Distribution of Epibenthic Microcrustaceans in Nearshore Lake Michigan

Epibenthic distributions at depths of 11, 17, and 23 m in nearshore Lake Michigan were investigated by sampling the sediments and water column on a monthly basis from May to November 1977. The taxa with a greater portion of the population occurring in/near the sediments than in the water column were Acanthocyclops vernalis, cyclopoid copepodids IV-V, and Eurytemora affinis. The taxa occurring mostly in the water column were early stage cyclopoids (nauplii and copepodids I-III) and Bosmina longirostris. For all epibenthic taxa except nauplii, abundances in/near the sediments declined as water depth increased; total mean densities at the 11, 17, and 23 m depths were 13,300; 3,500; and 1,500/m2, respectively. In contrast, abundances in the water column did not decline with increased depth; total mean densities at the three depths were 15,300; 17,000; and 14,400/m3. Based on concentrations in the sediments and vertical distributions in the water column, many taxa were more aggregated near the bottom in spring/early summer than later in the year. This coincided with the occurence of a fine layer of detritus over the sandy sediments at this time. Nighttime dispersal off the bottom into the water column was less apparent in spring/early summer than in midsummer. Our findings suggest that in any quantitative study of microcrustaceans in nearshore areas of the Great Lakes, the sediments as well as the water column must be considered.