INFLUENCE OF WASTE-WATER TREATMENT ON THE MICROBIAL ECOLOGY OF A LARGE, TEMPERATE RIVER SYSTEM - THE DANUBE RIVER

In an attempt to assess the influence of tertiary-treated sewage influ x on bacterial metabolism in the Danube River, bacterial abundance, ec toenzymatic activity, faecal coliforms, faecal streptococci, chlorophy ll a (chi a), concentrations of dissolved (DOG) and total organic carb on (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 a bundance as determined by epifluorescence microscopy was compared with plate counts of total heterotrophic bacteria. Significantly higher va lues were obtained at the stations downstream from the sewage influx o nly for faecal coliforms and faecal streptococci, for glucuronidase ac tivity and bacterial biovolume. All the other parameters were not sign ificantly different from values obtained at the upstream sampling site . Strong seasonal dependence was detectable for nitrate with high conc entrations during the winter (approximate to 250 mu M) and low concent rations during summer (approximate to 100 mu M). A distinct spring phy toplankton bloom was noticeable in the river reaching chi a concentrat ions of approximate to 70 mu g l(-1); during the remaining seasons chi a concentrations were <20 mu g l(-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 11.7:9.2:1 (for the downstream stations) of dissolved nutrients are rather low i ndicating severe carbon limitation of bacterioplankton. Based on our r esults we conclude that the heterotrophic bacterial community is not s ignificantly effected by the input of treated sewage but faecal contam ination is readily detectable over a comparatively long reach of 30 km .