Macro-zooplankter responses to simulated climate warming in experimental freshwater microcosms

1. We report data collected from 48 replicated microcosm communities created to mimic plant-dominated shallow lake and pond environments. Over a 2-year period, the microcosms were subjected to warming treatments (continuous 3 degreesC above ambient and 3 degreesC above ambient during summer only), a nutrient addition treatment and the presence or absence of fish. We tracked macro-zooplankter dynamics, censusing cladoceran populations at the species level, copepods at the order level and ostracods as a class. 2. Responses to warming were subtle. Cladoceran diversity and overall abundance were not significantly affected by warming, although measures of community evenness increased. Warming effects on patterns of population trajectories tended to be strongly seasonal and most apparent during periods of pronounced increase. Populations of the prevalent cladocerans, Chydorus sphaericus and Simocephalus vetulus, displayed idiosyncratic patterns, with evidence in the case of S. vetulus for a negative relationship between warming and body-size at maturity. Copepod populations were reduced in size by warming, but those of ostracods increased. 3. The effects of the nutrient addition and fish treatments were strong and consistent, interacting little with warming effects in statistical models. Zooplankter abundance tended to be the highest in the fish-free microcosms receiving additional nutrient inputs and lowest when fish were present and no nutrients were added. Both treatments reduced cladoceran diversity and community evenness. 4. We suggest that warming, independently, is unlikely to supplant the effects of changing nutrient loading and fish predation as the major driver of zooplankter dynamics in shallow lakes and ponds. Moreover, in the situations where warming was of significant influence in our experiment, the distinction between summer-only warming and year-around warming was blurred. This suggests that warming effects were most pervasive during the summer, at the upper end of the temperature spectrum.