Abstract

 The deepwater amphipod Diporeia hoyi has disappeared from Lake Erie and much of Lake Ontario at depths above 80 m. It comprised up to 80 % of the benthos, and was an important food resource supplying approximately 20 % of the fisheries energy budget. The community on soft sediments has changed with the exotic mussel Dreissena bugensis now constituting over 90% of the benthic biomass down to 60 m depth. Between 30 - 40 m, the filamentous bacterium Thioploca ingrica was found to occur commonly at sites from where the Diporeia has disappeared. Our laboratory has launched an extensive research project, which is designed to explore the probable causes of the disappearance of one of the important component of the fisheries food resource. Our investigation includes sediment chemistry, bacterial production and sediment-pseudofaeces assays with laboratory cultures of Diporeia hoyi, Hyalella azteca and Microtox. Results with sediment pore-water (Microtox) experiments showed no toxicity in sediments previously inhabited by amphipods. Presence of filamentous bacterium Thioploca had no effect on amphipod survival. Laboratory bioassays with Hyalella and Diporeia responded somewhat differently to test sediments and mussel pseudofaeces. Sediment from sites with dense Dreissena populations had lower Diporeia survival. Amphipod survival and growth was greatest in sediment that rapidly lost its Diporeia population in 1993.