THE IMPACT OF A STREAM-DWELLING HARPACTICOID COPEPOD UPON DETRITALLY ASSOCIATED BACTERIA

Natural densities of common stream-dwelling harpacticoid copepods, Attheyella spp., reduced the density and biomass and increased the area-specific and cell-specific production of detritally associated bacteria. Bacterial density was reduced by as much as 58% and rod-shaped bacteria were reduced as much as 27%. Attheyella selectively removed larger rod-shaped bacteria. This resulted in a significant change in the size-frequency distribution of rod-shaped bacteria and a decrease in median cell length by 17-30%. The removal of larger sized cells by Attheyella probably contributed to the observed reduction in bacterial biomass. With copepods present, bacterial biomass was reduced as much as 45%. Estimated rates of ingestion of bacterial carbon by copepods are comparable to values for marine harpacticoids. Bacterial biomass appears to be a more important food source to microdetritivores than to macrodetritivores in streams.