Bacterial abundance and production in river sediments as related to the biochemical composition of particulate organic matter (POM)

The major proportion of heterotrophic activity in running waters is localized on the solid surfaces of sediments in the benthic and hyporheic zone and is dominated by microorganisms. However, this assertion is based on the studies of small streams, and little is known about the microbial metabolism of organic matter in river ecosystems. We therefore explored the relationships between bacterial abundance and production and the gradients of organic matter quality and quantity in sediments of a sixth-order lowland river (Spree, Germany). We found vertical gradients of detrital variables (particulate organic matter (POM), particulate organic carbon (POC), nitrogen (PN), and protein) and of bacterial variables (abundance, production, turnover time, and proportion of bacterial carbon in total POC) in two different sediment types. These gradients were steeper in stratified sediments than in the shifting sediments. Detrital variables correlated strongly with bacterial abundance and production. The best correlation was found for detrital variables indicating substrate quantity and quality (r(S) = 0.90 for PN with abundance). Although bacterial biomass comprised only 0.7% of the POC (1.9% of PN, 3.4% of the protein) in sediments, the turnover of sedimentary organic carbon was fast (median = 62 d), especially in the shifting sediments. Our findings demonstrate that sediment dynamics significantly foster organic carbon metabolism in river systems. Thus, these sediments, which are typical for lowland rivers, strongly influence the metabolism of the whole ecosystem.