Zooplankton effects on phytoplankton in lakes of contrasting trophic status

Abstract

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 Daphnia 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 Daphnia on the nutrient‐limited algal assemblage. The cyanobacteria‐dominated algal community of Clear Lake was resistant to grazing impacts, responding relatively weakly only to the highest densities of Daphnia. Good correspondence between estimates of algal biomass made from chlorophyll measurements and microscopic examination were obtained, and, in Castle Lake, concordance between estimated community productivity turnover times made from species‐specific growth determinations and community productivity measurements was observed. The contrasting responses to experimental zooplankton manipulations in the three lakes provide support for the view that the coupling between zooplankton and phytoplankton is strongest in lakes of intermediate productivity and imply that food‐web alterations at the top of the food web are most likely to propagate to the level of the phytoplankton (and therefore lake water quality) in lakes of moderate trophic status.