The growth of heterotrophic bacteria in the surface waters of warm core rings1

The “seawater culture” approach is evaluated as a means of observing changes in cell numbers over time to estimate specific growth rates and production of heterotrophic bacterioplankton in oligotrophic open ocean waters. We use models of exponential growth limited by grazers, and of logistic growth limited by substrate availability, to estimate the specific growth rates of natural bacterial populations in warm core Gulf Stream rings. The two models yielded somewhat different rate estimates, but in general the results suggest that in situ specific growth rates for bacterial populations in oligotrophic ocean waters are as fast as rates in rich coastal waters and lakes (1–2 d −1 ). Estimates of hourly bacterial production averaged 10% of hourly primary production. Since bacteria grew at night, up to 40% of the daytime primary production was estimated to pass through the bacteria every 24 h. Estimates of removal rates by grazers are about the same as bacterial growth rates. However we estimate that only 3–7% of primary production was converted to protozoan biomass via ingestion of bacteria. Our data suggest that simple dilution of natural samples with cell‐free water is a useful, valid method for estimation of bacterial specific growth rates.