PREDATION ON CILIATES BY FRESH-WATER CALANOID COPEPODS - RATES OF PREDATION AND RELATIVE VULNERABILITIES OF PREY

1. The susceptibility of ciliates in a mesotrophic lake to predation by Epischura lacustris, Diaptomus minutus and D. pygmaeus was assessed during summer. Oligotrichs, particularly Strobilidium velox (c. 43 mu m), were removed efficiently by adult copepods and contributed substantially to the diet of female D. minutus. The presence of approximately 1.6 adult Epischura 1(-1), or sixteen adult female Diaptomus 1(-1), could halt the growth of S. velox populations in summer. 2. Factors affecting the rate at which copepods prey on ciliates were examined in experiments with D. pygmaeus and cultured ciliates. Rate of predation on S. velox, the preferred species, became saturated at 5 S. velox ml(-1) (45 ng C ml(-1)) and did not change with a 10-fold increase in alternative algal food. 3. Behavioural differences among ciliates, and the presence of other ciliates, contributed to differences in ciliate susceptibility to predation and suggest reasons why rates of removal of ciliates are not related to ciliate size. 4. By feeding selectively, at high rates, calanoids may suppress populations of some ciliates and thereby influence microzooplankton community structure.