Short-Term Effects of Zooplankton Manipulations on Phosphate Uptake

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Abstract
Samples of epilimnetic water were collected, manipulated to alter their zooplankton fauna, and incubated in situ for 24 h. We hypothesized that removal and concentration of zooplankton would decrease and increase phosphate turnover time, respectively, and that the presence of zooplankton would increase the accumulation of added 32PO4 by algae (particles greater than 1.0 μm) relative to bacteria (particles 0.2–1.0 μm). Although the treatments produced significant changes in turnover time and size distribution, neither hypothesis was supported; removal of zooplankton did not decrease turnover time, and it reduced the dominance of 0.2- to 1.0-μm particles in phosphorus uptake. However, concentrating microzooplankton frequently increased turnover time. Zooplankton removal caused a relative increase in ciliates, suggesting that these protozoa may be limited by their predators rather than by resource levels. Our results suggest that microzooplankton were the most important grazers and that the nutrient environment of phytoplankton was not directly affected by mesozooplankton. Phosphate dynamics were probably determined by bacteria, the protozoan predators of bacteria, and the supply of dissolved organic carbon.