Warming air temperatures alter the timing and magnitude of reservoir zooplankton biomass

Warming air temperatures are altering many physical, chemical, and biological processes in freshwater ecosystems. Process-based ecosystem models are important tools for predicting potential future changes to water quality due to warming by simulating complex ecological interactions. However, while previous studies have modeled climate-driven impacts on water quality (e.g., water temperature, dissolved oxygen, phytoplankton), few have included zooplankton, despite their critical role in freshwater ecosystems. Zooplankton functional groups can exhibit variable responses to warming temperatures, but the implications of these responses on freshwater ecosystems are not well understood. To understand the effects of warming on reservoir zooplankton and water quality, we configured and calibrated a process-based freshwater ecosystem model simulating three zooplankton functional groups and then applied multiple air temperature scenarios to explore ecosystem responses. We found that warming air temperature increased modeled rotifer biomass and decreased modeled cladoceran and copepod biomass. While the timing of annual rotifer peak biomass was not altered by warming air temperatures, annual copepod biomass peaks were delayed by 54–100 days within a year across warming scenarios. The timing of cladoceran biomass peaks was more variable in response to warming. Changes to the timing and magnitude of modeled zooplankton biomass were likely driven by changes in nutrients and phytoplankton, as we observed a trophic mismatch between phytoplankton and zooplankton biomass. These results highlight the importance of including zooplankton functional groups in process-based models when exploring the future effects of climate change on freshwater ecosystems, as changes in zooplankton communities can directly and indirectly alter ecosystem dynamics.