No abstract

ABSTRACT. To determine density changes in both the zebra mussel, Dreissena polymorpha, and native mussels, Unionidae, in Lake St. Clair, surveys were conducted in 1990, 1992

Summary We examined spatial patterns in population characteristics (density, biomass, mean body length) and physiological condition (lipid content, length‐weight) of the amphipod Diporeia spp. in Lake Michigan by collecting samples at up to 85 sites in late summer 1994 and 1995. Variables were examined relative to water depth and three lake regions: south, central and north. Most major river systems are found in the south, and this region is more nutrient‐enriched compared to the north. Over all sites, mean density was 5240‐2, biomass was 4.1g dry wt m‐2, and mean body length was 5 mm. While maximum densities were related to depth, with a peak at 30–70 m, greatest densities occurred on the west side of the lake, and low densities were found in the south‐east, north‐east and lower Green Bay. High densities in the west probably resulted from upwelling, and reduced densities in the south‐east may reflect food competition with Dreissena polymorpha (zebra mussel). Lipid content, weight per unit length, and mean length declined with increased water depth, but depth‐related trends were most evident in the south. Overall, mean lipid content and weight per unit length were significantly lower in the south (16.6% dry wt, 0.59 mg at 5 mm body length) compared to the north (23.7% dry wt, 0.78 mg at 5 mm body length). These regional differences may have resulted from greater diatom availability in the north and competition from D. polymorpha in the south. Triacylglycerols and phospholipids were the dominant lipid classes in all three regions. Although the mean proportion of triacylglycerols, the energy‐storage lipid, was lower in the south than in the north, regional differences in proportions of lipid classes were not significant. Mean lipid content and weight per unit length of Diporeia in the south were lower than values found in the late 1980s prior to the establishment of Dreissena. Mean lipid content of mature individuals is now at levels considered a minimum for successful reproduction.

Densities of the major benthic macroinvertebrate groups declined dramatically at sites shallower than 50 m in southern Lake Michigan between 1980 and 1993. Declines in Diporeia, Oligochaeta, and Sphaeriidae ranged from 40 to 75% at these depths. Total densities declined from 16 800·m-2 in 1980-1981 to 4800·m-2 in 1992-1993 at sites in the 16-30 m depth interval and from 21 300 to 11 300·m-2 at sites in the 31-50 m depth interval. Changes at sites deeper than 50 m were minimal; only sphaeriids declined to any extent. Declines in oligochaetes and sphaeriids occurred uniformly throughout the nearshore region and over the entire sampling period. Declines in these two groups were most likely related to reductions in phosphorus loads and a general decrease in productivity in the nearshore area over the sampling period. On the other hand, declines in Diporeia occurred mainly in the later portion of the sampling period and were greatest in the southeastern region of the lake. Densities at many sites in the southeastern region were < 100·m-2 in 1993, down from 3000-8000·m-2 in 1980-1987. We hypothesize that the filtering activities of large populations of Dreissena polymorpha in nearshore waters may be having a negative impact on Diporeia in deeper waters by decreasing the amount of food available to this surface-feeding detritivore.