Limnol. Oceanogr., 44(2), 1999, 259–272

Studies were conducted during August and March–April in the Sargasso Sea south of Bermuda to examine rates of bacterial growth and picoplankton consumption by microbial consumers. Bacterial growth rates were estimated from [3H]thymidine (TdR) incorporation rates, while grazing rates were determined using fluorescently labeled prey (FLP). In addition, net bacterial growth rates were calculated from changes in bacterial abundance during 24-h incubations. The latter measurements were compared to net growth rates obtained by subtracting picoplankton grazing mortality rates from bacterial TdR growth rate estimates (TdR minus FLP). Overall, bacterial growth rates determined by TdR uptake were similar to FLP grazing rates during the March–April cruise, indicating an approximate balance between production and removal processes. Bacterial growth rates during August, however, were approximately twice the rates of grazer removal. Net bacterial growth rates determined from TdR growth rates minus FLP grazing rates were similar to net growth rates estimated from changes in abundance for samples collected near the surface during both cruises. However, rates of change of bacterial abundances during incubations were generally greater than rates predicted from TdR minus FLP for samples collected in the deep euphotic zone during both cruises. These discrepancies might be explained by several factors, including the inclusion of prochlorophytes in the bacterial counts and/or the stimulation of bacterial growth during containment. The TdR conversion factor also was an important consideration when comparing net bacterial growth rates estimated from changes in bacterial abundance to net growth rates determined from TdR minus FLP. Small nanoplanktonic protists (,5 mm) were responsible for most of the picoplanktonic grazing activity. Doubling times of 0.9–18.3 d for the heterotrophic nanoplankton were estimated based on the removal rates of picoplankton. The complexity of the microbial food web of this oligotrophic ecosystem is such that relatively little carbon may be recovered from nonliving organic material and passed on to metazoa. Numerous studies have firmly established the importance of picoplanktonic microorganisms (organisms # 2 mm in size; primarily bacteria, cyanobacteria, prochlorophytes, and some protists) in energy flow and nutrient cycling in marine planktonic ecosystems. Phototrophic picoplankton contribute significantly to phytoplankton biomass and production, while nonphotosynthetic picoplankton are instrumental in carbon and nutrient transformation and remineralization. Consequently, research has focused on the rates of production of these minute primary and secondary producers in freshwater and marine habitats and on their fate in aquatic food webs. Trophic relationships involving picoplankton in oceanic environments have unique significance in light of investigations that have concluded that picoplankton biomass and production are highly important in oligotrophic oceanic ecosystems (Fogg 1995). Bacterial biomass constitutes a major