Epibenthic and benthic microcrustaceans (copepods, cladocerans, ostracods) from a nearshore area in southeastern Lake Michigan1

Cores of bottom sediments and overlying waters were collected from depths of 6 and 9 m in southeastern Lake Michigan, Copepod, cladoceran, and ostracod abundances were assessed in terms of known plankton. Species could be roughly divided into euplanktonic species showing no affinity for the sediments, epibenthic species occurring in the plankton but concentrated near the sediments, and benthic species inhabiting the sediments and rarely encountered in the plankton. While several epibenthic species left the sediments at night to enter the plankton, only a small fraction of the benthic species showed this migration. Epibenthic and benthic microcrustaceans in the lower 0.3 m of the water column accounted for 33-39s of the standing stock (by numbers) of the microcrustaceans within the 9-m depth contour. Preliminary information on the local food web indicates that the microcrustaceans have a significant role in the trophodynamics of the inshore area. Several of the species found in southeastern Lake Michigan are tolerant of eutrophic, mesotrophic, or moderately saline waters. Although the planktonic crustaceans of the open waters of the Great Lakes have been studied extensively, the epibenthic components have been largely overlooked, probably due to the technical difficulties in collecting from this habitat. Conventional zooplankton sampling gear (nets, water bottles) does not sample the organisms living on the sediments (benthic) or within a few centimeters of the sediments (epibenthic). Sampling gear such as dredges and grabs may collect epibenthic and benthic organisms, but the mesh sizes of screens in these devices and of the sieves used for washing samples are generally coarse (500 p) and do not retain the smallest organisms. Where special gear has been used, an abundant and diverse microfauna has been collected (Moore 1939; Cole 1955). Although the trophodynamics of the cpibenthic and benthic microfauna have not been well studied in temperate freshwaters, studies in Arctic ponds (Fenchel 1975), brackish waters (Jansson 1974)) and marine waters (McIntyre 1964) have shown that microfauna arc important in benthic communities both as competitors with the ’ This research was supported by the Indiana & Michigan Power Company. Great Lakes Research Division contribution 214. macrobenthos for food resources and as prey for benthic invertebrates and fish. In the Great Lakes, microbenthos probably have a similarly significant role. In summer 1974, we investigated the nearshore arca of southeastern Lake Michigan to determine the species composition and abundance of the epibenthic and benthic microfauna. The samples contained a diverse assemblage of organisms from a number of taxonomic groups. We here discuss primarily the copepods, cladocerans, and ostracods. The project was undertaken in cooperation with S. C. Mozley and we acknowledge his advice and help. Divers were J. Dorr and G. Gitschlag. Materials and methods Duplicate sediment cores were collected at five sites 80-100 m apart (Fig. 1) along two transects parallel to shore and extending southward from the intake structures of the Donald C. Cook Nuclear Plant in southeastern Lake Michigan. The plant was not operational in 1974 although water was occasionally drawn from the lake during periods of circulating-pump testing. Transect C (6-m depth contour) was sampled during the afternoon of 22 July 1974, LIMNOLOGY AND OCEANOGRAPHY 1059 NOVEMBER 1977, V. 22( 6) 1060 Evans and Stewart I Q DISCHARGE STRUCTURE