Significance of bacterial biomass in the nutrition of a freshwater isopod (Lirceus sp.).

The quantitative significance of bacterial biomass in the nutrition of detritivores remains equivocal. We have used tritiated thymidine to specifically label stable macromolecules in natural assemblages of sediment-associated and detritus-associated bacteria. This material was presented to the isopod (Lirceus sp.) and incorporation of bacterial biomass measured. The isopod incorporated roughly 1 ng bacterial carbon (mg wet wt.)-1 h-1 from leaf discs and about 6 ng mg-1 h-1 from sediment. Calculation of grazing rate from changes in cell counts yields grazing rates from 2.3-17.9 ng C mg-1 h-1. Even the maximum grazing rate, which is an overestimate of C assimilated, represents only 14.7% of C respired by the isopod.