The Summer (May through August) community of crustacean zooplankton and zebra mussel (Dreissena polvmorpha) larvae at a 15 m sample site in Lake Michigan near Michigan City was evaluated for the years 1990 through 1994. Yearly mean densities were compared with investigations done at the same sample location by Johnson (1972) and Ball State University (unpublished data) during 1970 and 1971, respectively to investigate possible changes in community composition during the past 25 years. Potential declines in crustacean zooplankton relating to the 1991 introduction of the filter-feeding zebra mussel were also evaluated.Overall declines in major species since 1970-71 included Bosmina longirostris, Daphnia retrocurva, and Diacyclops thomasi. Recent increases in the nearshore fish population of alewife (Alosa pseudoharengus) and spottail shiner (Notropis hudsonius) may have influenced the densities of these species in recent years. Copepod nauplii were the only crustaceans to increase in overall abundance since the period of 1970-71, as the number of invertebrate predators appeared to decrease. Other species did not show any noticeable change in long-term abundance or during the 1990-94 period.Zebra mussel larvae were first observed in the zooplankton complex studied on June 17, 1991. -Densities of larvae reached maximal levels for the period of study in 1994 with two large reproductive pulses of 195,000 and 210,000/m3 in June and August, respectively. The mean abundance during 1994 made up 65% of the total crustacean zooplankton/zebra mussel community complex. Evaluation of the vertical stratification of zebra mussel larvae in 1994 revealed a strong (r2=0.75, p<0.0005) tendency for veligers to remain in the upper 10 m of the water column due to thermal stratification. / Department of Biology

Zooplankton samples collected May through August, 1990 to 1996 at site M, 15 m depth in the Indiana waters of Lake Michigan were examined for microcrustaceans, rotifers, and zebra mussel (Dreissena polymorpha) veligers. Abundance numbers per m3 were combined with Coffman's (1995) data to quantify recent trends in the zooplankton.Rotifers dominated the community, with Polyarthra vulgaris, Colurella, Keratella crassa, and K. cochlearis the most prevalent species. Copepods, primarily nauplii and copepodid forms, were the next most abundant taxa. Except for 1994 and 1995, Cladocera abundance was higher than zebra mussel veliger abundance. The cladoceran assemblage was dominated by Bosmina Iongirostris, with very few Daphnia noted. A precipitous decrease in zebra mussels veligers from 1994 to 1996 warrants further investigation to determine the nature and the extent of the population decline.Regression analysis of zebra mussel veliger abundance against abiotic and biotic factors demonstrated veligers varied directly with rotifer abundance and water column temperature and indirectly with cladoceran and yellow perch (Perca flavescens) abundance. A similar regression model of rotifer abundance revealed direct correlation with calanoid, cladoceran, zebra mussel veliger, and yellow perch abundance. / Department of Biology