Limnol. Oceanogr., 44(8), 1999, 1871–1881

The role of the benthic bacterial community in the carbon cycle of a temperate, macrophyte-dominated oxbow was investigated in a 2-yr study from 1994 to 1996. Simultaneous incorporation of 3H-thymidine into DNA and 14C-leucine into proteins was monitored as a tool for the measurement of bacterial secondary production in the aerobic zone of the sediment along with bacterial numbers, morphotype composition, cell volumes, biomass, frequency of dividing cells (FDC), and abiotic variables. Bacterial numbers ranged from 2.5 3 109 to 8.5 3 109 cells ml21, corresponding to bacterial biomass values of 64 and 288 mg C ml21, respectively. Both bacterial production methods yielded similar and reliable results over the whole investigation period, varying between 42 and 2,350 mg C L21 h21 for the thymidine method and between 67 and 2,490 mg C L21 h21 for the leucine method, and also corresponded well with the FDC values. Only during late spring and early summer was the thymidine uptake uncoupled from leucine incorporation. Temperature was found to be of significant importance for the variation of all bacterial parameters. In addition carbon inputs from the water column were apparently crucial stimulators of benthic bacterial growth. The spring phytoplankton bloom together with higher temperatures led to a strong positive response of the bacterial community in the sediment. However, the planktonic algal production was not sufficient to cover the benthic bacterial C-demand and, as derived from mass-balance calculations, decaying macrophytes of the previous season seemed to provide the main energy source until the middle of spring. From the middle of August to the beginning of October, the period of maximal biomass of submerged macrophytes, all bacterial parameters exhibited their annual maximum. The benthic bacterial community was dependent on leachates and decay products of the submerged vegetation during this time. Temperature limitation was most probably the reason for the low biomass and production values observed during autumn and winter. Modification and decomposition processes of material in sediments are dominated by microorganisms that may be characterized as the driving force for the cycle of elements (Meyer-Reil 1994). The measurement of bacterial production is therefore of crucial importance in understanding the role of sediments for the carbon cycle in aquatic ecosystems. Soon after its introduction by Fuhrman and Azam (1980, 1982) in the early 1980s, the 3H-thymidine incorporation method became the most widely used approach for determining bacterial secondary production in aquatic environments, due to advantages, such as specificity of bacterial uptake at short incubation times and relatively inexpensive and quick handling. However, many assumptions (conversion factors for cellular carbon, produced cells per incorporated pmol or, for thymidine, content of DNA per cell) and additional investigations (isotope dilution, saturation, and time-course experiments) are needed to lead to reliable bacterial production estimates. A second approach, dealing with incorporation of labeled substances into bacterial cells is provided by the 14C-leucine incorporation method, developed by Kirchman et al. (1985) and elaborated for routine measurements by Simon and 1 Corresponding author.