The Relative Contribution of Benthic and Suspended Bacteria to System Biomass, Production, and Metabolism in a Low-Gradient Blackwater River

Abstract

We compare the relative contribution of bacteria in four suspended and benthic habitats to total system bacterial standing stocks, production, and respiration in the Ogeechee River, a sixth order, low-gradient, blackwater river. The habitats considered are water column, sandy main channel sediment, backwater sediment, and snags. Total system bacterial biomass ranged widely with a mean of 3240 mg C m^-2. It was greatest during winter and early summer, and peaks were dominated by bacteria in sandy sediments where from 21 to 98% of total bacterial biomass was found. Bacterial biomass in both backwater and snag habitats remained consistently low. Dye studies revealed considerable water movement into sandy sediments, which were aerobic to the greatest depth sampled (60 cm). System bacterial production was less variable than biomass, ranging from 0.84 to 8.54 mg C m^-2 n^-1 >80% of which occurred in main channel sediments. System bacterial production could not account for system biomass; hence allochthonous sources of bacteria appear to be important. Bacterial respiration was calculated from rates of production and growth efficiencies measured previously in laboratory studies. The calculated contribution of bacterial respiration to measured system respiration had a modal value of 97%, but varied widely (from 9 to 161%). Benthic bacteria appear to be a major driver of system respiration, although uncertainties in these calculations point to the critical need for a better understanding of temporal variability in bacterial growth efficiency in nature.