A DECADE OF PREDATORY CONTROL OF ZOOPLANKTON SPECIES COMPOSITION OF LAKE-MICHIGAN

From 1983 to 1992, 71 species representing 38 genera from the Calanoid a, Cladocera, Cyclopoida, Mysidacea, Rotifera, Mollusca and Harpactico ida comprised the offshore zooplankton community of Lake Michigan. Our data demonstrate that the composition and abundance of the calanoid c ommunity after 1983 is not unlike that of 1960s and that species diver sity of the calanoid community is more diverse than the cladoceran com munity in the 1990s as compared to the early 1980s. Even though the re lative biomass of the cladocerans has remained similar over the 1983-1 993 period, the species diversity and evenness of the Cladocera commun ity in the early 1990s is unlike anything that has been previously rep orted for Lake Michigan. Cladocera dominance is centered in one specie s, Daphnia galeata mendotae, and only three species of Cladocera were observed in the pelagic region of the lake in 1991 and 1992. Nutrient levels, phytoplankton biomass, and the abundance of planktivorous alew ife and bloater chub and Bythotrephes are examined as possible causes of these changes in zooplankton species composition. The increase in R otifera biomass, but not Crustacea, was correlated with an increase in relative biomass of unicelluar algae. Food web models suggest Bythotr ephes will cause Lake Michigan's plankton to return to a community sim ilar to that of the 1970s; that is Diaptomus dominated. Such a change has occurred. However, correlational analysis suggest that alewife and bloater chubs (especially juveniles) are affecting size and biomass o f larger species of zooplankton as well as Bythotrephes.