The River Danube was studied in the vicinity of Vienna (Austria) to examine the Interactions among suspended matter, bacterial colonization and dissolved organic matter on a medium temporal and spatial scale. The concentration of suspended matter, the total number of particles and the number of bacteria attached to particles correlated with discharge rates. In the River Danube, 39% of the particles stained with Alcian Blue were colonized by bacteria, but only a small number of particles were heavily colonized. About 9.5% of the total bacterial community was found to b e associated with particles, abundance of free-living bacteria varied between 2.4 and 7.8 X 106 cells ml-' and attached bactenal abundance varied from 0.1 to 1.4 X 106 cells ml-l On average, 8 bacteria particle-' were found in the River Danube. Smaller particles were colonized more densely by bacteria than larger particles In the River Danube, 60 % of the particles were found to be in the size range of > 9 to 100 pm2 and 57 ' % of the attached bacteria colonized thls slze class.

1. Microbial parameters were determined at five sampling sites in the River Danube upand downstream of Vienna, Austria, twice monthly over an annual cycle. Bacterial production (BP) was estimated from thymidine and leucine incorporations; additionally, the effect of turbulence on BP and the conversion factors for converting incorporation rates into bacterial ceil production were determined using the cumulative approach. 2. BP under turbulent conditions was not significantly different from that under stagnant conditions. For thymidine, a mean armual conversion factor of 3.2 X 10^*^ cells mol"^ thymidine incorporated was calculated. For leucine, the corresponding factor was 0.07 x 10'^ cells mol"' leucine. Average annual BP calculated by thymidine incorporatior\ was significantly higher than BP calculated from leucine incorporation and ranged from 47.2 to 77.5 |ag C 1"^ day' depending on the tracer and the conversion factor used. 3. Bacterial growth rates ranged from 0.1 day"' during winter to 1.7 day"' in the summer. A strong correlation was found between temperature as well as chlorophyll a and bacterial growth when temperature was greater than 5 °C; a major spring phytoplankton bloom at a temperature below 5 "C did not increase BP. 4. Dissolved organic carbon (DOC) concentrations varied between 2 and 7.2 mg C 1"' and comprised between 50 and 92% of the total organic carbon pool in the River Danube. Based on the EXDC concentration and an assumed bacterial growth yield of 20% we calculated mean EXXI turnover times of around 60 days in the winter and less than 8 days during the summer.

To determine the role of microbes in the degradation of organic matter in the River Danube, the biochemical oxygen demand (BOD) was followed over 2 and 5 d from July to December 1993, and the fluctuations of the microbial community were monitored in incubation flasks over the incubation periods of 2 and 5 d from January to December 1993. In the BOD flasks, bacterial abundance increased by up to 63% of the initial abundance during the first l 5 h of incubation followed by an increase in flagellates, reaching their maximum about 35 h after starting the incubation. After 60 h, bacterial abundance declined to 15 % of the initial density. This rapid fluctuation indicates that the BOD method routinely used to monitor water quality may be inappropriate for measuring the degradability of organic matter since the confined bacterial community is efficiently controlled by flagellates. The role and degradability of the organic coating of suspended solids was evaluated by Alcian blue staining and transmission light microscopy. Degradation experiments revealed that the number of particles decreased by up to 50% during the incubation period indicating rapid microbial utilization of this particulate matter.

In an attempt to assess the influence of tertiary-treated sewage influx on bacterial metabolism in the Danube River, bacterial abundance, ectoenzymatic activity, faecal coliforms, faecal streptococci, chlorophyll a (chl a), concentrations of dissolved (DOC) and total organic carbon (TOC) and inorganic nutrients were measured upstream of the sewage influx and compared with sampling sites downstream . Additional samples were taken near the outlet of the sewage treatment plant . Bacterial abundance as determined by epifluorescence microscopy was compared with plate counts of total heterotrophic bacteria. Significantly higher values were obtained at the stations downstream from the sewage influx only for faecal coliforms and faecal streptococci, for glucuronidase activity and bacterial biovolume . All the other parameters were not significantly different from values obtained at the upstream sampling site . Strong seasonal dependence was detectable for nitrate with high concentrations during the winter ( ;z~ ᗸ50 pM) and low concentrations during summer (. ::100 ttM) . A distinct spring phytoplankton bloom was noticeable in the river reaching chl a concentrations of . : 70 pg 1-1 ; during the remaining seasons chl a concentrations were <ᗸ0 pg 1 -1 . Highly significant correlations were found between faecal coliform counts and glucuronidase activity . C :N :P ratios of 13 .9 :10.7 :1 (for the upstream station) and 11 .7 :9 .ᗸ :1 (for the downstream stations) of dissolved nutrients are rather low indicating severe carbon limitation of bacterioplankton . Based on our results we conclude that the heterotrophic bacterial community is not significantly effected by the input of treated sewage but faecal contamination is readily detectable over a comparatively long reach of 30 km .

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