Planktonic community structure determines the fate of bacterial production in a temperate lake

We examined the fate of planktonic bacterial production and the balance between bacterial growth and grazing mortality in the surface waters of Upton Lake, New York. Growth rates were measured by the incorporation of [3H]thymidine into DNA. Grazing rates on bacteria were determined with small cells produced by a mutant strain of Escherichia coli and made either fluorescent or radioactive to monitor feeding. Bacterial community turnover times calculated from either growth or grazing rates ranged from 1.5 to 16 d. On the basis of these data and results from 29 other studies, most bacterial communities appear to have turnover times substantially >1 d. Our measurements of feeding rates on bacteria frequently exceeded estimates of growth. Limitations of precision and doubts about the accuracy of methods make attempts to balance measurements of bacterial growth and grazing with current techniques unrealistic. The fate of bacterial production depends on planktonic community structure. Flagellates were the primary consumers of bacteria in winter and fall. At other times, Daphnia galeata consumed most of the bacterial production. Ciliates and rotifers were never important bacterial grazers. In Upton Lake large populations of Daphnia effectively “break” the microbial loop and funnel bacterial production to higher consumers.