The influence of mixing on rotifer assemblages of Michigan lakes

Abstract

Seasonal changes in the rotifer assemblages of 42 lakes in northern lower Michigan was closely related to lake mixing characteristics, basin morphometry, and the presence of an oxygenated coldwater refuge. Three major classes of lakes (dimictic, discontinuous polymitic, and continuous polymictic) were evident by their capacity to maintain coldwater species as seasons progressed from winter through fall. The disappearance of coldwater assemblages from dimictic lakes coincided with oxygen depletion in the hypolimnion or with erosion of the hypolimnion through mixing. Coldwater species disappeared from large discontinuous polymictic lakes when deep epilimnetic mixing occured in late summer and fall. Species assemblages of nearly all stratified lakes converged with those of continuous polymictic lakes when the hypolimnetic refuge deteriorated in summer and fall. Local weather conditions, however, between years had a pronounced effect on the persistence of cold water species through the seasons by affecting the temperature and oxygen conditions of the hypolimnion. Large lakes of the region contain many of the coldwater species of the Laurentian Great Lakes but some taxa are conspicuosly absent. Cold stenothermal rotifers persist in the lakes of the region despite adverse environmental conditions. Their life histories and ability to form resting stages permit them to escape periods of oxygen depletion and thermal stress. In contrast, the crustacean glacial marine fauna (i.e. Mysis relicta, Limnocalanus macrurus, and Scenecella calanoides) was absent from all of the study lakes even though many of the present-day basins were once connected to the Laurentian Great Lakes. These species long life cycles, lack of diapausing stages, and limited dispersal may make them vulnerable to local extinction with the deterioration and loss of the coldwater refuge.