RESPONSES OF PLANKTON, TURBIDITY, AND MACROPHYTES TO BIOMANIPULATION IN A SHALLOW PRAIRIE LAKE

We evaluated effects of complete fish kill in a large, shallow, eutrophic lake in Minnesota. Low densities of Bosmina and Chydorus (<100.L-1) were replaced in the early spring/early summer by high densities of larger Daphnia galeata and D. pulex (>100.L-1) during the first year after the fish kill (1988). During peak daphnid abundance (May-June), chlorophyll a concentrations and edible phytoplankton were reduced, water transparency increased, and submerged macrophytes expanded. Orthophosphate and ammonia were detectable during clear-water phases, indicating that phytoplankton were not severely nutrient limited. Increased water transparency in subsequent years (1989-90) apparently was related to decreased sediment resuspension and lower algal biomass owing to the expansion of submerged plants. The fish kill invoked a strong initial response that transmitted to the phytoplankton level and increased water clarity. Higher transparency persisted during subsequent years because submerged macrophytes both responded to improved water clarity and subsequently prolonged it. These results support the contention that removal of fish from shallow lakes may invoke 'cascading impacts' that enhance water clarity, stimulate macrophytes, and establish a, new steady state.