Abundance and distribution of benthic invertebrates, with emphasis on Diporeia, along the Keweenaw Peninsula, Lake Superior

The benthic invertebrate community in Lake Superior is an important component in the fisheries food web. Among the Great Lakes, only Lake Superior contains populations of Diporeia spp. that have not been reduced or extirpated in the last 20 years. The objectives of this study were to determine the abundance and distribution of benthic invertebrates along the Keweenaw coast, and to investigate the reproductive ecology of Diporeia. The benthic community was sampled monthly from 1998 to 2000, using a ponar dredge along three transects with distinctly different bathymetry. Each transect contained shelf, slope, and profundal depth regions. Diporeia was the most abundant invertebrate, accounting for 48% of the invertebrate community, while chironomids composed 21.3%, oligochaetes 18.7%, and sphaeriids 8.4%. Only chironomids showed a statistically significant change in monthly density, being more abundant in September than earlier or later in the year. Diporeia and Sphaeriidae were most densely distributed along the slope, and the distribution of chironomids and oligochaetes was even more specific within the slope region, with peak densities occurring at 50 meters on all three transects. Diporeia reproduced when the water temperature was 4°C or colder. Along the shelf, Diporeia released their young in May, whereas in the slope and profundal regions young appeared in June and September, suggesting two periods of reproduction, late spring and summer. In the shelf area Diporeia grew at a faster rate than those offshore. There was a linear correlation between the length of female Diporeia and the number of eggs carried, similar to what has been shown for the other Great Lakes. The benthic invertebrate community peaks in abundance in the slope region of Lake Superior along the Keweenaw Peninsula. This region may provide important habitat and nutrients for other organisms in the Lake Superior food web