Benthic bacterial secondary production measured via simultaneous H-3-thymidine and C-14-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 temper ate, 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-leuc ine into proteins was monitored as a tool for the measurement of bacterial secondary production in the aerobic zone of the sediment along with bacteri al numbers, morphotype composition, cell volumes, biomass, frequency of div iding 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 va lues of 64 and 288 mu g C ml(-1), respectively. Both bacterial production m ethods yielded similar and reliable results over the whole investigation pe riod, varying between 42 and 2,350 mu g C L-1 h(-1) for the thymidine metho d and between 67 and 2,490 mu g C L-1 h(-1) for the leucine method, and als o corresponded well with the FDC values. Only during late spring and early summer was the thymidine uptake uncoupled from leucine incorporation. Tempe rature was found to be of significant importance for the variation of all b acterial parameters. In addition carbon inputs from the water column were a pparently crucial stimulators. of benthic bacterial growth. The spring phyt oplankton bloom together with higher temperatures led to a strong positive response of the bacterial community in the sediment. However, the planktoni c algal production was not sufficient to cover the benthic bacterial C-dema nd and, as derived from mass-balance calculations, decaying macrophytes of the previous season seemed to provide the main energy source until the midd le of spring. From the middle of August to the beginning of October, the pe riod of maximal biomass of submerged macrophytes, all bacterial parameters exhibited their annual maximum. The benthic bacterial community was depende nt on leachates and decay products of the submerged vegetation during this time. Temperature limitation was most probably the reason for the low bioma ss and production values observed during autumn and winter.