Predation by three glacial opportunists on natural zooplankton communities

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Abstract
We examine the influence of three glacial opportunist predators, Mysis relicta, Limnocalanus macrurus, and Senecella calanoides, on natural zooplankton communities of central Ontario [Canada] through a series of feeding experiments in small enclosures (23.6 L). Estimates of in situ clearance rates by M. relicta match previously determined rates, with the following gradation of values: Asplanchna sp. > daphnids and bosminids > Epischura lacustris > large cyclopoids, Chydorus sphaericus and small Diaptomus sp. > L. macrurus > S. calanoides. Clearance rats by M. relicta are similar for all daphnids and bosminids. Hypolimnetic species like Daphnia longiremis and Eubosminia longispina are apparently eliminated by M. relicta, while similarly vulnerable species survive because they have an epilimnetic refuge from M. relicta. Limnocalanus macrurus and S. calanoides prey primarily on copepods, Diaphanosoma spp., and rotifers in the hypolimnion. When both clearance rates and population densities of M. relicta, L. macrurus, and S. calanoides are taken into account, the total predatory impact of M. relicta is much larger than that of the two relict copepods. Based on a theoretical comparison of measured clearance rates by predators with estimated rates of prey recruitment, we conclude that differences in species composition and abundance between relict and nonrelict lakes described in a previous survey are due principally to predation by M. relicta.

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