Zooplankton effects on phytoplankton in lakes of contrasting trophic status

Studies were carried out in three lakes of strongly contrasting trophic state to evaluate how the effects of zooplankton on phytoplankton vary as a function of lake productivity. Chlorophyll and total P concentrations differed by 2-3 orders of magnitude among ultra-oligotrophic Lake Tahoe, meso-oligotrophic Castle Lake, and strongly eutrophic Clear Lake. Three experimental designs involving short-term (4 d) manipulations of nutrients, ambient zooplankton, and the common crustacean grazer Duphnia were performed in each lake. Algal responses were assessed at both the “aggregate, ” community (Chl, primary productivity, nutrient status) and species levels. Experiments in Lake Tahoe revealed a high degree of nutrient sensitivity but negligible grazing impacts by the very low densities of ambient zooplankton. Daphnia grazing had substantial impacts on Tahoe phytoplankton, however, indicating high susceptibility to grazing in this assemblage of relatively small-sized algal species. Castle Lake experiments revealed strong direct and indirect impacts of both ambient zooplankton and Duphnia on the nutrient-limited algal assemblage. The cyanobacteria-dominated algal community of Clear Lake was resistant to grazing impacts, re-sponding relatively weakly only to the highest densities of Duphnin. Good correspondence betwee