LIPID-CONCENTRATION AND SIZE VARIATION OF BYTHOTREPHES (CLADOCERA, CERCOPAGIDAE) FROM LAKES ERIE, HURON, AND MICHIGAN

Lipid concentrations of Bythotrephes cederstroemi were compared among three Great Lakes, Erie, Huron and Michigan, in an effort to investiga te the phenotypic plasticity in size displayed among the lakes. Four d evelopmental stages were measured in Lakes Erie and Huron and two stag es were studied in Lake Michigan. With a gravimetric extraction method , the total lipid concentration range (mu g lipid mu g dry weight(-1), expressed as percent)for Bythotrephes was estimated to be 10-19%. Sta tistically significant differences were found in lipid concentrations of Bythotrephes among lakes and developmental stages. Lake Erie had si gnificantly higher lipid concentration values than Lake Huron for stag es 2 through 4, and had similar values to Lake Michigan for the analyz ed stages I and 4. The fir-st instar had indistinguishable lipid conce ntrations among Lakes Erie, Huron, and Michigan. Even though animals f rom Lake Erie were significantly smaller, the data suggest that they w ere nor less well nourished. We hypothesize that selective mortality i mposed by visual predators on larger Bythotrephes and the lack of deep water refuges in Lake Erie has encouraged the smaller size of Bythotr ephes found there in comparison to those found in Lakes Huron and Mich igan.