U.S. Geological Survey, Lake Superior Biological Station, 2800 Lake Shore Drive East, Ashland, WI 54806

Summary

         Spring (early May to mid-June) crustacean zooplankton communities were monitored over a 12-year period (1989-2000) across four ecoregions of Lake Superior. Composition, density, areal biomass and mean zooplankter size varied across ecoregions. Of 12 species of zooplankton that were identified, three copepods comprised ~98% of the community: the calanoids Diaptomus sicilis and Limnocalanus macrurus, and the cyclopoid Diacyclops thomasi (listed in order of importance). On average, the largest zooplankton and greatest density and areal biomass were found in the eastern ecoregions, Keweenaw Bay and Whitefish Bay and were intermediate in the western ecoregion, Minnesota North Shore. The zooplankton community in the western Apostle Islands was the most divergent; on average, zooplankton density, areal biomass, mean zooplankter size, and the proportion of larger calanoid copepods were lower than in other ecoregions. Consistent with predictions of the top-down effect of size-selective planktivory, we found that spatial and temporal patterns of zooplankton biomass and mean size were related to temporal and spatial variation in the biomass of the principal planktivore, lake herring (Coregonus artedi). During 1988-1990, large cohorts of lake herring were produced across Lake Superior and the maturation of these cohorts in the early 1990s coincided with subsequent declines in density, areal biomass, and mean size of the calanoid copepods, particularly the larger L. macrurus. The appearance of a moderate 1998 year class of lake herring only in western Lake Superior was correlated with sharply different zooplankton community structures in eastern vs. western ecoregions. Under conditions of low lake herring abundance, zooplankton biomass increased in the eastern ecoregions largely as a result of increased density and mean size of the larger calanoid, L. macrurus. The build-up of zooplankton biomass observed in the eastern Lake Superior in the late 1990s suggests that the relatively small resident lake herring populations were not food-limited in this portion of the Lake, but their principal predator, lake trout may be food limited. A greater understanding of zooplankton productivity and the trophic links between zooplankton, lake herring and lake trout will help managers determine the size of lake trout populations that can be sustained over the long term in Lake Superior.