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Northern lakes are facing rapid environmental alterations—including warming, browning, and/or changes in nutrient concentrations—driven by climate change. These environmental changes can have profound impacts on the synthesis and trophic transfer of polyunsaturated fatty acids (PUFA), which are important biochemical molecules for consumer growth and reproduction. Zooplankton are a key trophic link between phytoplankton and fish, but their biochemical responses to environmental change are not well understood. In this study, we assess the trends in fatty acid (FA) composition of zooplankton taxa among 32 subarctic and temperate lakes across broad climate‐productivity and ecosystem size gradients. We found that genus‐level taxonomy explained most FA variability in zooplankton (54%), suggesting that environmental changes that alter the taxonomic composition also affect the FA composition of zooplankton communities. Furthermore, the FA responses and their underlying environmental drivers differed between cladocerans and copepods. Cladocerans, including widespread Bosmina spp. and Daphnia spp., showed pronounced responses across the climate‐productivity gradient, with abrupt declines in PUFA, particularly eicosapentaenoic acid and arachidonic acid in warmer, browner, and more eutrophic lakes. Conversely, calanoid copepods had high and relatively stable PUFA levels across the gradient. In addition, all zooplankton taxa increased in stearidonic acid levels in larger lakes where PUFA‐rich cryptophytes were more abundant. Overall, our results suggest that climate‐driven environmental alterations pose heterogeneous impacts on PUFA levels among zooplankton taxa, and that the negative impacts of climate warming are stronger for cladocerans, especially so in small lakes. [ABSTRACT FROM AUTHOR]

Physical defenses and grazer avoidance of the bloom-forming microalga Gonyostomum semen may reduce the direct coupling between phytoplankton and higher trophic levels and result in an increased importance of alternative basal food resources such as bacteria and heterotrophic protozoans. To assess the importance of algal and heterotrophic food resources for zooplankton during G. semen blooms and the effects of zooplankton diets on a higher consumer, we analyzed the fatty acid composition of zooplankton and the invertebrate predator Chaoborus flavicans from eight lakes along a gradient in the predominance of G. semen relative to other algae and the duration of G. semen blooms. The proportion of fatty acids of bacterial origin increased significantly along the G. semen gradient in all consumers studied. In addition, the proportion of polyunsaturated fatty acids (PUFA) decreased in cladocerans. These results suggest that heterotrophic pathways can compensate for a reduced trophic coupling between phytoplankton and filter-feeding zooplankton. The lower PUFA content in cladoceran prey from lakes at the higher end of the G. semen gradient did not affect the PUFA content of the predator C. flavicans, suggesting selective assimilation and retention of PUFA and/or feeding on other, more PUFA-rich prey. [ABSTRACT FROM AUTHOR]

In a field study, we analyzed the fatty acid (FA) composition of the benthic generalist Asellus aquaticus collected from boreal lakes, ponds, and streams across gradients in ambient nutrient levels. In laboratory feeding experiments, we tested the diet-quality and seasonal effects on somatic growth and FAs of spring- and autumn- collected Asellus that were fed four different diets containing increasing concentrations of polyunsaturated FAs (PUFA): conditioned leaf litter, algal flakes, mixed litter and algal flakes (Mixed), or Mixed plus fish-food flakes. Ambient nutrients were strong determinants of FA variation of field Asellus, explaining > 44% in total. The ratios of eicosapentaenoic acid (EPA) to total FAs, EPA:ω3, and vω:ω6 of Asellus increased up to four times with increasing trophic state and decreasing humic matter content, likely because dietary ω3 FAs were more prevalent in benthic habitats of eutrophic than of oligotrophic systems. In the feeding trials, growth of Asellus collected in both seasons was markedly lower on leaf litter than on higher PUFA diets. However, autumn-collected Asellus fed a Mixed or Mixed+fish-food diet grew 3-10 times faster, but retained < 50% EPA and PUFA than spring counterparts. Asellus optimized PUFA accumulation in spring but somatic growth in autumn. Our field survey suggests ambient nutrient concentrations modify dietary PUFA supply from basal resources, while laboratory studies show that growth response and PUFA accumulation of Asellus differ between seasons, likely due to its season-specific physiological status and diet quality. An increase in nutrients will release benthic consumers from growth limitation and favor more efficient trophic transfer. [ABSTRACT FROM AUTHOR]