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Filter-feeding black fly larvae in lake outlet streams convert suspended bacteria and algae into benthic biomass, available to fish. Positive correlations between larval abundance and both bacterioplankton and phytoplankton were significant at the p < 0.10 level among 5 outlet streams of varying trophic status in the Eastern Townships of Quebec, and either food type alone explained &sim;28% of the variance in larval abundance among streams (Chapter 1), suggesting bacterioplankton and phytoplankton to play similar roles in larval distribution. However, when larvae were removed from 3 of 6 oligotrophic outlet streams examined in the Mauricie region of Quebec, calculated ingestion rates revealed larvae to ingest roughly 20 times more phytoplankton carbon than bacterial carbon (Chapter 2). Phytoplankton ingestion rates were high enough to satisfy daily carbon requirements, but bacterivory was low (Chapter 2), indicating that bacterial biomass contributes little to black fly diets in most systems, and will be less useful than phytoplankton biomass when using food to predict larval distribution. The majority of bacteria in streams form a carbon link to fish through sediment food webs, rather than through black fly larvae.

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