Experimental removal of early instar black fly larvae from 3 oligotrophic lake-outlet streams in southern Quebec showed that larvae reduced phytoplankton biomass, but had little effect on bacterioplankton biomass. The rates of decline in bacterioplankton and phytoplankton biomass with distance downstream from 6 lake outlets were determined before the insecticide Bacillus thuringiensis var. israelensis was used to remove black fly larvae from 3 of the streams. The rates of decline in bacterioplankton and phytoplankton biomass were measured again after the larvae were removed, and the differences in the rates of decline between before and after were used as indicators of the rates of biomass ingestion by the black fly larvae. The proportion of bacterioplankton ingested/m stream length was <0.081%/m in all streams, but the proportion of phytoplankton ingested/m stream length was 0.35%/m and 0.24%/m in 2 of the 3 manipulated streams. Downstream declines in both bacterioplankton and phytoplankton persisted in the absence of black fly larvae. Factors other than black fly feeding (such as biofilm adhesion and ingestion by other filter-feeders) were responsible for 50 to 90% of the total downstream loss rate of bacteria, whereas factors other than black fly feeding were responsible for 0 to 55% of the total downstream loss rate of phytoplankton. Apparently, free-living bacteria were not ingested to a great extent by black fly larvae in these oligotrophic lake-outlet streams, and the link between bacteria in the plankton and their potential black fly predators seemed weak. The relative importance of algivory over bacterivory is expected to be greater in eutrophic lake-outlet streams, where the ratio of bacterial C to phytoplankton C is smaller than in oligotrophic lake-outlet streams, and in systems with a greater abundance of later-instar larvae, which are less efficient at capturing bacteria than early instar larvae.
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