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 104colony‐forming units (cfu)/ml near the source of the river, to 2 times 105cfu/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, thePseudomonas—Agrobacterium—Alcaligenesgroup and theFlavobacterium—Cytophaga—Flexibactergroup. 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 ofVmaxvalues for acetate ranged from 0.2 to 30 μg/1/h. Fluctuations inVmaxvalues from the upland site were not seasonal but were instead linked to faecal pollution,Vmaxvalues from this site range from 0.
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