Assessing the strength of top-down influences on plankton abundance in unmanipulated lakes

The trophic structure of aquatic ecosystems can potentially be controlled by two sets of factors: resource availability and predation. The importance of nutrients was demonstrated by two observations: first, that plankton abundance among lakes is strongly correlated with resource levels, and second, that experimental manipulations of resources lead to changes in plankton abundance. In contrast, while experimental manipulations have shown that predators can affect prey abundance, it is less clear whether variation in plankton abundance among lakes is related to variation in predators. To test this, we sampled 29 lakes in Quebec and Ontario. We found strong relationships between plankton biomass and resource levels. Effects of predation were evident between zooplankton and phytoplankton. However, neither chlorophyll nor zooplankton biomass was related to piscivorous fish presence, after controlling for phosphorus. Other published studies that have reported correlations between plankton abundance and characteristics of the fish community failed to control for trophic status. Thus, there appears to be no published correlative evidence that variance in plankton abundance among lakes is related to variation in fish assemblages, even though the link is mechanistically plausible.