Seasonal Variation in Heterotrophic Activity and Population Density of Planktonic Bacteria in a Clean River

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Abstract
(1) Water samples were collected from two sites on the unpolluted R. Hull, North Humberside, at intervals over 2 yr, and the following variables measured: heterotrophic activity as indicated by potential for glucose mineralization (Vm); density of total bacteria by direct counting; the mean activity of individual bacteria as indicated by Vm per bacterium; and density of viable bacteria. (2) Heterotrophic activity (Vm) showed marked seasonal periodicity, with highest values in summer. These high values were partly explicable by high density of total bacteria and partly by high activity of the individual bacteria. Low Vm values in winter tended to be a result of low density and low activity of individual bacteria. (3) It is suggested that the high density of total bacteria sometimes found in summer was a result of high temperature and low river discharge, while the high summer activity of individual bacteria was related to high temperature. (4) The absence of a positive correlation between density of total bacteria and discharge or rainfall indicated that allochthonous bacteria were not important. (5) Density of viable bacteria varied considerably, but showed no definite seasonal pattern.