Material fluxes through the procaryotic compartment in a eutrophic backwater branch of the River Danube

A seasonal study on the quantification of energy fluxes through the microbi al compartment and on bacterial morphotype succession was performed in a hy pertrophic shallow backwater branch, which had shifted from a macrophyte-do minated de ar-water to a phytoplankton-dominated turbid-water state from 19 92 to 1994. Filamentous cyanobacterial species dominated the phytoplanktoni c compartment during summer. Bacterial numbers ranged from 2.7 x 10(9) to 9 .8 x 10(9) cells l(-1), corresponding to biomass values of 35 and 119 mu g C l(-1), respectively. Temperature, dissolved organic carbon (DOC), primary production and soluble phosphorus were found to explain most of the variat ion of bacterial numbers and biomass in the system. Bacterial morphotypes e xhibited a seasonal succession pattern with rods and vibrios as the most ab undant morphotypes. Vibrios dominated during the bloom of cyanobacteria in summer, while rods were found to increase rapidly after the breakdown of th e bloom in autumn and winter. Filamentous bacteria with cell lengths of up to 120 mu m bloomed during a short period in spring, making up more than 60 % of the total bacterial biomass. With principal component analysis we cou ld extract 2 main factors influencing the variation of bacterial morphotype s, namely an abiotic/inorganic factor, containing temperature, oxygen, nitr ate and ammonium, and a trophic factor, containing DOG, total nitrogen, tot al phosphorus and chlorophyll a. Variation of filamentous bacteria, however , could not be explained by these 2 factors. Bacterial secondary production amounted on average to 3.1 mu g C l(-1) h(-1) with a range from 0.5 to 7.0 mu g C l(-1) h(-1). The impact of top-down factors like grazing and viral lysis is expected to influence both the occurrence of filamentous bacteria and bacterial production rates. Enzyme kinetics of beta-glucosidase, leu-am inopeptidase and alkaline phosphatase were established and the relations to the other investigated parameters are described. By integrating the phosph atase and aminopeptidase activity into a phosphorus and nitrogen budget for the bacterial and phytoplanktonic compartment in the Alte Donau, we were a ble to show that there was no P limitation over the whole year, while N was possibly limiting at the beginning of algal blooms. On an annual basis pri mary production exceeded by far the bacterial carbon demand, but periods wi th the reverse situation occurred from October to May. Temperature and carb on supply were seen as the main factors for Limiting bacterial growth in th e Alte Donau during the cold months. The importance of viral lysis and pred ation in controlling bacterial growth during the summer months was pointed out. A comparison of the investigated bacterial parameters with those of a mesotrophic but macrophyte-dominated branch of the same backwater system le d us to the conclusion that the equilibrium shift of the Alte Donau has res ulted in high primary production of the autotrophic procaryotic compartment but not in the expected increase of energy flux through the compartment of the heterotrophic bacterioplankton.