The deepwater amphipod Diporeia hoyi has disappeared from Lake Erie and much of Lake Ontario at depths < 80 m. This amphipod had supplied 20 percent of the fisheries energy budget in the Great Lakes. The exotic mussel Dreissena bugensis now forms most of the benthic biomass above 60 m depth, but Diporeia is absent over large areas where Dreissena are rare. The filamentous bacterium Thioploca ingrica is now common at many sites between 30 and 40 m where Diporeia has disappeared. Fisheries and Oceans, Canada, investigated the causes of the decline by examining the sediment chemistry, bacterial production and conducted sediment bioassays using Diporeia, Hyalella and Microtox® Microtox® showed no evidence of toxicity in sediments now devoid of Diporeia. Amphipod survival and growth was greatest in sediment that rapidly lost its Diporeia population in 1993. Presence of Thioploca had no effect on Diporeia survival. Hyalella was more sensitive than Diporeia to test sediments and to filtered water from mussel cultures. Sediment from sites with dense Dreissena populations had lower Diporeia survival. A diet of mussel pseudofaeces caused significantly lower survival in both Hyalella and Diporeia. The exact mechanism causing lower survival is currently unknown and may be related to a nutritional problem or associated waste metabolites.

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