Phytoplankton responses to nutrient and grazer manipulations among northeastern lakes of varying pH

Short-term nutrient enrichment and zooplankton exclosure experiments were conducted at 14 lakes representing various stages of acidification (pH 4.6-6.8). We measured changes in chlorophyll as an indicator of the severity of nutrient limitation and grazing intensity and compared these with independent measures of P limitation (cell P quotas and phosphatase activity) and grazing (zooplankton densities and inferred community grazing rates). Results from nutrient enrichment experiments showed good correspondence to measured phosphatase activity but not cell P quotas. Phytoplankton in acidic lakes (pH < 5.0) responded more strongly to nutrient enrichment and exhibited fourfold higher biomass-specific phosphatase activity compared with nonacidic lakes. Phytoplankton responses to the removal of macrozooplankton did not exhibit any consistent pattern among lakes of varying acidity. Although the largest herbivores (Daphnia) were more abundant in nonacidic lakes, their absence in acidic lakes was compensated for by increased abundance of smaller species (principally Diaptomus minutus and Bosmina longirostris). The magnitude of the response to grazer removal was positively correlated with lake zooplankton densities at the time of the experiment, and the use of a nonlinear model enabled us to explain 84% of the variation in this response among the 14 sites.