In this thesis I quantified abundance and production of sediment bacteria in a 6th-order lowland river (Spree, Germany). Additionally, the influence of the amount and the biochemical composition of dissolved and particulate organic matter, and of the sediment structure as controlling factors for bacteria were examined. The Spree is characterised by high nutrient and phytoplankton loads. Its sediments are dominated by sands with shifting ripples and dunes. I hypothesised that the activity of bacteria attached to sediment surfaces is a major factor in the metabolism of this river ecosystem.

Bacterial production was measured in various habitats of the Spree using the [14C]leucine incorporation technique that I specifically adapted for the application to sediment and epiphytic samples. I found that this method could measure bacterial production in a wide range of aquatic habitats, including fluvial sediments, if substrate saturation and isotope dilution were determined. In order to quantify the contribution of sediments to total system metabolism of the Spree, I mapped the dominating habitats and compared bacterial production in sediments, epiphyton, and the pelagic zone. The area production in the pelagic zone mostly was higher than the area production in the epiphyton. However, bacterial biomass in the uppermost 2 cm of sediments exceeded pelagic biomass by a factors of 6 - 22, and sedimentary bacterial production was 17 - 35 times higher than in the overlying water column.

Using a combination of size exclusion chromatography and organic carbon detection, I measured the retention of various fractions of dissolved organic carbon (DOC) from river water in perfused sediment cores. High proportions of low-molecular-weight substances and polysaccharides, and low but significant proportions of humic substances were retained in sediment cores after perfusion. Bacterial abundance, production and turnover times were correlated with the retention of various DOC fractions, especially polysaccharides, indicating a differential microbial utilization of DOC. I also determined the characteristics of particulate organic matter (POM) in vertical gradients of two differing sediment types of the Spree. These detrital variables correlated strongly with bacterial abundance and production, with the best correlation found for those variables indicating organic matter quality and quantity (rs = 0.90 for particulate nitrogen (PN) and rs = 0.81 for protein with bacterial abundance). Although bacterial biomass comprised only 0.7 % of the POC (1.9 % of the PN, 3.4 % of the protein) in sediments, the turnover of sedimentary organic carbon was fast (median = 62 d), especially in the shifting sediments

The system metabolism of the Spree was dominated by processes located within the sediments. However, only the uppermost sediment layer showed significant bacterial activities in stratified sediments. In contrast, high bacterial activity was found in sediment depths down to 20 cm within shifting sands. Thus, these sediments which are typical for lowland rivers have a significant influence on the whole ecosystem metabolism. Seasonal dynamics of bacterial activity were controlled by temperature and by the supply with and the composition of organic substances. On the other hand, bacteria influenced the biogeochemistry of the Spree by selectively utilizing different fractions of dissolved and particulate organic matter.