Measurements of bacterial secondary production were carried out during 13 diel studies at one coastal marine station and in five lakes differing with respect to nutrient concentration and primary production. Bacterial secondary production was measured in situ every 3 to 5 h by [3H]thymidine incorporation into DNA. In some of the diel studies, these results were compared with results obtained from dark 14CO2 uptake and frequency of dividing cells. Only minor diel changes were observed. The rate of [3H]thymidine incorporation into DNA and the frequency of dividing cells varied from 23 to 194% of the diel mean. The dark CO2 uptake rate varied from 12 to 259% of the diel mean. An analysis of variance demonstrated that no specific time periods during 24 h showed significantly different production rates, supporting the idea that bacterial activities in natural assemblages are controlled by a variety of events. The best correction (r2 = 0.74) was obtained between the [3H]thymidine incorporation and frequency of dividing cells procedures from the lake water samples. The actual production rates calculated by [3H]thymidine incorporation into DNA were appreciably lower than those obtained by the frequency of dividing cells and the dark CO2 uptake techniques. Diel rates of bacterial production are discussed in relation to sampling frequency, statistical errors, and choice of method.
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