Benthic amphipods are a keystone species in food webs of marine and freshwater systems. The amphipod in the phylogroup Diporeia found in the Great Lakes has historically been a dominant member of the benthic community and is critical to the fisheries food web. This study investigates water depth, life stage, sex, production, lipid content, and gut fullness as influences and outcomes of Diporeia distribution in Lake Superior. Samples were collected at 2- to 3-week intervals from May to October 2003 from ten stations along one transect in Lake Superior near the Keweenaw Peninsula of Michigan to determine seasonal trends in abundance and condition. Diporeia comprised, on average, 52% of the benthic macroinvertebrate community. Abundance and productivity of Diporeia is greatest within the slope region (depths of 30 to 125 m) with estimated biomass and annual P/B (production/biomass) values of 0.57 and 0.73 g/m2 for this habitat. Young-of-the-year Diporeia were the most abundant life stage in the shelf in the spring while adults were most abundant in the slope and profundal regions in fall. The greatest mean percent gut fullness occurred in September and October while the least full and empty gut occurrences were observed in June and July. The slope of the length-weight relationship for Diporeia in Lake Superior is lower (less biomass per unit length) than that observed for lakes Huron, Michigan, and Ontario. The greatest lipid content was 41% DW in July at 20 m with the overall average lipid content of Diporeia from Lake Superior in 2003 at 32% with a decline observed from September to October. The description of Diporeia abundance, distribution, and life history presented here for Lake Superior, and compared with that of other Great Lakes, will be of use in support of studies of lower food web bioenergetics.
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Vol. 35 • No. 4