Date of this Version
Published in J. Great Lakes Res. 29(1):14–33.
Trends in benthic macroinvertebrate populations were examined in inner and outer Saginaw Bay, Lake Huron, from 1987 to 1996. These years represent the time period after phosphorus abatement, but immediately before (1987 to 1990) and after (1991 to 1996) colonization of the bay by the zebra mussel, Dreissena polymorpha. In 1987 to 1990, densities of the major macroinvertebrate groups in the inner and outer bay were not significantly different from, or were greater than, densities reported just prior to abatement efforts in the early 1970s. Oligochaete densities in the deepwater/silt region of the inner bay were trending downward between 1988 and 1991, but pollution-tolerant forms dominated the community, indicating the system was eutrophic just prior to Dreissena colonization. Dreissena impacts on the macroinvertebrate community varied depending on the particular habitat. At shallow-water/sand sites in the inner bay, Gammarus increased, and sphaeriids declined after Dreissena colonization, but no changes were observed in oligochaetes and chironomids, and overall species diversity showed little change. At deepwater/silt sites in the inner bay, densities of oligochaetes and chironomids declined just after the peak in Dreissena, but then returned to levels generally similar to those found prior to Dreissena. The oligochaete trophic index at deepwater/silt sites indicated a shift from eutrophic to more oligotrophic indicator species after Dreissena became established, and species diversity increased. In the outer bay, Diporeia and sphaeriids declined after Dreissena peaked, but few other changes were observed. Total non-dreissenid macroinvertebrate biomass (AFDW) in the inner bay, and in shallow areas of the outer bay, did not change as a result of Dreissena colonization. On the other hand, biomass in the deeper regions of the outer bay decreased because of the loss of Diporeia. Changes in the inner and outer bay typify the growing dichotomy between nearshore and offshore communities in the Great Lakes since Dreissena became established.