Suzanne McGowan

Hydrological disconnection from main channels (either via natural siltation or due to construction of hydrological infrastructures) is modifying biogeochemical cycling in river-floodplain systems. Knowledge on how this process influences phytoplankton composition and harmful algal blooms (HABs) in floodplain lakes is quite scant due to the lack of long-term water quality monitoring and the concurrent influence of multiple drivers of change. Here, chlorophyll and carotenoid pigment biomarkers from dated sediment cores were analyzed from Dongting Lake (China's second largest freshwater lake) and one of its satellite lakes (Donghu) in the Yangtze floodplain, to evaluate the long-term influence of hydrological isolation on algal community composition and HABs. The results showed that pigment concentrations and the ratio of canthaxanthin/diatoxanthin (which reflects the relative abundance of cyanobacteria to diatoms) increased after the 1910s in Donghu Lake, when it was separated from Dongting Lake due to siltation. In contrast, significant increases in pigments started from the 1980s in Dongting Lake. Variance partitioning analysis revealed that the combined influence of hydrology, temperature and anthropogenic pollutants explained the largest proportion of variance (33.4%) in the pigment assemblages in Donghu Lake, followed by the joint effects of anthropogeny pollutants and hydrology (23.6%) and the sole effects of anthropogenic pollutants (14.9%) and hydrology (11.2%). In Dongting Lake, anthropogenic pollutants explained 24.5% of the variance in pigment assemblages solely, followed by the additive effects of anthropogenic pollutants and temperature (17.8%). These long-term analyses therefore demonstrate that, in combination with anthropogenic pollutants and warming, hydrological isolation from the main channel may stimulate algal production and the prevalence of cyanobacteria, whereas free hydrological connection with the Yangtze main channel seems to alleviate such HABs in these Yangtze floodplain lakes.

Tropical floodplain lakes are increasingly impacted by human activities, yet their pathways of spatial and temporal degradation, particularly under varying hydrological connectivity regimes and climate change, remain poorly understood. This study examines surface-sediment samples and ²¹⁰Pb-dated sediment cores from six floodplain lakes, representing a gradient in hydrological connectivity in the lower Magdalena River Basin, Colombia. We analysed temporal and spatial variations in several sediment biogeochemical indicators: the concentration and flux of nutrients, heavy metals, and organic matter (OM), and redox conditions, flooding and erosion. Multiple factor analysis (MFA) of surface-sediments identified redox conditions, OM, flooding, heavy metals and lake connectivity as the main contributors to spatial variability within- and between-lakes sediments, accounting for 48 % of the total variation. Additionally, no clear distinction was found between littoral and open-water sediment characteristics. Isolated lakes sediments exhibited reductive conditions rich in OM and nutrients, whereas connected lakes sediments showed greater heavy metal enrichment and higher concentrations of coarse river-fed material. Generalised additive models identified significant changes in the biogeochemical indicators since the late 1800s, that accelerated post-1980s. Shifts in OM, erosion, flooding, redox conditions, land-cover change, heavy metals and climate were identified by MFA as the main drivers of change, explaining 60 %-71 % of the variation in the connected lakes and 53 %-72 % in the isolated lakes. Post-1980s, connected lakes transitioned from conditions of higher accumulation of OM and little erosion to higher accumulation of heavy metals and river-fed material. Conversely, isolated lakes, shifted from detrital-heavy metal-rich sediments to OM-, and nutrient-rich, reductive sediments. Sedimentation rates also surged post-1980s, particularly in highly connected lakes, from 0.14 ± 0.07 g cm² yr⁻¹ to 0.5 ± 0.5 g cm² yr⁻¹, with elevated fluxes of metals, OM and nutrients. These changes in sediment biogeochemistry align with deforestation, river regulation and prolonged dry periods, highlighting the complexities behind establishing reliable reference conditions for pollution assessments in large, human-impacted tropical river systems.

Water bodies located in floodplains and tropical forests are known to be important carbon stores, but many are subjected to intensive pressures from damming, land use and climate changes. Sedimentary records preserve long-term archives for understanding how such changes affect the quantity and quality of carbon stores. We analysed sediment cores from seven sites across a flood-pulse multi-basin wetland, Tasik Chini in Peninsular Malaysia (for percentage LOI₅₅₀, sediment density and spheroidal carbonaceous particles), and conducted more analyses on three ²¹⁰Pb-dated cores (X-ray fluorescence of elements, grain size analysis, carbon isotopes, C/N ratios, carotenoid pigments) to gain an understanding of the drivers of organic carbon accumulation rates (OCARs) since 1860 ce. The median OCAR of 85 g m⁻² a⁻¹ for the basin since 1945 ce was higher than in other floodplain and temperate lakes and in line with other tropical forest lakes. However, we found evidence for different mechanisms of OC deposition across the basin. In ‘autochthonous mode’, the site with minimal local land disturbance had lowest OCARs and OC was derived mainly from autochthonous production, which rose slightly around 1940 ce when regional land disturbance increased nutrient influx to the basin. The site with the most long-term and intensive land disturbance through forest removal (1940s) and then conversion to rubber and oil palm farming (1980s) functioned mainly in ‘allochthonous mode’; that is, increases in OCARs after 1940 ce were driven by deposition of soil-derived OC. The highest OCARs were in the basin that was converted to oil palm after the 1980s and had increased iron mining activity in the 2000s; because this site was located distal from the flood pulse and became increasingly hydrologically disconnected after a low rainfall period in the 1970s, the lake responded strongly in ‘autochthonous mode’, through encroachment of fringing swamp, the spread of benthic algae and macrophytes, and efficient sediment retention. Weir installation in 1995 ce raised water levels and increased lentic conditions, promoting autochthonous OC production and sedimentation across all basins. The long-term fate of this more recently deposited OC remains uncertain because it is more labile. Overall Tasik Chini has responded strongly to land use changes since at least the 1940s, earlier than anticipated in this region of Southeast Asia, and the sedimentary proxies indicate large changes in the ecosystem function and capacity for C storage over the past ca. 80 years. Most of these shifts have increased OC accumulation by strengthening autochthonous production or allochthonous OC fluxes, but the implications for other aspects of the C cycle, including catchment soil C loss and greenhouse gas production, need to be accounted for when evaluating the overall impacts of land and hydrological disruption.

The Red River originating from Yunnan province, China is the second largest river in Vietnam in terms of length and discharge. Combination of water chemistry monitoring data of 4 years (2018–2022) from different sub-basins of the Red River (the Da, Lo, Thao, Tra Ly, and Day) with historical datasets indicates a decline in pH from 8.1 in 2000 to 7.7 in 2021, greater CO₂ concentrations and a shift from waters naturally dominated by carbonate weathering to waters dominated by evaporite weathering. Such changes were most apparent in the delta area where heavy human activities have increased influxes of most dissolved chemicals, except SiO₂. Evaporite weathering is particularly enhanced by mining and deforestation occurring in upstream regions of both China and Vietnam. Pyrite oxidation, alongside silicate weathering, is enhanced along the Red River Fault Zone but reduced in tributaries with a higher proportion of hydropower reservoirs. Longer water residence times in these large reservoirs (total volume > 2.7x10¹⁰ m³) located in the Da and Lo sub-basins have also increased primary productivity, leading to higher evasion/uptake of CO₂ and SiO₂, lower total dissolved solids (TDS), and higher pH. The total physical and chemical denudation rates of upstream mountain tributaries ranged between 0.107 ± 0.108 and 0.139 ± 0.137 mm yr⁻¹, mainly due to reservoir implementation and instream aquatic biogeochemistry changes. Our findings demonstrate that anthropogenic activities are profound factors impacting the water chemistry of the Red River system.

Dam construction for social-economic benefits has raised substantial biological and ecological concerns. However, contrasting findings have been reported regarding the role of hydrological modification in floodplain lake ecosystems. Here, we evaluated the influence of hydrology and dam construction on the eutrophication of floodplain lakes over the last 200 years by studying Zn/Al (an indicator of industrial activities), nutrient influxes (total phosphorus, TP), cyanobacteria production (canthaxanthin) and aquatic invertebrate dynamics (chironomids) in ²¹⁰Pb dated sediment cores from two Ramsar Wetlands of International Importance in the middle Yangtze floodplain, one dammed (Wanghu) and the other freely connected (Poyang) with the Yangtze River. The results show that Wanghu Lake transitioned to a macrophyte-dominated clear water state (as indicated by the increases in pigment derived ultraviolet radiation index and dominance of macrophyte-related chironomid taxa) after local dam construction when anthropogenic nutrient loadings were relatively low. With increases in nutrient loadings, phytoplankton increased in both lakes, but water clarity declined and macrophyte-related chironomids decreased only in the lake which was locally dammed. Our study reveals that local damming facilitates the response of floodplain lake ecosystems to eutrophication and decouples it from the effects of hydrological variability. This study highlights the potential influence of hydrology and damming on the eutrophication of floodplain lakes by influencing water clarity and macrophyte coverage, implying that evaluating the role of local dam construction on ecosystem states should be based on knowledge of nutrient conditions in floodplain lakes.

Endorheic lakes, lacking river outflows, are highly sensitive to environmental changes and human interventions. Central Asia (CA) has over 6000 lakes that have experienced substantial water level variability in the past century, yet causes of recent changes in many lakes remain unexplored. Modelling hydrological processes for CA lakes poses challenges in separating climatic change impacts from human management impacts due to limited data and long-term variability in hydrological regimes. This study developed a spatially lumped empirical model to investigate the effects of climate change and human water abstraction, using Shortandy Lake in Burabay National Nature Park (BNNP) as a case study. Modelling results show a significant water volume decline from 231.7x10⁶m³ in 1986 to 172.5x10⁶m³ in 2016, primarily driven by anthropogenic water abstraction, accounting for 92% of the total volume deficit. The highest rates of water abstraction (greater than 25% of annual outflow) occurred from 1989 to 1993, coinciding with the driest period. Since 2013, the water volume has increased due to increased precipitation and, more importantly, reduced water abstraction. Despite limited observational data with which to calibrate the model, it performs well. Our analysis underscores the challenges in modelling lakes in data-sparse regions such as CA, and highlights the importance and benefits of developing lake water balance models for the region.

In many places around the world, anthropogenic activities have resulted in nitrate (NO₃⁻) pollution and changes in the metabolic state of aquatic ecosystems. Here we combined stable isotope and physico-chemical monitoring to assess the sources of NO₃⁻ and the overall metabolic state within the Red River delta, Vietnam. River water stable isotope compositions (δ¹⁸O-H₂O) ranged between −11.2 and −2.7 ‰, δ¹⁸O-NO₃⁻ between −7.1 and + 29.7 ‰ and δ¹⁵N-NO₃⁻ between −3.9 and + 14.0 ‰. We identified the dominant NO₃⁻ sources as: 1) soil leachate, 2) domestic waste flushed from urban areas, and 3) NH₄⁺ fertilizers washed from paddy fields. The relative impact of each source depends on geographical location within the delta and the time of year, due to dilution and concentration effects during wet and dry seasons. The primary NO₃⁻source upstream is natural soil leachates, predominantly from tributaries connected to the Red River's main stream. Within the middle-lower section of Red River delta, urban pollution from manure and septic waste reaches as high as 50 % of the total NO₃⁻ load during dry season. NO₃⁻ leached from fertilizers is also high at sites in the middle of the delta, related to agricultural activities. Dissolved oxygen isotope (δ¹⁸O-O₂) values calculated from δ¹⁸O-H₂O and δ¹⁸O-NO₃⁻ values indicate that the aquatic metabolism is net autotrophic (oxygen from primary production exceeds consumption by respiration), but high inputs of biodegradable organic matter from untreated domestic waste and high rates of sediment oxygen demand (SOD) and chemical oxygen demand (COD) have resulted in the whole river system becoming undersaturated in oxygen. High NO₃⁻ loads and low DO saturation are of critical concern and require mitigation practices to improve water quality for millions of people.

This paper examines a series of connected and isolated lakes in the UK as a model system with historic episodes of heavy metal contamination. A 9-year hydrometeorological dataset for the sites was identified to analyse the legacy of heavy metal concentrations within the selected lakes based on physico-chemical and hydrometeorological parameters, and a comparison of the complementary methods of multiple regression, time series analysis, and artificial neural network (ANN). The results highlight the importance of the quality of historic datasets without which analyses such as those presented in this research paper cannot be undertaken. The results also indicate that the ANNs developed were more realistic than the other methodologies (regression and time series analysis) considered. The ANNs provided a higher correlation coefficient and a lower mean squared error when compared to the regression models. However, quality assurance and pre-processing of the data were challenging and were addressed by transforming the relevant dataset and interpolating the missing values. The selection and application of the most appropriate temporal modelling technique, which relies on the quality of available dataset, is crucial for the management of legacy contaminated sites to guide successful mitigation measures to avoid significant environmental and human health implications.

Water quality of floodplain lakes in the Yangtze region which supports ca. 450 million people is being severely compromised by nutrient pollution, climate change and dam installation resulting from intensive socio-economic development. However, due to a lack of long-term monitoring data, the onset and causes of ecosystem degradation are unclear. Here, we used chlorophyll and carotenoid pigments in dated sediment cores from six lakes spanning the region to reconstruct changes in algae and cyanobacterial HAB (harmful algal bloom) taxa alongside sedimentary nutrient flux measurements and historical archives. Sedimentary N fluxes are linked to changes in agriculture, while urbanization has had greater influences on P fluxes. Over the last 70 years algal and N₂-fixing HAB pigments increased markedly in lakes (Luhu, Wanghu) that are strongly influenced by urbanization/industrialization. Algal assemblages in two other lakes (Futou, Honghu) changed gradually and responded primarily to agriculture and associated N fluxes; diazotrophic HAB pigments were absent and the lakes retained macrophyte cover. Local dam installation had no discernible effect on pigment assemblages in three of the four lakes in the past 70 years, but in the two hydrologically-open lakes (Poyang, Dongting), increasing algal production was significantly related to the upstream installation of the Three Gorges Dam (TGD) and to urban/industrial and agricultural stressors. Temperature only influenced phototrophs in the most degraded lakes (Luhu, Wanghu). This spatial and temporal overview identifies that nutrient pollution is the primary regional driver of lake phototrophs, but that diazotrophic HABs are stimulated by P-enriched urban wastewater pollution, and agriculturally-derived N pollution favors non-N₂-fixing cyanobacteria. Despite negative effects of the Three Gorges project, free connection to the river appears to help mitigate excess HABs in freely connected lakes. Management thus needs to be tailored appropriately to specific lake conditions and palaeolimnology can be valuable in identifying appropriate strategies.

The onset of agriculture improved the capacity of ecosystems to produce food, but inadvertently altered other vital ecosystem functions. Plant traits play a central role in determining ecosystem properties, therefore we investigated how the onset of agriculture in Europe changed plant trait composition using 78 pollen records. Using a novel Bayesian approach for reconstructing plant trait composition from pollen records, we provide a robust method that can account for trait variability within pollen types. We estimate an overall four-fold decrease in plant size through agriculture and associated decreases in leaf and seed size. We show an increase in niche space towards the resource-acquisitive end of the leaf economic spectrum. Decreases in leaf phosphorus might have been caused by nutrient depletion through grazing and burning. Our results show that agriculture, from its start, has likely been gradually impacting biogeochemical cycles through altered vegetation composition.

Understanding 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.

Tropical 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.

Carbon 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.