Spatial distribution and respiration of bacteria in stream-bed sediments

Bacterial abundance, biomass, and activity were determined in the hyporheic zone of a third-order mountain stream (Steina, Black Forest, Germany). Two layers of the sediment biofilm were analysed separately: POM which was loosely associated to sediment surfaces (LAPOM), and POM associated strongly (SAPOM). Abundance, biomass, and activity of bacteria were strongly correlated with the amount of LAPOM in sediments. In October, mean bacterial abundance in the hyporheic zone was 7.2 x 10 8 /cm 3 and showed a significant vertical gradient. Mean bacterial biomass amounted to 2.7 g C for a standard hyporheic sediment volume of 1 m 2 area and 40 cm depth. The percentage of respiring cells was significantly higher in LAPOM (11.4%) than in SAPOM (4.2%). Microbial respiration in the standard sediment volume was 1.95 g O 2 /d, which also correlated with the LAPOM content. Bacterial production was estimated at 0.27 g C/d in the standard sediment volume. Our results suggest that there is a distinctive vertical zonation of POM content, microbial biomass, and respiration rate within the hyporheic sediments. In the microscale, hyporheic biofilms show a functional layering concerning bacterial abundance and activity.