Benthic bacterial secondary production measured via simultaneous 3H-thymidine and 14C-leucine incorporation, and its implication for the carbon cycle of a shallow macrophyte-dominated backwater system

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 H-3-thymidine into DNA and C-14-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 x 10(9) to 8.5 x 10(9) cells ml(-1), corresponding to bacterial biomass values of 64 and 288 mu g C ml(-1), respectively. Both bacterial production methods yielded similar and reliable results over the whole investigation period, varying between 42 and 2,350 mu g C L-1 h(-1) for the thymidine method and between 67 and 2,490 mu g C L-1 h(-1) 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.