Phytoplankton succession in microcosm experiments under simultaneous grazing pressure and resource limitation

The influence of grazing pressure on the occurrence and outcome of nutrient competition among planktonic algae was studied in two-chamber microcosms where there was a flow in both directions between a light reactor without zooplankton and a dark reactor with zooplankton (Daphnia fongi-spina and Daphnia magna). The phytoplankton inoculum was a mixed, natural assemblage. Zoo-plankton could influence the dynamics of phytoplankton both by selective grazing and by differential excretion of limiting nutrients. Grazing pressure did not prevent the occurrence of nutrient limi-tation in algae and, hence, of nutrient competition between them. Zooplankton did, however, influence the outcome of competition by lowering Si: P ratios. A comparison with my previous experiments shows that diatoms need higher Si: P supply ratios for dominance over green algae in the presence of grazers than in grazing-free competition with steady or weekly pulsed nutrient SUPPlY. Phytoplankton ecology has long been dominated by the view that community dy-namics are controlled by physical factors