Considerable current effort in limnology focuses on the contributions of nutrient supply and food web structure to the regulation of productivity in planktonic ecosystems. This effort promotes a synthesis of community and ecosystem perspectives. The main goal of this proposal is to investigate an important question that has emerged from such recent syntheses: how does the strength of phytoplankton-zooplankton coupling change between lakes of differing trophic status? We propose to answer this question by performing experimental manipulations of zooplankton and nutrients in mesocosms in four lakes which encompass a broad range of trophic states. Factorial manipulations of zooplankton and nutrients will directly assess grazing impacts and the extent of enhancement of algal growth by zooplankton nutrient regeneration. Exposure of phytoplankton to a broad range of zooplankton densities will reveal non-linear algal responses and evaluate their responses to zooplankton densities that might arise from seasonal or inter-annual variation. Finally, by evaluating algal responses at the species, community, and aggregate ("ecosystem") levels, we will try to integrate approaches characteristic of population, community, and ecosystem ecology. The results of this study will assist both theoretical and applied limnologists in understanding the complex interaction between phytoplankton and zooplankton. This interaction represents the crucial link which transfers photosynthetic production to consumers higher in the food web and which translates changes in food web structure into changes in ecosystem properties such as primary productivity.
