Square Lake, Minn., mimics an ideal of the biomanipulation approach. The lake's zooplankton community is dominated by large (body size) herbivorous cladocerans (Daphnia pulicaria) whose collective grazing abilities have maintained a reduced standing crop of algae, apparently since 1926. The physical and chemical environmental factors that defineDaphnia'sniche in Square Lake are discussed and include stable stratification, a large hypolimnetic volume, and phosphorus concentrations within certain threshold limits. These environmental limitations seem to be generally important for providing a metalimnetic refuge forDephnia. Future biotic reconstructions that aim to encourage large-bodiedDaphniashould consider these environmental limitations.