation in West Greenland
Holocene changes in effective precipitation (precipitation-evaporation; P-E) were reconstructed for the Søndre Strømfjord region of southwest Greenland using the sediment records of two neighbouring closed-basin 'saline' lakes. Past lakewater conductivities (a proxy for P-E balance) were estimated using a diatom-inferred conductivity model. Broadly similar changes in both lake records corroborate the use of DI conductivity as a regional climate proxy. An increasing DI conductivity trend through the Holocene indicated net negative or balanced P-E ratios in this region, even during most of the Neoglacial period (post-4000 yr BP). This contrasts with other regions of West Greenland, where conditions became more humid in the Neoglacial period. The reconstructions presented here therefore suggest a high degree of spatial variability in the P-E balance, likely caused by region-specific orographic features. While interpretation of the mid-Holocene P-E balance was limited by dissolution of the diatom record, other sediment indicators suggest a period of extremely negative effective precipitation (∼7000-5600 yr BP), followed by a period of positive effective precipitation lasting until 4700 yr BP. This contrasts markedly with the later Holocene, after c. 4000 yr BP, when high-frequency oscillations in DI conductivity probably reflect short-term climatic variations (amplified by in-lake processes connected with meromixis) but no long-term trends in the P-E balance.https://doi.org/10.1191/0959683603hl610rpDevelopment and evaluation of a diatom-conductivity model from lakes in West Greenland
1. The area around Kangerlussuaq (Søndre Strømfjord; West Greenland, 67°N 51°W) contains thousands of lakes ranging from coastal, dilute (conductivity < 30 μS cm-1) oligotrophic systems to subsaline (∼4000 μS cm-1), closed basin lakes close to the ice sheet margin. In closed basins, salinity (or conductivity) is often a proxy for effective moisture, and thus palaeorecords of lake conductivity can provide valuable palaeoclimatic data. Little or nothing is known about the recent history of these lakes and hence it is difficult to evaluate how they will respond to effects of future changes. 2. Over 100 lakes have been sampled (1996-2000) between the ice sheet and the outer coast for a variety of water chemical and limnological variables. Surface sediments were taken from a subset of 40 lakes and analysed for diatoms. Diatom responses to 28 environmental variables were analysed by multivariate ordination techniques and indicate that the main gradient is highly correlated to conductivity (explaining ∼12% of species variance). Despite the relatively short gradient (24-4072 μS cm1), diatom assemblages exhibit a clear response to conductivity. The most saline lakes do not contain a true saline flora. 3. We developed a range of weighted averaging (WA) and weighted-averaging partial least squares (WA-PLS) models from this training data set and found two component WA-PLS models performed best. The effects of data transformation and omission of dissolution susceptible species (Diatoma spp.) on model performance were also examined. The error statistics for the preferred WA-PLS (2) model (r2jack = 0.88, root mean square error of prediction (RMSEP) = 0.217 log μS cm-1) compare well with other published models. 4. A 210Pb-dated short core from a meromictic, subsaline lake (Braya Sø; location 67°N, 51°W, max. depth 23 m; conductivity 2600 μS cm-1) was analysed for diatoms. Diatom preservation is poor and some taxa (e.g. Diatoma spp.) are badly corroded. Lake water conductivity was reconstructed using WA-PLS models. Diatom-inferred conductivity ranges from 1800 to 4400 μS cm-1 over the last 600 years (extrapolated 210Pb chronology). 5. The Kangerlussuaq area of West Greenland is an important area for palaeoclimatic research, located as it is between the Greenland ice sheet (ice core records) and the Davis Straits to the west. The development of a statistically robust transfer function for diatoms and conductivity will enable the reconstruction of conductivity from the many subsaline lakes around the head of the fjord and, hence, regional estimates of changing palaeo precipitation.https://doi.org/10.1046/j.1365-2427.2002.00832.xSediment evidence of early eutrophication and heavy metal pollution of Lake Mälaren, central Sweden
Lake Mälaren is the water supply and recreation area for more than 1 million people in central Sweden and subject to considerable environmental concern. To establish background data for assessments of contemporary levels of trophy and heavy metal pollution, sediment cores from the lake were analyzed. Diatom-inferred lake-water phosphorus concentrations suggest that pre-20th century nutrient levels in Södra Björkfjärden, a basin in the eastern part of Mälaren, were higher (c. 10-20 μg TP L-1) than previously assumed (c. 6 μg TP L-1). Stable lead isotope and lead concentration analyses from 3 basins (S, Björkfjärden, Gisselfjärden and Asköfjärden) show that the lake was polluted in the 19th century and earlier from extensive metal production and processing in the catchment, particularly in the Bergslagen region. The lake has experienced a substantial improvement of the lead pollution situation in the 20th century following closure of the mining and metal industry. The lead pollution from the old mining industry was large compared to late-20th century pollution from car emissions, burning of fossil fuels and modern industries.https://doi.org/10.1579/0044-7447-30.8.496Coring of laminated lake sediments for pigment and mineral magnetic analyses, Søndre Strømfjord, southern West Greenland
Lake sediments are natural archives that provide longterm records of past changes in climate-catchment processes as well as changes in biological communities in lakes. The thousands of lakes in West Greenland are poorly studied but ideally suited to palaeoclimate and lake-catchment interaction projects because of the minimal level of anthropogenic impact and their tight links to regional climate. In 1999, therefore, activity in the Kangerlussuaq / Søndre Strømfjord area (Fig. 1) continued with both a summer limnological sampling programme (Brodersen & Anderson 2000, this volume) and a late winter to early spring expedition to retrieve sediment cores from lakes at the head of the fjord. In our previous coring trips along Søndre Strømfjord we identified a number of lakes with laminated sediments (Anderson & Bennike 1997; Anderson et al. 1999). Initially, it was thought that these finely resolved laminated sediments were restricted to lakes with permanently stratified water columns (meromictic lakes). However, it appears that lakes with laminated sediments are more widespread in West Greenland that hitherto thought.https://doi.org/10.34194/ggub.v186.5220Ancient blue-green blooms
https://doi.org/10.4319/lo.1999.44.2.0436Recent decades have seen a large increase in surface scums (blooms) of cyanophytes (blue-green algae and blue-green bacteria) in inland waters. These are potentially toxic to mammals, including humans, and have caused considerable public concern in Europe, Australasia, and North America. They are often associated with eutrophication, and much has been invested in their control. Not all blooms, however, are necessarily the results of human interference with lakes. Scattered paleolimnological evidence indicates that some blooms may be associated with pristine conditions, though this message has largely been ignored. Evidence is given here of a long history of blooms in Whitemere, U.K., from extraction and identification of specific carotenoids from dated sediment cores. Whitemere is representative of a large group of lakes in the West Midlands of the U.K. and is likely to be representative also of those similar postglacial kettle hole lakes in North America and Eurasia, which are groundwater fed with long retention times and thermally stratified. Blue-green blooms may thus be a normal feature of such lakes and not necessarily a pathology to be controlled.
Physiological tests and bioassays: Aids or superfluities to the diagnosis of phytoplankton nutrient limitation? A comparative study in the Broads and the Meres of England
Relationships between phytoplankton growth and supplies of phosphorus and nitrogen have been variously investigated in three lakes in two contrasted English lake areas using physiological nutrient indices (alkaline phosphatase activity, phosphorus debt, surplus phosphorus, phosphorus uptake kinetics, dark ammonium uptake, ammonium uptake kinetics, stimulation of 14C uptake, and batch bioassay) and by inspection of water chemistry. None of the lakes was significantly affected by wastewater effluent but all lay in agricultural catchments. Physiological indices and water chemistry suggested limitation of phytoplankton biomass mostly by phosphorus, and to some extent by nitrogen in summer in North Ormesby Broad, by nitrogen in summer and to some extent by phosphorus in Lily Broad and by nitrogen in White Mere. Indices did not always accord with one another, though were not severely misleading. They added little to conclusions that could be drawn from water chemistry alone. Reasons for the differential relative importance of nitrogen and phosphorus in the two lake areas are discussed. The paradigm of phosphorus limitation, though unchallenged in upland waters and those on poorly weathered rocks, may be less relevant in lowland lakes.https://doi.org/10.1017/S0967026299002218Vertically-challenged limnology; contrasts between deep and shallow lakes
Previous work on a set of small lakes, of varying depth, the meres of North West England, has shown that nitrogen availability controls the summer phytoplankton populations in the deeper ones (max depth > 3 m) and zooplankton grazing in shallow ones. The mere have generally high total phosphorus concentrations and this may be a natural phenomenon dependent on the local geochemistry. Some antrophogenic eutrophication has occurred, however, and from a chain of three meres, sewage effluent was diverted in 1991. The upper lake, Mere Mere, lying above the point of discharge, has not changed in any systematic way since effluent diversion. The middle lake, the very shallow Little Mere, has changed markedly in water chemistry but not fundamentally in ecosystem structure. It was and remains a clear-water, macrophyte dominated lake. The third lake, the deep Rostherne Mere, has shown no response in chlorophyll α concentrations in four years since effluent diversion though in the past two years there appears to be a downward trend in total phosphorus. The reasons for this are explored in terms of our understanding of lake eutrophication. Comparisons are made with White Mere, a deep groundwater fed lake with a long retention time and a very high total phosphorus concentration. The deep meres may add a new dimension to our understanding of natural and anthropogenic eutrophication.https://doi.org/10.1007/978-94-011-5648-6_27Determination of phytoplankton crops by top‐down and bottom‐up mechanisms in a group of English lakes, the West Midland meres
Twenty-four lake basins formed in glacial drift ranged in maximum depth from 1.5 to 31 m and were characterized by generally low inorganic N concentrations and very high total P concentrations. The high P values were in some cases related to farm effluent pollution but in many others, despite annual mean values up to 1.46 mg liter−1, appear natural and due to the local mineralogy.https://doi.org/10.4319/lo.1994.39.5.1020
For the entire group, there were no significant relationships between mean growth season chlorophyll a concentration and any measured chemical, morphometric, or zooplankton variable. When the group was divided on the basis of maximum depth and presence or absence of thermal stratification in summer into a shallow group (<3 m) and a deep group (>3 m), strong inverse correlation was obtained between chlorophyll a and cladoceran abundance in the shallow group and strong direct correlation with inorganic N in the deep group. Grazer control in the shallow group is probably linked with the dominance of submerged macrophytes and the refuges they provide for grazers. Some of the controversy presently surrounding the relative importance of top-down vs. bottom-up control of phytoplankton populations may thus be removed if consideration is given to the morphometry of the lakes.
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