Bacterial abundance and production in river sediments as related to the biochemical composition of particulate organic matter (POM) HELMUT FISCHER*, SUSANNE C. WANNER and MARTIN PUSCH Department Limnology of Lowland Rivers and Shallow Lakes, Institute of Freshwater Ecology and In- land Fisheries, Müggelseedamm 301, 12587 Berlin, Germany; *Author for correspondence (e-mail: fischer@igb-berlin.de) Received 9 February 2001; accepted in revised form 31 August 2001 Key words: Bacterial production, C/N ratio, Particulate organic matter (POM), Protein, River, Sedi- ments Abstract. 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 metabo- lism 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 sedi- ments. Detrital variables correlated strongly with bacterial abundance and production. The best correla- tion 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. Introduction The role of heterotrophic microbial processes is now central to current concepts in river ecology (Wetzel 1992; Meyer 1994). In small streams, most of the het- erotrophic activity is localized on the solid surfaces of sediments of the benthic and hyporheic zone, where it is dominated by microorganisms (Pusch and Schwoerbel 1994; Naegeli and Uehlinger 1997; Pusch et al. 1998). However, the level of these processes may vary by several orders of magnitude within and between streams, and their contribution to whole system metabolism has rarely been examined in larger rivers (Edwards et al. 1990; Schwoerbel 1994a; Fischer and Pusch 2001). The content of organic matter has been proposed as a major determinant of het- erotrophic activities in sediments (Lock 1993; Pusch and Schwoerbel 1994; Fuss Biogeochemistry 61: 37–55, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.