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Natural populations of bacteria in water samples from the lower Rhine River were tested for their sensitivity to selected toxicants. Bacterial growth, measured as [3H-methyl] thymidine incorporation, was affected by additions of 5 pg Cu L-'; these concentrations were similar to the copper concentrations found in the river. Concentrates of the organic fraction from river water, obtained via XAD-columns, affected thymidine incorporation in samples of river water when added in concentrations corresponding to ambient levels. The exoenzymatic activities of phosphatase and protease decreased systematically at concentrations higher than 5 pg Cu L-' . Potassium dichromate (K2Cr207) at a concentration of approximately 3 mg L-' and a detergent tetrapropylbenzene sulfonate (TPBS) at a concentration of 5 mg L-' had detectable effects on microbial activity. Atrazine affected the growth rate but not the enzymatic activity of bacterial populations when this herbicide was added in concentrations of 50 to 200 pg L-I. Thymidine incorporation was more sensitive to all test compounds than the enzymatic activities. It is concluded that the present pollution levels, although decreased, are still likely to affect microbial processes in the river.

The effects of a quaternary ammonium compound, ditallowdimethylammonium chloride (DTDMAC), on natural populations of bacteria and phytoplankton from the lower River Rhine were examined to estimate their sensitivity to the discharges of cationic surfactants in the river basin. In short-term experiments, significant decreases in the growth rate of bacterioplankton and in the photosynthetic rate of phytoplankton were observed at a nominal concentration of 0.03 to 0.1 mg of DTDMAC liter-'. Nitrification was measured with an ion-selective electrode and by the rate of acid production in ammonium-spiked river water and was found to be only sensitive to the addition of concentrations higher than 1 mg of DTDMAC liter-'. This does not support an earlier suggestion that ammonium-oxidizing bacteria are specifically sensitive to quaternary ammonium compounds. The effect of DTDMAC on thymidine incorporation was shown to depend strongly on the concentration of suspended material, which varied with the sampling date. This effect was also quantified in experimental manipulations with Rhine water. Calculations on the partitioning of DTDMAC between water and suspended matter confirmed the role of suspended solids and showed that an increase of the dissolved DTDMAC concentration in Rhine water by circa 0.01 mg liter-' leads to a slight inhibition of the growth of heterotrophic bacteria. It is concluded that a total concentration of circa 0.01 mg of DTDMAC liter-' measured in the River Rhine is likely to have biological consequences. * Corresponding author. MATERIALS AND METHODS Water samples containing their natural communities of bacteria and phytoplankton were collected from the lower River Rhine at station Lobith, close to the German-Dutch border (863 km from Lake Constance). Samples of 20 to 40

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