Declines in benthic macroinvertebrate populations in southern Lake Michigan, 1980-1993

Densities of the major benthic macroinvertebrate groups declined dramatical ly 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 1980-198 1 to 4800.m(-2) in 1992-1993 at sites in the 16-30 m depth interval and fro m 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 exten t. Declines in oligochaetes and sphaeriids occurred uniformly throughout th e nearshore region and over the entire sampling period. Declines in these t wo groups were most Likely related to reductions in phosphorus loads and a general decrease in productivity in the nearshore area over the sampling pe riod. 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 ne arshore waters may be having a negative impact on Diporeia in deeper waters by decreasing the amount of food available to this surface-feeding detriti vore.