In a 29 month study of bacterial populations at three sites on the Welsh River Dee, the aerobic heterotrophic bacteria increased from an average basal level of about 1.2 times 104 colony‐forming units (cfu)/ml near the source of the river, to 2 times 105 cfu/ml in the lower reaches. The ratio, total bacterial cell count: viable count, decreased from 70 in the upland reaches to 10 in the lower parts of the river. There was no apparent seasonal variation in bacterial numbers but on occasions the bacterial populations in both upland and lowland reaches of the river increased simultaneously. Fluctuations in bacterial numbers over a 50‐fold range were observed in this study. Bacterial isolates from both upland and lowland sites were predominated by two groupings of bacteria, the Pseudomonas—Agrobacterium—Alcaligenes group and the Flavobacterium—Cytophaga—Flexibacter group. Results suggested that the latter group may have been part of the autochthonous population. Seasonal variation in heterotrophic potential (Vmax) for acetate uptake was shown to occur over a 30‐fold range in the lowland reaches of the River Dee. Peaks in activity at the lowland site occurred during the summer months, the range of Vmax values for acetate ranged from 0.2 to 30 μg/1/h. Fluctuations in Vmax values from the upland site were not seasonal but were instead linked to faecal pollution, Vmax values from this site range from 0.04 to 3 μg/1/h.

A method is described for the isolation of IncH plasmid‐dependent bacteriophages present in very low numbers in sewage. An isogenic plasmid‐lacking host was used to absorb the majority of non‐specific phages. This resulted in an enrichment of 100‐to 400‐fold of the IncH‐dependent phages. A total of 27 IncH‐dependent phages were isolated and all were shown to be temperature‐sensitive RNA phages which produced turbid plaques.

Water samples collected every 8 d from three sites on the Welsh River Dee over 29 months were examined chemically and morphologically and their heterotrophic activities assessed. Bacterial population estimates were regressed on 21 independent environmental variables using multiple linear regression analysis. Prediction equations calculated from 1975–1976 data accounted for 70% of the total variation from an upland site but only 40% for a lowland site. The validity of the prediction equations was examined by correlating the predicted and observed count data of 1977–1978. The correlations of plate count values were significant for the upper (r= 0.64, P < 0.001) and middle reach (r= 0.62, P < 0.01) sites but not significant for the lowland site. The direct count correlation was significant for all three sites. The permanganate value, a measure of organic matter, was the most dominant variable in the prediction equations and was shown by principal component analysis (PCA) to be linked to particulate matter and river flow. Prediction equations constructed from the PCA data accounted for less variation than the original equations but, in general, there was an agreement between the two methods of multivariate analysis. The logarithm of the heterotrophic potential (Vmax) from the lowland site was shown to be a linear function of temperature (r= 0.7, P < 0.001). A specific activity index calculated for the lowland site data gave a median value of 8.4 times 10‐10μg/h/bacterial cell and was highly sensitive to temperature change (r= 0.74, P < 0.001) whereas the median value for the upland site was 2.6 times 10‐10μg/h/bacterial cell and did not respond significantly to temperature change (r= 0.44, P > 0.05).

In a l‐year study at Ironbridge on the Welsh River Dee the Escherichia coli popu‐lation varied over a 200‐fold range and the coliphage populations grown at 37°C (H. T. phage) and at 22°C (L. T. phage) each varied by over 100‐fold. Both the E. coli and H. T. phage counts were shown to have highly signiflcant seasonal fluctuations, which included troughs in summer and peaks in winter. The L. T. phage count appeared to maintain a baseline population in summer and had a 20‐fold less sig‐nificant association with temperature than the H. T. phage population.

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