Background/Question/Methods   Rising temperatures, declining predator populations and eutrophication are three pervasive global changes taking place in aquatic ecosystems. These processes may interact in a number of ways that are likely to vary seasonally as temperature affects consumer-resource interactions and metabolic processes. We used a year-long experiment in 1000 L mesocosms to investigate the combined effects of warming (3°C), elevated nutrient loads and planktivorous fish (threespine sticklebacks, Gasterosteus aculeatus) on freshwater communities. We monitored responses at multiple trophic levels by sampling phytoplankton biomass weekly and zooplankton community structure bimonthly. Multiple ecosystem processes were also measured, including gross primary production, periphyton growth, and leaf litter decomposition.

Results/Conclusions We observed strong top-down and bottom-up effects on phytoplankton communities. Nutrient addition enhanced phytoplankton biomass and the effect was strongest when fish were present, driven primarily by fish predation on large Daphnia species. Both warming and elevated nutrients enhanced the strength of the trophic cascade but these interactions were not apparent until the onset of winter. Warming reduced phytoplankton biomass in the absence of fish, largely by increasing grazing pressure by zooplankton, while the effects were neutral or positive when fish were present. Elevated temperature allowed zooplankton to persist throughout the winter and exert top-down control over phytoplankton except when fish predation was intense. We found no evidence for any interaction between warming and enrichment. Our results suggest that climate warming may intensify trophic cascades by strengthening the top-down effects of zooplankton, especially during winter in temperate climates. Short-term studies conducted only in one season may not reflect the effects of ecological stressors or interactions among them.