Seasonal patterns of grazing and nutrient limitation of phytoplankton in a eutrophic lake

Observations and experiments on zooplankton grazing and nutrient (N and P) limitation of phytoplankton of Lake Mendota, Wisconsin, revealed that the relative strength of the two factors varied markedly during the seasonal succession of phytoplankton. Furthermore, seasonal changes in grazing and nutrient limitation were caused not only by seasonal changes in zooplankton community structure and nutrient availability but also by changes in phytoplankton community structure, which led to changes in edibility to zooplankton. The spring phytoplankton bloom of diatoms and small flagellates was highly vulnerable to grazing by both cyclopoid copepods and Daphnia , although Daphnia had a much stronger effect. Increased Daphnia grazing in late spring resulted in low phytoplankton biomass (the clear‐water period) and a phytoplankton community dominated by colonial green algae — taxa vulnerable to grazing by Daphnia but not other zooplankton. Nutrients (N and P) did not limit phytoplankton growth during the spring bloom or the clear‐water period. After the clear‐water period, the summer phytoplankton community was dominated by blue‐greens and Ceratium . Grazing effects by both Daphnia and copepods were low in summer, while nutrient limitation (both N and P) became severe. Reduced grazing impacts in summer may have resulted from prior intense grazing impacts, which led to dominance of grazing‐resistant taxa. These results suggest that seasonal variation in the strength of grazing and nutrient controls in eutrophic lakes results from changes in zooplankton biomass and community structure, nutrient availability, and phytoplankton community structure interacting to determine phytoplankton seasonal succession.