Mesocosm studies linking phytoplankton diversity and zooplankton nutrition: The role of essential fatty acids in complex natural communities

Abstract This multi‐year study highlights the ecological role of diversity and fatty acid availability in phytoplankton communities and their complex interactions with zooplankton. These findings, achieved through large‐scale mesocosm experiments, provide novel evidence of the critical role of biochemical components in shaping zooplankton community composition in natural environments, which short‐term and less complex laboratory studies cannot reveal. Over several years in a meso‐oligotrophic lake, we investigated how phytoplankton diversity affects zooplankton growth and community composition via fatty acid availability. Results show that changes in phytoplankton fatty acid profiles influenced zooplankton abundance. Physiologically essential polyunsaturated fatty acids—arachidonic, eicosapentaenoic, and docosahexaenoic acids—were crucial for the presence of key zooplankton groups, including copepods and Daphnia . The study also revealed that different zooplankton species exhibited varied responses to fatty acids, highlighting the importance of dietary complementarity within zooplankton communities. While the findings largely support previous laboratory studies, they also reveal unique complexities in natural systems, where other factors may modulate the effects of fatty acids. This research underscores the significance of considering fatty acid profiles in understanding phytoplankton–zooplankton interactions and offers experimental evidence on the ecological consequences of changes in phytoplankton diversity in natural complex communities.