Ecological theory suggests that community structure determines how ecosystems respond to perturbation. However, this idea is not well-tested empirically, in part because it is difficult to define the appropriate reference state for evaluating post-perturbation dynamics in field experiments. In this study, we (1) evaluated two different methods of determining the reference state, and (2) tested hypotheses derived from theory and previous experiments about the effects of herbivore community structure on autotroph responses to nutrient pulses in freshwater plankton communities. We hypothesized that (1) phytoplankton are less sensitive to a nutrient pulse with increasing zooplankton biomass, size, and Daphnia abundance, and (2) phytoplankton recover more quickly from the nutrient pulse with increased zooplankton biomass and increased abundance of taxa with rapid numerical responses, especially small-bodied cladocerans. We tested these hypotheses in field mesocosm experiments, which crossed four zooplankton community structures with two nutrient pulse treatments in a fully randomized factorial design. We found that phytoplankton sensitivity decreased with increased zooplankton biomass, size, and Daphnia biomass regardless of the method used to define the reference state, supporting our first hypothesis and suggesting that results for sensitivity are robust to methodology. This result, together with previous work, clearly demonstrates that zooplankton have a strong effect on phytoplankton sensitivity. Further, as predicted, return times were shorter with increased zooplankton biomass and biomass of small cladocerans. However, contrary to expectations, return times also declined with Daphnia and copepod biomass, suggesting that the effects of zooplankton on return time are more complex than we hypothesized. Moreover, conclusions for return time depended somewhat on methodology, indicating that we may need to reevaluate whether the concept of return time is relevant for field experiments.
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