Comparison of bacterial production in sediments, epiphyton and the pelagic zone of a lowland river

Abstract

1. The microbial metabolism of organic matter in rivers has received little study compared with that of small streams. Therefore, we investigated the rate and location of bacterial production in a sixth‐order lowland river (Spree, Germany). To estimate the contribution of various habitats (sediments, epiphyton, and the pelagic zone) to total bacterial production, we quantified the contribution of these habitats to areal production by bacteria. 2. Large areas of the river bottom were characterized by loose and shifting sands of relatively homogenous particle size distribution. Aquatic macrophytes grew on 40% of the river bottom. Leaf areas of 2.8 m2 m−2 river bottom were found in a 6.6 km river stretch. 3. The epiphyton supported a bacterial production of 5–58 ng C cm−2 h−1. Bacterial production in the pelagic zone was 0.9–3.9 μg C L−1 h−1, and abundance was 4.0–7.8 × 109 cells L−1. Bacterial production in the uppermost 2 cm of sediments ranged from 1 to 8 μg C cm−3 h−1, and abundance from 0.84 to 6.7 × 109 cells cm−3. Bacteria were larger and more active in sediments than in the pelagic zone. 4. In spite of relatively low macrophyte abundance, areal production by bacteria in the pelagic zone was only slightly higher than in the epiphyton. Bacterial biomass in the uppermost 2 cm of sediments exceeded pelagic biomass by factors of 6–22, and sedimentary bacterial production was 17–35 times higher than in the overlying water column. 5. On a square meter basis, total bacterial production in the Spree was clearly higher than primary productivity. Thus, the lowland river Spree is a heterotrophic system with benthic processes dominating. Therefore, sedimentary and epiphytic bacterial productivity form important components of ecosystem carbon metabolism in rivers and shallow lakes. 6. The sediments are focal sites of microbial degradation of organic carbon in a sand‐bottomed lowland river. The presence of a lowland river section within a river continuum probably greatly changes the geochemical fluxes within the river network. This implies that current concepts of longitudinal biogeochemical relationships within river systems have to be revised.

Theoretical and empirical conversion factors for determining bacterial production in freshwater sediments via leucine incorporation

“…This approach was taken during recent studies (this article; Marxsen 1996Marxsen , 1999Marxsen , 2001; Buesing and Gessner 2003;Fischer and Pusch 1999). It has the advantage that there is no measurable isotope dilution that has to be taken into account.…”

“…This is especially true if the data are compared, which were achieved when empirical CFs were determined at leucine saturation levels as well as the data for theoretical CFs. Therefore, high bacterial production determined in freshwater sediments by the leucine uptake method in recent years (e.g., Marxsen 1999Marxsen , 2001; Fischer and Pusch 2001;Buesing 2002) were not overestimates caused by inappropriate use of CFs from marine systems, but represent true high bacterial production in these environments.…”

“…Bacterial production measurement-Subsamples from each of the three batch cultures were incubated with 3 H leucine with a final concentration of 50 µmol L -1 (Marxsen 1996;Fischer and Pusch 2001;Buesing and Gessner 2003). Incorporation of leucine was stopped after 1 h by adding trichloroacetic acid (TCA) to a final concentration of 5% w/v.…”

“…Growth rates and production of bacteria can be estimated from the incorporation of radiolabeled nucleic or amino acids into bacterial DNA or protein, respectively (Fuhrman and Azam 1980;Kirchman et al 1985). Despite some theoretical advantages of the thymidine technique, which follows the incorporation of thymidine into bacterial DNA (Robarts and Zohary 1993), many researchers have preferred the incorporation of radiolabeled leucine into protein as a superior technique (e.g., Riemann and Azam 1992; Thomaz and Wetzel 1995;Marxsen 1996; Fischer and Pusch 2001;Buesing and Gessner 2003). One reason for this preference is that the leucine method is more sensitive because the production of a bacterial cell requires 10 times the incorporation of leucine into protein than thymidine into DNA (Riemann and Bell 1990).…”

“…In most of these studies, the theoretical CF from a marine system was used to calculate bacterial carbon production from leucine incorporation into protein. Especially in freshwater sediments, surprisingly high bacterial production values were obtained (e.g., Marxsen 1996Marxsen , 2001; Fischer and Pusch 2001;Buesing 2002). Thus, the question arises whether these high production values are overestimates resulting from the application of inappropriate conversion factors from marine systems.…”