Competition between rotifers and cladocerans of different body sizes.

We conducted laboratory experiments to test the hypothesis that rotifers could coexist with small (<1.2 mm) but not large (>1.2 mm) cladocerans. Keratella cochlearis was excluded in <8 days by the large cladocerans Daphnia pulex and D. magna, probably through both interference and exploitative competition. On the other hand, K. cochlearis persisted for 8 weeks with two small cladocerans (Bosmina longirostris and Ceriodaphnia dubia) and excluded a third small cladoceran (D. ambigua). Similarly, Synchaeta oblonga coexisted with B. longirostris for >7 weeks, and K. testudo coexisted with D. ambigua for >4 weeks. Coexistence of small cladocerans and rotifers was always accompanied by suppression of one or both populations, probably primarily if not exclusively by exploitative competition for limiting food resources. These results indicate that the competitive dominance of cladocerans over rotifers decreases markedly with cladoceran body size and that factors other than body size may determine the competitive outcome between rotifers and small cladocerans. Our study provides a mechanistic explanation for a commonly observed pattern in natural zooplankton communities: planktonic rotifers often are abundant when only small cladocerans occur but typically are rare when large cladocerans are present.