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1. Lake stratification strengthens with increasing surface water temperatures, thereby reducing the depth of the mixed layer. Phytoplankton communities are not only exposed to different nutrient availability within a mixed water column, but also to different light quality. We conducted controled laboratory and mesocosm experiments to investigate phytoplankton, especially cyanobacteria, responses to different light quality and mixing depths.2. Our mesocosm experiment allowed the manipulation of mixing depth in situ by a mesocosm approach and to follow the effects of changing mixing depth on the phytoplankton community composition. Our laboratory experiment allowed the control of temperature and light quantity. To investigate the effect of light quality on phytoplankton, we created a light gradient from full photosynthetic active radiation to a reduced blue spectrum.3. In both experiments, shifts in phytoplankton community composition from eukaryote to cyanobacteria occurred at shallow mixing depth with higher availability of photosynthetic active radiation. Our results from the mesocosm experiment support the idea that reduced mixing depth can promote cyanobacterial abundance. With our laboratory experiment, we were able to manipulate light quality independent of temperature, available nutrients and light intensity influencing phytoplankton abundance. Results from the laboratory experiments support our hypothesis that a shift in light spectrum alone is a driver, strong enough to enhance cyanobacteria occurrence.4. Most of the previous studies dealing with cyanobacterial blooms have investigated temperature and eutrophication effects. Certainly, these are major factors for the growth of phytoplankton, but our experiments show that other aspects, such as the quality of light, must be also taken into account to explain cyanobacterial blooms. Such shifts in the phytoplankton community from eukaryote to cyanobacteria dominated communities will have strong consequences for food web dynamics. Several cyanobacteria specific traits, (e.g., toxin production, lack of essential fatty acids, and inedibility through production of large colonies) reduce transfer efficiencies of energy and matter between phyto- and zooplankton and therefore can influence higher trophic levels such as fish. published

The growth rate hypothesis (GRH) posits that variation in organismal stoichiometry (C:P and N:P ratios) is driven by growth-dependent allocation of P to ribosomal RNA. The GRH has found broad but not uniform support in studies across diverse biota and habitats. We synthesise information on how and why the tripartite growth-RNA-P relationship predicted by the GRH may be uncoupled and outline paths for both theoretical and empirical work needed to broaden the working domain of the GRH. We found strong support for growth to RNA (r

The human-caused proliferation of cyanobacteria severely impacts consumers in freshwater ecosystems. Toxicity is often singled out as the sole trait to which consumers can adapt, even though cyanobacteria are not necessarily toxic and the lack of nutritionally critical sterols in cyanobacteria is known to impair consumers. We studied the relative significance of toxicity and dietary sterol deficiency in driving the evolution of grazer resistance to cyanobacteria in a large lake with a well-documented history of eutrophication and oligotrophication. Resurrecting decades-old Daphnia genotypes from the sediment allowed us to show that the evolution and subsequent loss of grazer resistance to cyanobacteria involved an adaptation to changes in both toxicity and dietary sterol availability. The adaptation of Daphnia to changes in cyanobacteria abundance revealed a sterol-mediated gleaner–opportunist trade-off. Genotypes from peak-eutrophic periods showed a higher affinity for dietary sterols at the cost of a lower maximum growth rate, whereas genotypes from more oligotrophic periods showed a lower affinity for dietary sterols in favour of a higher maximum growth rate. Our data corroborate the significance of sterols as limiting nutrients in aquatic food webs and highlight the applicability of the gleaner–opportunist trade-off for reconstructing eco-evolutionary processes.

Using sodium chloride (NaCl) for de-icing roads is known to have severe consequences on freshwater organisms when washed into water bodies. N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, also known as 6PPD, is an antiozonant mainly found in automobile tire rubber to prevent ozone mediated cracking or wear-out. Especially the ozonated derivate, 6PPD-quinone, which is washed into streams after storm events, has been found to be toxic for coho salmon. Studies on other freshwater organisms could not confirm those findings, pointing towards distinct species-specific differences. Storm events result in greater run-offs from all water-soluble contaminants into freshwater bodies, potentially enhancing the concentrations of both chloride and 6PPD during winter. Here we show that these two contaminants have synergistic negative effects on the population growth of the rotifer Brachionus calyciflorus, a common freshwater herbivore. Hence, while only high concentrations of 6PPD and even higher concentrations of 6PPD-quinone, beyond environmentally relevant concentrations, had lethal effects on rotifers, the addition of NaCl enhanced the sensitivity of the rotifers towards the application of 6PPD so that their negative effects were more pronounced at lower concentrations. Similarly, 6PPD increased the lethal effect of NaCl. Our results support the species-specific toxicity of 6PPD and demonstrate a synergistic effect of the antiozonant on the toxicity of other environmentally relevant stressors, such as road salt contamination.

Diel vertical migration (DVM) is a common predator avoidance strategy of zooplankton. Migration to deeper water layers during the day to escape visually hunting predators is most likely to require physiological adaptations to periodically changing temperatures. Polyunsaturated fatty acids (PUFA) play crucial roles in membrane temperature acclimation. Exposure to cold temperatures typically results in an increase in the relative abundance of PUFA in cell membranes and PUFA requirements of Daphnia increase with decreasing temperatures. To assess the role of dietary PUFA in coping with temperature fluctuations experienced during DVM, we reared Daphnia magna at either constantly warm or fluctuating temperatures, simulating DVM both with and without dietary PUFA supplementation. We show that the well‐known positive effect of dietary eicosapentaenoic acid supplementation on offspring production and population growth rates of D. magna is more pronounced at alternating temperatures than at constantly warm temperatures. Exposure to alternating temperatures caused modification in body PUFA concentrations and, consequently, increased lipid peroxidation. However, detrimental effects of lipid peroxidation were not evident. Our data demonstrate that the capacity to cope with the distinct temperature fluctuations experienced during DVM increases with dietary eicosapentaenoic acid supplementation, suggesting that an adequate dietary PUFA supply is crucial especially for migrating Daphnia populations. A dietary deficiency in long‐chain PUFA may thus severely constrain predator evasion, potentially resulting in increased mortality and cascading effects on lower trophic levels.

Exploring the capability of organisms to cope with human-caused environmental change is crucial for assessing the risk of extinction and biodiversity loss. We study the consequences of changing nutrient pollution for the freshwater keystone grazer, Daphnia, in a large lake with a well-documented history of eutrophication and oligotrophication. Experiments using decades-old genotypes resurrected from the sediment egg bank revealed that nutrient enrichment in the middle of the 20th century, resulting in the proliferation of harmful cyanobacteria, led to the rapid evolution of grazer resistance to cyanobacteria. We show here that the subsequent reduction in nutrient input, accompanied by a decrease in cyanobacteria, resulted in the re-emergence of highly susceptible Daphnia genotypes. Expression and subsequent loss of grazer resistance occurred at high evolutionary rates, suggesting opposing selection and that maintaining resistance was costly. We provide a rare example of reversed evolution of a fitness-relevant trait in response to relaxed selection., Anthropogenic changes, such as eutrophication from lake pollution, can lead to rapid evolution. Comparing Daphnia resurrected from generations adapted to historical pollution to contemporary, post-cleanup populations finds that Daphnia rapidly reversed their evolved resistance to harmful cyanobacteria.

Daphniids act as keystone species in lake ecosystems by controlling phytoplankton biomass and experiencing intense fish predation. However, the importance of single daphniid species as trophic links between phytoplankton and fish remains unclear, especially compared with other zooplankton taxa. To disentangle the role of individual zooplankton taxa in the food web of a large lake, we performed an in-situ mesocosm experiment with natural phytoplankton and zooplankton communities in three treatments with native, invasive or no fish predators, respectively. A large daphniid, Daphnia longispina, was the zooplankter most strongly predated by both fish species, and also had the highest top-down effects on phytoplankton. All other zooplankton taxa, including a small daphniid species, had minor roles in terms of both predation by fish and grazing on phytoplankton. We suggest that daphniid species with large body sizes can strongly link higher and lower trophic levels in lake food webs, and thus function as keystone species in trophic cascades from fish to phytoplankton. published

Salinization of freshwater ecosystems is a growing hazard for organisms and ecosystem functioning worldwide. In northern latitudes, road salt that is being transported into water bodies can cause year-round increases in lake salinity levels. Exploring the environmental factors driving the susceptibility of freshwater zooplankton to road salt is crucial for assessing the impact of salinization on food web processes. We studied the role of essential lipids, i.e., sterols and long-chain polyunsaturated fatty acids (PUFAs), in mediating salt tolerance of the freshwater keystone herbivore Daphnia. Sterols and PUFAs are involved in regulating ion permeability of biological membranes and thus we hypothesized that the susceptibility to salt is affected by the dietary sterol and PUFA supply. Life history experiments revealed opposing effects of sterol and PUFA supplementation on salt tolerance, i.e., tolerance increased upon sterol supplementation but decreased upon PUFA supplementation, which is consistent with their proposed impact on membrane permeability. Our results suggest that the susceptibility of freshwater zooplankton to salinization strongly depends on the dietary lipid supply and thus the phytoplankton community composition. Hence, trophic state related differences in the phytoplankton community composition need to be considered when assessing the consequences of salinization for freshwater ecosystem functioning.