RELATIVE VULNERABILITIES OF NATURAL ROTIFER AND CILIATE COMMUNITIES TO CLADOCERANS - LABORATORY AND FIELD EXPERIMENTS

1. Two experiments with plankton communities from Storrs Pond (NH), one conducted in the laboratory and one in field enclosures, assessed the impact of different cladocerans on rotifers and ciliated protozoa. 2. The smallest cladoceran, Bosmina longirostris, did not depress rotifer or ciliate growth rates while the intermediate sized cladoceran, Daphnia galeata mendotae, reduced ciliate growth rates in the enclosure experiment but had only a marginal effect in the jar experiment. D. galeata mendotae had no effect on any of the rotifers in either experiment. 3. In both experiments the largest cladoceran, Daphnia pulex, depressed the growth rates of ciliates and those rotifers known to be vulnerable to interference competition. Polyarthra vulgaris, previously shown to be resistant to cladoceran interference, was the only rotifer unaffected by D. pulex in the field experiment but was depressed by the much higher densities of this cladoceran in the laboratory experiment. 4. Cladocerans did not affect phytoplankton or bacterioplankton abundance in either experiment. Therefore the mechanism most likely to be responsible for the suppressive effect of cladocerans on rotifers and ciliates in these experiments is direct mechanical interference or predation, rather than exploitative competition.