ABSTRACT: A seasonal study on the quantification of energy fluxes through the microbial compartment and on bacterial morphotype succession was performed in a hypertrophic shallow backwater branch, which had shifted from a macrophyte-dominated clear-water to a phytoplankton-dominated turbid-water state from 1992 to 1994. Filamentous cyanobacterial species dominated the phytoplanktonic compartment during summer. Bacterial numbers ranged from 2.7 x 10⁹ to 9.8 x 10⁹ cells l^-1, corresponding to biomass values of 35 and 119 µg C l^-1, respectively. Temperature, dissolved organic carbon (DOC), primary production and soluble phosphorus were found to explain most of the variation of bacterial numbers and biomass in the system. Bacterial morphotypes exhibited a seasonal succession pattern with rods and vibrios as the most abundant morphotypes. Vibrios dominated during the bloom of cyanobacteria in summer, while rods were found to increase rapidly after the breakdown of the bloom in autumn and winter. Filamentous bacteria with cell lengths of up to 120 µm bloomed during a short period in spring, making up more than 60% of the total bacterial biomass. With principal component analysis we could extract 2 main factors influencing the variation of bacterial morphotypes, namely an abiotic/inorganic factor, containing temperature, oxygen, nitrate and ammonium, and a trophic factor, containing DOC, total nitrogen, total 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 µg C l^-1 h^-1 with a range from 0.5 to 7.0 µg 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 b-glucosidase, leu-aminopeptidase and alkaline phosphatase were established and the relations to the other investigated parameters are described. By integrating the phosphatase and aminopeptidase activity into a phosphorus and nitrogen budget for the bacterial and phytoplanktonic compartment in the Alte Donau, we were able 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 primary production exceeded by far the bacterial carbon demand, but periods with the reverse situation occurred from October to May. Temperature and carbon supply were seen as the main factors for limiting bacterial growth in the Alte Donau during the cold months. The importance of viral lysis and predation 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 led us to the conclusion that the equilibrium shift of the Alte Donau has resulted 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.

KEY WORDS: Backwater system · Eutrophication · Alternative equilibria · Nutrients · Bacterioplankton · Morphotypes · Enzyme activities · Bacterial carbon demand

Published in AME Vol. 17, No. 3 (1999) on June 18
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 1999