Ellen van Donk

Eukaryotic phytoplankton form the basis of aquatic food webs and play a key role in the global carbon cycle. Many of these evolutionarily diverse microalgae are also capable of feeding on other microbes, and hence simultaneously act both as primary producers and consumers. The net ecosystem impact of such mixotrophs depends on their nutritional strategy which is likely to alter with environmental change. Many temperate lakes are currently warming at unprecedented rates and are simultaneously increasing in water color (browning) due to increased run-off of humic substances. We hypothesized that the resulting reduction in light intensity and increased bacterial abundances would favor mixotrophic phytoplankton over obligate autotrophs, while higher temperatures might boost their rates of bacterivory. We tested these hypotheses in a mesocosm experiment simulating a gradient of increasing temperature and water color in temperate shallow lakes as expected to occur over the coming century. Mixotrophs showed a faster increase in abundance under the climate change scenario during spring, when they dominated the phytoplankton community. Furthermore, both bacterial abundances and rates of phytoplankton bacterivory increased under future climate conditions. Bacterivory contributed significantly to phytoplankton resource acquisition under future climate conditions, while remaining negligible throughout most of the season in treatments resembling today's conditions. Hence, to our knowledge, we here provide the first evidence for an increasing importance of bacterivory by phytoplankton in future temperate shallow lakes. Such a change in phytoplankton nutritional strategies will likely impact biogeochemical cycles and highlights the need to conceptually integrate mixotrophy into current ecosystem models.

Abstract The assessment of the risk posed by invasive alien species (IAS) to the environment is a component of increasing importance for Pest Risk Analysis. Standardized and comprehensive procedures to assess their impacts on ecosystem services have been developed only recently. The invasive apple snails (Pomacea canaliculata and P. maculata) are used as a case study to demonstrate the application of an innovative procedure assessing the potential impact of these species on shallow freshwater ecosystems with aquatic macrophytes in Europe. The apple snail, Pomacea maculata, recently established in the Ebro delta in Spain resulting in a serious threat to rice production and wetlands, having also a high risk to spread to other European wetlands. Here, the population abundance of apple snails is regarded as the main driver of ecosystem change. The effects of ecosystem resistance, resilience and pest management on snail population abundance are estimated for the short (5 years) and the long (30 years) term. Expert judgment was used to evaluate the impacts on selected ecosystem services in a worst-case scenario. Our study shows that the combined effects of apple snails are estimated to have profound effects on the ecosystem services provided by shallow, macrophyte-dominated ecosystems in Europe. This case study illustrates that quantitative estimates of environmental impacts from different IAS are feasible and useful for decision-makers and invasive species managers that have to balance costs of control efforts against environmental and economic impacts of invasive species.

Summary 1.We investigated how dynamics in phytoplankton biomass are driven through light and nutrient limitations in the Mwanza Gulf, Lake Victoria (Tanzania). 2.We measured light attenuation, chlorophyll a and water quality parameters along a phytoplankton biomass gradient in the Mwanza Gulf at six sampling stations in three different seasons from August 2009 to March 2011. 3.We found a higher rate of attenuation of photosynthetic available radiation than in other gulfs of Lake Victoria which could only partially be explained by phytoplankton biomass, measured as chlorophyll a. Suspended particulate detritus strongly affected light attenuation, particularly in shallow water. 4.In shallow water in the southern part of the Mwanza Gulf, nutrients were in excess and phytoplankton biomass was limited by light. In deeper water near the entrance of the gulf, light was not limiting when nitrogen probably was, as indicated by the N:P ratio. 5.Low abundance of N-fixing phytoplankton species suggests that N-fixation was low in shallow water. Phytoplankton biomass was lower in Mwanza Gulf than in northern gulfs of Lake Victoria, but might increase in the future as a result of nutrient enrichment caused by an increase in intensive agriculture and rapid growth of the city of Mwanza.

Intraguild predators both feed on and compete with their intraguild prey. In theory, intraguild predators can therefore be very effective as biological control agents of intraguild prey species, especially in productive environments. We investigated this hypothesis using the mixotrophic chrysophyte Ochromonas as intraguild predator and the harmful cyanobacterium Microcystis aeruginosa as its prey. Ochromonas can grow photoautotrophically, but can also graze efficiently on Microcystis. Hence, it competes with its prey for inorganic resources. We developed a mathematical model and parameterized it for our experimental food web. The model predicts dominance of Microcystis at low nutrient loads, coexistence of both species at intermediate nutrient loads, and dominance of Ochromonas but a strong decrease of Microcystis at high nutrient loads. We tested these theoretical predictions in chemostat experiments supplied with three different nitrogen concentrations. Ochromonas initially suppressed the Microcystis abundance by >97% compared to the Microcystis monocultures. Thereafter, however, Microcystis gradually recovered to ∼20% of its monoculture abundance at low nitrogen loads, but to 50–60% at high nitrogen loads. Hence, Ochromonas largely lost control over the Microcystis population at high nitrogen loads. We explored several mechanisms that might explain this deviation from theoretical predictions, and found that intraspecific interference at high Ochromonas densities reduced their grazing rates on Microcystis. These results illustrate the potential of intraguild predation to control pest species, but also show that the effectiveness of their biological control can be reduced in productive environments. Read More: http://www.esajournals.org/doi/abs/10.1890/13-0218.1

Theory predicts that intraguild predation leads to different community dynamics than the trophic cascades of a linear food chain. However, experimental comparisons of these two food-web modules are rare. Mixotrophic plankton species combine photoautotrophic and heterotrophic nutrition by grazing upon other phytoplankton species. We found that the mixotrophic chrysophyte Ochromonas can grow autotrophically on ammonium, but not on nitrate. This offered a unique opportunity to compare predator–prey interactions in the presence and absence of intraguild predation, without changing the species composition of the community. With ammonium as nitrogen source, Ochromonas can compete with its autotrophic prey for nitrogen and therefore acts as intraguild predator. With nitrate, Ochromonas acts solely as predator, and is not in competition with its prey for nitrogen. We parameterized a simple intraguild predation model based on chemostat experiments with monocultures of Ochromonas and the toxic cyanobacterium Microcystis. Subsequently, we tested the model predictions by inoculating Ochromonas into the Microcystis monocultures, and vice versa. The results showed that Microcystis was a better competitor for ammonium than Ochromonas. In agreement with theoretical predictions, Microcystis was much more strongly suppressed by intraguild predation on ammonium than by top–down predation on nitrate. Yet, Microcystis persisted at very low population densities, because the type III functional response of Ochromonas implied that the grazing pressure upon Microcystis became low when Microcystis was rare. Our results provide experimental support for intraguild predation theory, and indicate that intraguild predation may enable biological control of microbial pest species.

1.Rising atmospheric CO2 concentrations might affect the primary production and community composition of freshwater ecosystems. 2.We investigated these potential effects in laboratory mesocosms (Limnotrons), using monoculture experiments and competition experiments with the green alga Scenedesmus obliquus and the cyanobacterium Synechocystis sp. PCC6803. The Limnotrons were sparged with ambient air (controls, 380 parts per million volume (ppmv) CO2), moderately elevated CO2 levels (3000 ppmv CO2) or highly elevated CO2 levels (18 800 ppmv CO2). 3.Growth at ambient air led to the depletion of dissolved CO2 during algal bloom development and hence a high pH. In contrast, growth at elevated CO2 levels resulted in high concentrations of dissolved CO2 and dissolved inorganic carbon (DIC), lower pH and low concentrations of nitrate and soluble reactive phosphorus. Elevated CO2 levels did not have a significant effect on the biomass of the algal species, but shifted their elemental composition towards higher carbon-to-nutrient ratios. 4.Competition experiments at ambient air were driven mainly by competition for inorganic carbon. In this case, the cyanobacterium Synechocystis was displaced by the green alga Scenedesmus. Elevated CO2 alleviated the community from carbon limitation, which shifted the species interactions towards competition for nitrogen and phosphorus, and resulted in coexistence of the two species. Hence, our findings do not support the hypothesis that cyanobacteria are generally better competitors for inorganic carbon than green algae. 5.In conclusion, our results show that elevated CO2 levels may cause major changes in freshwater ecosystems, including a reduction in pH, shifts in resource limitation patterns, and changes in the ecological stoichiometry and species composition of phytoplankton communities.

The metabolic theory of ecology predicts that temperature affects heterotrophic processes more strongly than autotrophic processes. We hypothesized that this differential temperature response may shift mixotrophic organisms towards more heterotrophic nutrition with rising temperature. The hypothesis was tested in experiments with the mixotrophic chrysophyte Ochromonas sp., grown under autotrophic, mixotrophic and heterotrophic conditions. Our results show that (1) grazing rates on bacterial prey increased more strongly with temperature than photosynthetic electron transport rates, (2) heterotrophic growth rates increased exponentially with temperature over the entire range from 13 to 33 °C, while autotrophic growth rates reached a maximum at intermediate temperatures and (3) chlorophyll contents during mixotrophic growth decreased at high temperature. Hence, the contribution of photosynthesis to mixotrophic growth strongly decreased with temperature. These findings support the hypothesis that mixotrophs become more heterotrophic with rising temperature, which alters their functional role in food webs and the carbon cycle.

Dominance by cyanobacteria hampers human use of lakes and reservoirs worldwide. Previous studies indicate that excessive nutrient loading and warmer conditions promote dominance by cyanobacteria, but evidence from global scale field data has so far been scarce. Our analysis, based on a study of 143 lakes along a latitudinal transect ranging from subarctic Europe to southern South America, shows that although warmer climates do not result in higher overall phytoplankton biomass, the percentage of the total phytoplankton biovolume attributable to cyanobacteria increases steeply with temperature. Our results also reveal that the percent cyanobacteria is greater in lakes with high rates of light absorption. This points to a positive feedback because restriction of light availability is often a consequence of high phytoplankton biovolume, which in turn may be driven by nutrient loading. Our results indicate a synergistic effect of nutrients and climate. The implications are that in a future warmer climate, nutrient concentrations may have to be reduced substantially from present values in many lakes if cyanobacterial dominance is to be controlled.

Many organisms have developed defences to avoid predation by species at higher trophic levels. The capability of primary producers to defend themselves against herbivores affects their own survival, can modulate the strength of trophic cascades and changes rates of competitive exclusion in aquatic communities. Algal species are highly flexible in their morphology, growth form, biochemical composition and production of toxic and deterrent compounds. Several of these variable traits in phytoplankton have been interpreted as defence mechanisms against grazing. Zooplankton feed with differing success on various phytoplankton species, depending primarily on size, shape, cell wall structure and the production of toxins and deterrents. Chemical cues associated with (i) mechanical damage, (ii) herbivore presence and (iii) grazing are the main factors triggering induced defences in both marine and freshwater phytoplankton, but most studies have failed to disentangle the exact mechanism( s) governing defence induction in any particular species. Induced defences in phytoplankton include changes in morphology (e.g. the formation of spines, colonies and thicker cell walls), biochemistry (such as production of toxins, repellents) and in life history characteristics (formation of cysts, reduced recruitment rate). Our categorization of inducible defences in terms of the responsible induction mechanism provides guidance for future work, as hardly any of the available studies on marine or freshwater plankton have performed all the treatments that are required to pinpoint the actual cue(s) for induction. We discuss the ecology of inducible defences in marine and freshwater phytoplankton with a special focus on the mechanisms of induction, the types of defences, their costs and benefits, and their consequences at the community level.

Diatoms form large spring blooms in lakes and oceans, providing fuel for higher trophic levels at the start of the growing season. Some of the diatom blooms, however, are not grazed by filter-feeding zooplankton like Daphnia due to their large size. Several of these large diatoms are susceptible to chytrid infections. Zoospores of chytrids appeared to be excellent food for Daphnia, both in terms of size, shape, and quality (PUFAs and cholesterol). Thus, zoospores of chytrids can bridge the gap between inedible diatoms and Daphnia. In order to examine the effects of diatoms and chytrids on the survival of copepods, we performed one grazing and one survival experiment. The grazing experiment revealed that the diatom, Asterionella formosa, was not grazed by the copepod, Eudiaptomus gracilis, even after being infected by the chytrid Zygorhizidium planktonicum. However, carbon and nitrogen concentrations were significantly reduced by E. gracilis only when A. formosa was infected by Z. planktonicum, indicating that the chytrids might facilitate material transfer from inedible diatoms to the copepods. The survival experiment revealed that E. gracilis lived shorter with A. formosa than with the cryptophyta Cryptomonas pyrenoidifera. However, the survival of E. gracilis increased significantly in the treatment where A. formosa cells were infected by Z. planktonicum. Since E. gracilis could not graze A. formosa cells due to their large colonial forms, E. gracilis may acquire nutrients by grazing on the zoospores, and were so able to survive in the presence of the A. formosa. This provides new insights into the role of parasitic fungi in aquatic food webs, where chytrids may improve copepod survival during diatom blooms.

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While most experiments investigating zooplankton grazing on harmful cyanobacteria have been carried out with metazoan plankton, several protozoa can also feed efficiently on cyanobacteria. We investigated grazing by the mixotrophic flagellate Ochromonas sp. on the toxic cyanobacterium Microcystis aeruginosa. Ochromonas sp. grew rapidly on M. aeruginosa and had a strong impact on the population density of its prey. However, specific growth rates of Ochromonas sp. decreased over time, possibly indicating a negative impact on Ochromonas sp. mediated by M. aeruginosa. Grazing did not have any effect on the intracellular microcystin content of M. aeruginosa, and the ingested microcystins did not accumulate within Ochromonas sp. We studied the functional and numerical response of Ochromonas sp. grazing on the microcystin-producing strain M. aeruginosa PCC 7806 and its microcystin-deficient mutant. Ochromonas sp. showed a Type 3 functional response of very similar shape on both the toxic and non-toxic M. aeruginosa strain. Ingestion rates of Ochromonas sp. were even slightly higher on the toxic M. aeruginosa strain. We therefore found no indication of microcystins acting as a defence against mixotrophic flagellates.

Many short-lived or univoltine organisms at high latitudes and altitudes face the challenge to complete their life-cycle within a brief growing season. This means that they need to maintain a high growth rate at low temperatures, and one way of doing this is to allocate limiting resources like phosphorus to RNA in order to maximize protein synthesis. We here explore the allocations of phosphorus to RNA relative to DNA, and the potential bearings on growth rate and life history traits of polyploid (high-Arctic) and diploid (temperate) Daphnia pulex. The polyploid clone matured earlier at low temperature (8°C) but later than the diploid clone at high temperature (18°C). Juveniles of Arctic Daphnia had both higher specific levels of RNA and higher growth rates at low temperature compared with the temperate clone of Daphnia. We hypothesize that Arctic Daphnia may overcome growth constraints posed by low temperature and polyploidy by increasing their allocation of resources to RNA. The prevalence of polyploidy in Arctic populations strongly suggests that the potential drawbacks of polyploidy are counteracted by an increased allocation of resources to RNA to keep a high rate of protein synthesis even under low temperatures.

Fine organic suspended sediments (refractory detritus) play an important role in the underwater light attenuation of large shallow lakes with a peat origin and a eutrophication history. Wind driven resuspension of this material, its flocculation in the water column and the settlement of the formed flocs are the main processes governing the dynamics of this material. For restoration of the macrophyte community in such lakes, reduction of this refractory detritus to improve underwater light climate is the key process when eutrophication reduction measures alone are not effective enough. The shallow Lake Loosdrecht (The Netherlands) was used as case study to illustrate the effects of artificially created deepenings on suspended matter concentrations and the consequences for the underwater light climate. Suspended sediment balances were created for the current situation and the situation with deepenings. Field measurements were taken to quantify various processes and results of the calculations from the suspended sediment balances were used to quantify the effects on light climate and potential habitat for macrophytes. These calculations show that creating deepenings (three sections with a total surface area of 120 ha and a depth of 12 m on a 10% section of the lake) decreases the concentration of organic detritus by 25% and decreases attenuation coefficients from 2.5 m-1 to 2.2 m-1. P-load reductions affecting chlorophyll-a levels lead to a change of attenuation coefficients from 2.5 m-1 to 2.0 m-1. The combination of deepenings with P-load reduction measures gives the most optimal result and leads to a predicted attenuation coefficient of 1.6 m-1. These improvements of the underwater light climate are a first step to the recovery of the submerged macrophyte community.

The combined influence of a pesticide (carbaryl) and a cyanotoxin (microcystin LR) on the life history of Daphnia pulicaria was investigated. At the beginning of the experiments animals were pulse exposed to carbaryl for 24 h and microcystins were delivered bound in Microcystis’ cells at different, sub-lethal concentrations (chronic exposure). In order to determine the actual carbaryl concentrations in the water LC–MS/MS was used. For analyses of the cyanotoxin concentration in Daphnia’s body enzyme-linked immunosorbent assay (ELISA) was used. Individual daphnids were cultured in a flow-through system under constant light (16 h of light: 8 h of dark), temperature (20°C), and food conditions (Scenedesmus obliquus, 1 mg of C l−1). The results showed that in the treatments with carbaryl egg numbers per female did not differ significantly from controls, but the mortality of newborns increased significantly. Increasing microcystin concentrations significantly delayed maturation, reduced size at first reproduction, number of eggs, and newborns. The interaction between carbaryl and Microcystis was highly significant. Animals matured later and at a smaller size than in controls. The number of eggs per female was reduced as well. Moreover, combined stressors caused frequent premature delivery of offspring with body deformations such as dented carapax or an undeveloped heart. This effect is concluded to be synergistic and could not be predicted from the effects of the single stressors.

In a previous field study (Flik & Vijverberg 2003) we showed that in an oligotrophic-mesotrophic lake in the Netherlands (L. Maarsseveen) two co-occurring Daphnia species, Daphnia pulicaria and Daphnia galeata × hyalina performed distinct vertical migration behaviour during summer which is induced by a high fish biomass of young perch. During late spring and summer, D. galeata × hyalina exhibits diel vertical migration, whereas D. pulicaria is staying down day and night in the hypolimnion. This difference in migration behaviour results in contrasting predation mortalities caused by fish, low for D. pulicaria and relatively high for D. galeata × hyalina. In the present study, we measured in the laboratory the effects of fish released infochemicals on five life history traits in four genetically distinct D. galeata × hyalina and three genetically distinct D. pulicaria clones collected during summer at day-time from the hypolimnion. We tested the hypothesis that the species which behaviour is providing the best protection against fish predation (i.e. D. pulicaria) is less protected by life history traits induced by fish released infochemicals than the species which by its behaviour is less well protected against fish predation (i.e. D. galeata × hyalina). Our results show that D. galeata × hyalina responded in three out of five traits differently to fish infochemicals than D. pulicaria. In all these three traits D. galeata × hyalina responded significantly to fish-released infochemicals, whereas D. pulicaria did not show any significant response at all. We conclude that in D. pulicaria behavioural defences trade-off against life history defences.

Colony forming and toxic cyanobacteria form a problem in surface waters of shallow lakes, both for recreation and wildlife. Zebra mussels, Dreissena polymorpha, have been employed to help to restore shallow lakes in the Netherlands, dominated by cyanobacteria, to their former clear state. Zebra mussels have been present in these lakes since they were created in the 19th century by the excavation of peat and are usually not considered to be an invasive species. Most grazing experiments using Dreissena have been performed with uni-cellular phytoplankton laboratory strains and information on grazing of larger phytoplankton taxa hardly exists. To gain more insight in to whether D. polymorpha is indeed able to decrease cyanobacteria in the phytoplankton, we therefore performed grazing experiments with zebra mussels and two species of cyanobacteria, that greatly differ in shape: colony forming strains of Microcystis aeruginosa and the filamentous species Planktothrix agardhii. For both species a toxic and a non-toxic strain was selected. We found that zebra mussels cleared toxic Planktothrix at a higher rate than non-toxic Planktothrix, toxic or non-toxic Microcystis. Clearance rates between the other strains were not significantly different. Both phytoplankton species, regardless of toxicity, size and shape, were found in equal amounts (based on chlorophyll concentrations) in the excreted products of the mussels (pseudofaeces). The results show that zebra mussels are capable of removing colonial and filamentous cyanobacteria from the water, regardless of whether the cyanobacteria are toxic or not. This implies that the mussels may be used as a biofilter for the removal of harmful cyanobacterial blooms in shallow (Dutch) lakes where the mussels are already present and not a nuisance. Providing more suitable substrate for zebra mussel attachment may lead to appropriate mussel densities capable of filtering large quantities of cyanobacteria.

Zebra mussels (Dreissena polymorpha) are an important component of the foodweb of shallow lakes in the Netherlands, amongst others in Lake IJsselmeer, an international important wetland. Large numbers of ducks feed on these mussels in autumn and winter. The mussels are filter feeders and are exposed to high densities of cyanobacteria in summer and autumn. Mussels and cyanobacteria both thrive in Lake IJsselmeer. Apparently the mussels are somehow protected against accumulation of harmful quantities of cyanobacterial toxins. In this study, we investigated the assimilation of the cyanobacterial toxin microcystin–LR (MC–LR) in zebra mussels when fed the toxic cyanobacterium Microcystis aeruginosa as sole food or in a mixture with the eustigmatophyte Nannochloropsis limnetica. After 3 weeks of assimilation we studied the depuration of MC–LR during 3 weeks when the food of the mussels was free of cyanobacteria. These assimilation/depuration experiments were combined with grazing experiments, using the same food treatments. Microcystins were analyzed using liquid chromatography–mass spectrometry (LC–MS); in addition, covalently bound MC were analyzed using the MMPB method. The mussels showed higher clearance rates on Microcystis than on Nannochloropsis. No selective rejection of either phytoplankton species was observed in the excretion products of the mussels. Zebra mussels fed Microcystis as single food, assimilated microcystin–LR relatively fast, and after 1 week the maximum value of free unbound microcystin assimilation (ca. 11 µg g DW-1) was attained. For mussels, fed with the mixed food, a maximum of only 3.9 µg g DW-1 was recorded after 3 weeks. Covalently bound MC never reached high values, with a maximum of 62% of free MC in the 2nd week of the experiment. In the depuration period microcystin decreased rapidly to low values and after 3 weeks only very low amounts of microcystin were detectable. The amount of toxin that accumulated in the mussels would appear to be high enough to cause (liver) damage in diving ducks. However, death by exposure to microcystin seems unlikely. Mussels seem efficient in minimizing the assimilation of microcystin. If it were not for this, mass mortalities of ducks in shallow lakes in the Netherlands would presumably occur on a much more widespread scale than is currently observed. [KEYWORDS: Clearance rate; Cyanotoxins; Diving ducks; Food web; LC–MS; Microcystis; Phytoplankton; Pseudo-faeces]

We studied the occurrence of colony formation within 40 different strains of Scenedesmaceae (Chlorococcales, Chlorophyta) in response to grazing-released infochemicals from the herbivorous zooplankters Brachionus calyciflorus Pallas (Rotifera) and Daphnia magna Strauss (Cladocera). With the exception of two strains, all strains showed similar responses to both B. calyciflorus and D. magna infochemicals, either no response or inducible colony formation. Colony size was found to increase with B. calyciflorus infochemical concentration and could be described by a sigmoid function. The increase in colony size was more pronounced in the Scenedesmus species tested than in Desmodesmus species, which was probably due to higher threshold infochemical concentrations for colony induction in Desmodesmus. Therefore, the adaptivity of colony formation to the herbivory threat only holds above the threshold concentration for colony induction and as long as maximum colony size has not been attained. Taking this into account, our results suggest that inducible colony formation is a common adaptive response of many Scenedesmaceae to the threat of herbivory. [KEYWORDS: Brachionus calyciflorus; Daphnia magna; Desmodesmus; herbivory; inducible defences; infochemicals ; Scenedesmus]

Semiochemicals play an important role in interactions between living organisms in aquatic environments. Although the presence of chemical cues is confirmed in more and more systems, the chemical structures remain predominantly elusive. To create more accurate prey–predator interaction models and to advance the research on chemical communication, it is essential to identify these compounds. A literature overview of cues involving Daphnia (either as producer or receiver) is given and the progress towards their isolation and structure elucidation is described. Most of the research so far has concentrated on the elucidation of kairomones produced by predators of Daphnia (especially Chaoborus and several species of fish). Although some progress has been made, these cues have not been isolated and identified yet. Additionally new results on the isolation and identification of the kairomone responsible for the colony formation in Scenedesmus using differential diagnosis and bioassay-directed fractionation of Daphnia exudates are presented. The importance of suitable and well performing bioassays herein cannot be underestimated. Some preliminary results with solid-phase extraction with C18 proved to be reproducible for extracting the active compound from Daphnia water, although it was not possible to get the biological activity into a single fraction. The cue was not extractable with an anion exchanger (SAX). Subjecting the extract to HPLC led to one active fraction [KEYWORDS: semiochemicals, Scenedesmus, Daphnia, Solid-Phase Extraction, chemical communication, bioassay]

The effects of chemicals released from crowded congeners and conspecifics on life history parameters of the freshwater zooplankters Daphnia cucullata and Daphnia pulex were examined. Length and age at maturity of D. pulex were affected by crowding chemicals. Reproduction was lower in crowded medium, and ephippia were produced. Newborn D. pulex in crowded medium were significantly longer than the controls. The intrinsic rate of population increase of D. pulex was 14 and 25% lower than the control when exposed to crowded medium from D. cucullata and D. pulex, respectively. Neither urea nor ammonia (at 1 mg l-1) seemed to be responsible for these effects in D. pulex. In D. cucullata, no significant effect of crowding infochemicals on length and age at maturity was found. However, crowding chemicals reduced reproduction. No ephippia were produced in crowded medium, but up to 83% non-developing eggs were observed in D. cucullata. Newborns were similarly sized in crowded and standard medium. The intrinsic rate of population increase of D. cucullata was 44 and 96% lower than the control when exposed to crowded medium from D. cucullata and D. pulex, respectively. Clearance rates of D. pulex were significantly reduced in crowded media compared with standard medium, which could partly explain why the animals exposed to crowding chemicals reacted as if they were food limited

Molecular techniques have become a valuable tool in phytoplankton studies over the past decades. The appropriate choice of a technique from an increasing array of methods can be rather complex, because different techniques are suitable for different questions or problems in ecology and evolution. Each technique has its particular strengths and weaknesses and is based upon different (theoretical) assumptions. Our aim is to give a better insight in the (correct) use of various molecular techniques in phytoplankton research, with special emphasis on the fields of strain identification, differentiation of populations and the establishment of phylogenetic relationships. The basic steps in the development of molecular techniques like allozyme electrophoresis, RFLP, DGGE, SSCP, RAPD, AFLP and microsatellites, and the application of these techniques in phytoplankton research, are discussed. Furthermore, recent developments in molecular biology, that have so far only found limited application in phytoplankton studies, such as single-cell PCR, PCR assays combined with molecular probes (Heteroduplex Mobility Assays or DNA arrays), Real-time PCR, complete genome sequencing, multi-gene expression studies using microarrays, and Single Nucleotide Polymorphism (SNPs), are discussed. We emphasise the relevance of fundamental and applied molecular studies on phytoplankton for a wider community of ecologists and evolutionary biologists [KEYWORDS: algae, cyanobacteria, genetic variation, phylogenetics, AFLP, microsatellites, microarrays, genomics, review]

In order to disentangle the causes of variations in water chemistry among European shallow lakes, we performed standardised sampling programs in 86 lakes along a latitudinal gradient from southern Spain to northern Sweden. Lakes with an area of 0.1 to 27 000 ha and mean depth of 0.4–5.6 m located in low to high altitudes were investigated within the EC project ECOFRAME 1–4 times during June–October 2000–2001. Several variables like conductivity, alkalinity, abundance of submerged plants, concentrations of suspended solids, total nitrogen and phosphorus were latitude-dependent decreasing from south to north. Secchi depth, concentrations of total nitrogen, total phosphorus, suspended solids, and chlorophyll a correlated strongly with the presumed quality classes of the lakes. We came to the conclusion that the variability of shallow lakes in Europe is still mostly dependent on natural differences. Variables connected to lake morphometry, seasonality, basin geology and climate explained altogether nearly half of the total variability of lakes. The trophic state factor, describing mostly the human influence on lakes, was the strongest single factor responsible for nearly a quarter of the total variability of the studied European lakes [KEYWORDS: ecological status, European shallow lakes, hydrochemistry, latitude]

1. The European Water Framework Directive requires the determination of ecological status in European fresh and saline waters. This is to be through the establishment of a typology of surface water bodies, the determination of reference (high status) conditions in each element (ecotype) of the typology and of lower grades of status (good, moderate, poor and bad) for each ecotype. It then requires classification of the status of the water bodies and their restoration to at least good status in a specified period. 2. Though there are many methods for assessing water quality, none has the scope of that defined in the Directive. The provisions of the Directive require a wide range of variables to be measured and give only general guidance as to how systems of classification should be established. This raises issues of comparability across States and of the costs of making the determinations. 3. Using expert workshops and subsequent field testing, a practicable pan-European typology and classification system has been developed for shallow lakes, which can easily be extended to all lakes. It is parsimonious in its choice of determinands, but based on current limnological understanding and therefore as cost-effective as possible. 4. A core typology is described, which can be expanded easily in particular States to meet local conditions. The core includes 48 ecotypes across the entire European climate gradient and incorporates climate, lake area, geology of the catchment and conductivity. 5. The classification system is founded on a liberal interpretation of Annexes in the Directive and uses variables that are inexpensive to measure and ecologically relevant. The need for taxonomic expertise is minimized. 6. The scheme has been through eight iterations, two of which were tested in the field on tranches of 66 lakes. The final version, Version 8, is offered for operational testing and further refinement by statutory authorities [KEYWORDS: lakes Water Framework Directive typology ecotypes ecological status quality]

Submerged macrophytes are crucial for the stabilization of the clear water state in shallow, mesotrophic and eutrophic lakes. Especially, charophytes often play an important role because they are typically rapid colonizers and because charophyte meadows are believed to have a particularly strong positive effect on water transparency compared to other macrophytes. Several mechanisms may contribute to the impact of submerged macrophytes on the planktonic food web. In this paper, the available literature on these mechanisms is briefly reviewed and special attention is paid to the impact of charophytes on the structure and dynamics of phyto- and zooplankton communities. The paper focuses on allelopathy, and possible differences between charophytes and other macrophytes, as well as gaps in our knowledge are discussed. [KEYWORDS: Macrophyte; Charophyte; Allelopathy; Zooplankton and phytoplankton]

1. Removal of zooplanktivorous fish (mainly bream) in 1987 from a shallow eutrophic lake in the Netherlands, Lake Zwemlust, resulted in a quick switch from a turbid state with cyanobacteria blooms to a clear state dominated by macrophytes. 2. The clear state was not stable in the long term, however, because of high nutrient loadings. 3. In 1999, another removal of zooplanktivorous fish (mainly rudd) had similar effects as in 1987, although macrophytes returned more slowly. 4. In the years directly following both interventions there was a 'transition period' of very clear water with high densities of zooplanktonic grazers in the absence of macrophytes; low oxygen concentrations indicate that during those years primary production was low relative to heterotrophic activity. 5. The transition period appears to provide the light climate necessary for the return of macrophytes. 6. Reduction of nutrient loading is necessary to improve water quality in Lake Zwemlust in the long term. In the short term, repeated fish stock reduction is a reasonable management strategy to keep Lake Zwemlust clear.

The feeding behaviour of the zebra mussel (Dreissena polymorpha) was studied in the laboratory on different combinations of food, including a green alga Chlamydomonas reinhardtii) and toxic and non-toxic strains of the cyanobacterium Microcystis aeruginosa. 2. The highest clearance rate of phytoplankton by zebra mussels was found when the mussels were feeding on a mixture of Chlamydomonas and non-toxic Microcystis, the lowest on a mixture of Chlamydomonas and toxic Microcystis. 3. The differences found in the clearance rates between food combinations can be partly explained by the production of pseudofaeces containing live phytoplankton cells. Zebra mussels expelled significantly more live phytoplankton cells in the presence of toxic Microcystis than in the presence of non-toxic Microcystis. The pseudofaeces contained predominantly live Chlamydomonas cells. Proportionately much less live Microcystis cells were encountered in the pseudofaeces. 4. Consequently, grazing of zebra mussels on a combination of Chlamydomonas and Microcystis may finally result in a dominance of Chlamydomonas over Microcystis. The presence of toxic Microcystis may even strengthen this shift.

We studied population dynamics, phosphorus balance, and individual growth to evaluate limitation of Daphnia cucullata by dietary phosphorus (P) in three interconnected, hypereutrophic Dutch lakes. Consistent with the P-limitation hypothesis, declines in egg production and population density coincided with seasonal increases in the seston C:P ratio from 300 to 400-500 (molar) in all three lakes. Adult Daphnia collected from Lake Breukeleveen exhibited a linear increase in specific P content during the first 36 h of feeding on a P-rich diet in the laboratory (r(2) = 0.99), which confirms that field Daphnia experienced P deficits in their tissues. Laboratory experiments tested the growth of juvenile D. cucullata in controls with natural seston and in seston supplemented with phosphate or the P-rich cyanobacterium Synechococcus. Growth was also estimated in filtered lake water with Scenedesmus (2 mg C L-1) as food. This design allowed us to partition differences between the control and Scenedesmus growth rates into limitation by phosphorus, energy and residual factors. In each of 10 experiments, D. cucullata exhibited improved growth in response to the phosphate and Synechococcus supplements. In agreement with predictions, Daphnia exhibited the lowest control growth rate and the strongest response to P addition in seston from the lake with the highest seston C:P ratio. However, animals in seston from two lakes with very similar seston C:P ratios differed in their growth responses. Our results confirm dietary P limitation but show that energy limitation and residual factors also contributed to between- lake differences in Daphnia growth. [KEYWORDS: Unsaturated fatty-acids; food quality; mineral limitation green-algae; zooplankton; carbon; cyanobacterium; phytoplankton; nitrogen; nutrition]

Oscillatoria spp. are generally very abundant in many shallow, eutrophic lakes in the Netherlands. However, this is less true for Daphnia galeata. The main aim of this study was to investigate whether the edibility of live Oscillatoria limnetica by Daphnia galeata, and of the detritus derived from this filamentous cyanobacterium, was, among others, an important limiting factor for the daphnids. We measured the consumption and assimilation rates of Daphnia using dual- labelling radio-tracer techniques (C-14 and P-32) to label separately the live Oscillatoria filaments and detritus prepared from these filaments. The two food types were mixed in different proportions, and both the food ingestion and food incorporation rates by daphnids were measured. The main findings of this study were that specific clearance rates of Daphnia on shorter Oscillatoria filaments were significantly higher than on the longer filaments, in other words the weight- specific ingestion rates were higher on the shorter Oscillatoria filaments than on the longer filaments from the batch cultures, The longer Oscillatoria filaments are more like v to clump and, therefore, are more liable to be rejected by Daphnia during the food collection and ingestion processes. The shorter filaments, in comparison, are apparently less prone to clumping and, therefore, are cleared by the daphnids at higher rates than the longer filaments. Feeding the daphnids on double-labelled (C-14 and P-32) shorter filaments revealed that the assimilation efficiency of shorter Oscillatoria filaments was generally higher for P than for C, probably because of a high CIP ratio of Oscillatoria. Daphnia (0.75-1.85 mm in size)fed significantly better on dead (detritus) Oscillatoria filaments than on live Oscillatoria filaments, even if the relative proportion of detritus in the food was only about one- quarter that of the live filaments. This preference for detritus over live Oscillatoria, as indicated by Chesson's selectivity coefficient a, was apparently a passive process, rather than a case of active food selection. This 'selective' feeding on detritus was apparently facilitated by the relatively greater rejection of the live filaments than the detrital filaments. At very high food concentrations (15-25 mg Cl-1), with the share of the live Oscillatoria filaments 2 to 2.5 times greater than that of the detritus, the ingestion rates increased proportionally less with increase in daphnid size than was expected on the basis of the allometric relationship between the length and weight of these animals. This implies that the larger animals had greater interfering effects of the Oscillatoria filaments on the food collection and ingestion processes. [KEYWORDS: Zooplankton-cyanobacteria interactions; lake lake loosdrecht; eutrophic lake; filamentous cyanobacteria; fresh-water; long-term; algal; discrimination; cladocerans; food]

Incubation experiments demonstrated a differential sensitivity to natural UV-radiation among the dominant phytoplankton species from three Arctic lakes, situated near Ny-Angstrom lesund, Spitsbergen (79 degrees N). The growth of small chlorophytes, diatoms and picocyanobacteria from two oligotrophic lakes was inhibited primarily by the shorter wavelength UV components, while the growth of the larger colony-forming species (cyanobacteria, Planktothrix sp., Woronichinia sp. and the chrysophyte, Uroglena americana Calkins) apparently was stimulated. These colonies (not easily eaten by daphnids) dominated at the end of the experiment in those treatments where the short wavelength UV components were not excluded. For the two oligotrophic localities, 70 and 61%, respectively, of total phytoplankton biovolume were edible in the treatments excluding short wavelength UV, compared to only 13 and 19%, respectively, in the treatments including such radiation. For the third, more productive and less transparent lake, the percentage of edible species in the treatments with and without short wavelength UV radiation did not differ (ca. 75% for both treatments). [KEYWORDS: cell size; cyanobacteria; daphnids; foodweb; grazers; inedible algae; Svalbard; UV-B Uv-b radiation; ozone depletion; chlamydomonas-reinhardtii; community responses; solar uvb; inhibition; growth; assemblages; algae; zooplankton]

The Loosdrecht Lakes comprise three shallow, hypereutrophic lakes in The Netherlands. Research conducted over the past two decades suggests that absence of large-bodied Daphnia from these lakes can be explained by size-specific effects of both poor food quality and predatory mortality. The phytoplankton is dominated by filamentous cyanobacteria which interfere with the feeding of large-bodied Daphnia. Moreover, dense populations of planktivorous fish are expected to feed selectively on larger prey. More recent research, however, suggests that the growth of the small native Daphnia species, Daphnia cucullata, is limited by low dietary phosphorus (P). In this study, we compared the growth and feeding rates of four Daphnia taxa representing a gradient in body size. In each of 10 growth experiments. native D. cucullata feeding in natural seston exhibited improved growth in response to additions of phosphate and the P-rich cyanobacterium Syncehococcus. The two largest species, Daphnia magna and Daphnia galeata, however, failed to grow in natural seston, even when the seston was supplemented with phosphate, Synechococcus, or Scenedesmus A D. galeata X cucullata hybrid exhibited weak growth in natural seston and no response to the supplements. Feeding experiments with polystyrene beads show that feeding inhibition in natural seston increased markedly with increased Daphnia size. Our results verify that Daphnia differing in size face different food quality constraints. The growth of native D. cucullata (adult size 0.6-0.8 mm) is reduced by P deficiency, whereas larger Daphnia appear to be excluded by interfering filaments of cyanobacteria. [KEYWORDS: Blue-green filaments; loosdrecht lakes eutrophic lake zooplankton; netherlands; selection; consumers; carbon]

Grazer-induced colony formation in the common green alga Scenedesmus acutus may be interpreted as an anti-grazer defense. Costs are to be expected because otherwise the protected colonial morph would be the norm. Analysis of growth rates, light harvesting in terms of photosystem II (PSII) efficiency using Xe-PAM fluorescence measurements and sedimentation rates were compared among unicellular and induced colonial populations. No differences in growth rates and PSII efficiency were detected between unicellular and colonial populations. However, colonial populations had higher settling velocities than unicellular populations. Hence, costs may be attributed to enhanced sinking out of the euphotic zone resulting in reduced growth due to lower light and temperature in deeper water layers. [KEYWORDS: Morphological-changes; inducible defenses; crucian carp; green- algae; in-situ; phytoplankton; daphnia; chlorophyceae; sinking; growth]

The genetic distances between several strains of Senedesmus obliquus (Turp,) Kutz,, S, acutus Hortobagyi, and S, naegelii Chod. calculated from ITS-2 sequences were found to be smaller than the genetic distances within other strains of Scenedesmus-that is, in S, acuminatus (Lagerh,) Chod, and S, pectinatus Meyen. These results confirm that the studied strains were not properly identified and should be renamed S, obliquus, as already suggested in other studies. [KEYWORDS: ITS, phylogeny, Scenedesmus acutus, Scenedesmus naegelii, Scenedesmus obliquus]

Scenedesmus acutus Meyen was cultured at four temperatures (9.5 degrees, 16.5 degrees, 24 degrees, and 29 degrees C) in standard medium or in medium with filtered water from a Daphnia culture, Growth was significantly reduced at low temperatures. At 9.5 degrees C it took more than a week before formation of eight-celled coenobia occurred in both the absence and presence of water from a Daphnia culture, At higher temperatures, formation of four- and eight-celled coenobia occurred more rapidly and was already observed in the presence of Daphnia water within 2 days, As cultures aged, also in the absence of Daphnia water, four-celled coenobia became dominant. At cold temperature, cell volume initially was significantly larger but declined after 3-4 weeks. Grazer-induced colony formation had occurred independent of the incubation temperature, but the number of cells per colony was increased with declining temperature. The morphological expression may be interpreted as a cyclomorphosis driven by nutrients, temperature, and chemical cues from gazers. [KEYWORDS: colony formation; cyclomorphosis; Daphnia; ecomorph; growth; infochemicals; phenotypic plasticity; Scenedesmus acutus; temperature Phenotypic plasticity; morphological-changes; daphnia-pulex; phytoplankton; growth; cyclomorphosis; light; transformation; infochemicals; consequences]

1. The growth and feeding of Daphnia pulex De Geer on different algal species was examined. The green algae Chlamydomonas reinhardtii Dangeard and Scenedesmus acutus Meyen, the diatom Synedra tenuissima Kutzing, the cryptophyte Cryptomonas pyrenoidifera Geitler and the cyanobacterium Microcystis aeruginosa Kutzing were cultured in nonlimiting and in N- or P-limiting medium and used as food for D. pulex. 2. Growth limitations were reflected in the elemental and biochemical composition and the morphological characteristics of the algal resources. 3. The clearance rates of D. pulex feeding on nutrient-limited algae were reduced. This was not observed when nutrient-limited mutant Chlamydomonas cells without cell walls were used as food, indicating that the cell wall may play an important part. 4. In comparison with animals grown on nutrient-sufficient cells, nutrient-limited algae resulted in smaller body length, reduced brood sizes, reduced size at maturity, increased age at first reproduction and, consequently, in reduced Daphnia population growth rates. 5. Daphnia population growth rates (r) were negatively correlated with the C:P ratio and the carbohydrate content of the food. Moreover, significant correlations between r and clearance rates were found. 6. The observed differences in the grazing and the life history parameters of Daphnia feeding on non-limited and nutrient-limited algae may be the result of both reduced nutritional value and reduced digestibility of nutrient-limited algae. [KEYWORDS: FRESH-WATER ZOOPLANKTON, FOOD QUALITY, CHEMICAL-COMPOSITION, MINERAL LIMITATION, VARYING QUALITY, GROWTH-RATE, DISCRIMINATION, MAGNA, ALGAE, SCENEDESMUS]

Biochemical substances released from Daphnia galeata induced colony formation in the green alga Scenedesmus acutus. Normally this strain consisted mainly of single cells in cultures. However, if exposed for 48 h to either water with live Daphnia or to O.1-mu m filtered water from a culture with Daphnia present, these unicellular "Chodatella" stages were induced to form colonies (coenobia). Colony induction was not unique to Daphnia; other zooplankters (rotifers and copepods) were able to induce colonies in S. acutus as well. This morphological response could also be evoked when Scenedesmus was exposed to 0.1-mu m filtered lake water during high zooplankton abundances. Especially during early spring, a clear relationship was found between rotifer abundance and colony formation in our test alga in the laboratory. Filtered lake water, incubated nonaxenically for at least 2 d at 20 degrees C, did not induce colony formation, possibly due to microbial degradation. The morphological changes in Scenedesmus could promote grazing resistance in small zooplankters and can be interpreted as an adaptive antipredator strategy. [KEYWORDS: DIEL VERTICAL MIGRATION; BLUE-GREEN-ALGAE; DAPHNIA; INDUCTION; CHLOROPHYCEAE; DEFENSES; FISH]

1. The effects of Daphnia infochemicals on the morphology and biochemical composition of Scenedesmus were studied and subsequently their influence on Daphnia life history. Three species of Scenedesmus (S. acutus, S. obliquus and S. subspicatus) were tested for Daphnia-induced colony formation. Life history experiments were performed with Daphnia cucullata and D. magna feeding on unicellular or colonial S. acutus. 2. Colony formation was promoted when S. acutus and S. obliquus were exposed to filtered water from a D. magna culture. S. subspicatus did not form colonies when exposed to culture water, but showed some colony formation when exposed to live D. magna. 3. No clear differences were found in total lipid, protein or carbohydrate content between the unicellular and colonial ecomorphs of the three Scenedesmus species. However, fatty acid (FA) concentration and composition were changed. Total FA as a percentage of frozen dry weight (DW) were increased in colonies. The ratio of monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA) and the ratio of omega 3:omega 6 fatty acids decreased in colonies. 4. A short-term grazing experiment with three size classes of Scenedesmus revealed lower clearance rates for small Daphnia when feeding on large colonial Scenedesmus. 5. Life history parameters of both Daphnia species were influenced by food type. The intrinsic rate of population increase (r) was significantly lower for daphnids feeding on colonies. 6. The negative influence of colonial Scenedesmus on population growth of Daphnia seems to be determined by algal morphology rather than biochemical composition. [KEYWORDS: Life-history shifts; mineral limitation; herbivore daphnia; flos-aquae; growth; zooplankton; phytoplankton; cladoceran; inhibition; magna]