Warming advances top‐down control and reduces producer biomass in a freshwater plankton community

Global warming has been shown to affect ecosystems worldwide. Warming may, for instance, disrupt plant herbivore synchrony and bird phenologyin terrestrial systems, reduce primary production in oceans, and promote toxic cyanobacterial blooms in freshwater lakes. Responses of communities will not only depend on direct species-specific temperature effects, but also on indirect effects related to bottom-up and top-down processes. Here, we investigated the impact of warming on freshwater phytoplanktoncommunity dynamics, and assessed the relative contribution of nutrient availability, fungal parasitism, and grazing therein. For this purpose, we performed an indoor mesocosmexperiment following seasonal temperature dynamics of temperate lakes and a warmed (+4°C) scenario from early spring to late summer. We assessed phytoplanktonbiomass, C:N:P stoichiometry and community composition, dissolved nutrient availabilities, fungal parasite (i.e., chytrid) prevalence, and zooplanktonabundance. Warming led to an overall reduction in phytoplanktonbiomass as well as lower C:P and N:P ratios, while phytoplanktoncommunity composition remained largely unaltered. Warming resulted in an earlier termination of the diatom spring bloom, and an epidemic of its fungal parasite ended earlier as well. Furthermore, warming advanced zooplankton phenology, leading to an earlier top-down control on phytoplanktonin the period after the spring bloom. Linear model analysis showed that most of the observed variance in phytoplanktonbiomass was related to seasonal temperature dynamics in combination with zooplanktonabundance. Our findings showed that warming advanced grazer phenologyand reduced phytoplanktonbiomass, thereby demonstrating how bottom-up and top-down related processes may shape future phytoplanktondynamics.