Picoplankton and nanoplankton and their trophic coupling in surface watersof the Sargasso Sea south of Bermuda

Studies were conducted during August and March-April in the Sargasso Sea so uth of Bermuda to examine rates of bacterial growth and picoplankton consum ption by microbial consumers. Bacterial growth rates were estimated from [H -3]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 incub ations. The latter measurements were compared to net growth rates obtained by subtracting picoplankton grazing mortality rates from bacterial TdR grow th rate estimates (TdR minus FLP). Overall, bacterial growth rates determin ed by TdR uptake were similar to FLP grazing rates during the March-April c ruise, indicating an approximate balance between production and removal pro cesses. Bacterial growth rates during August, however, were approximately t wice the rates of grazer removal. Net bacterial growth rates determined fro m 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 duri ng incubations were generally greater than rates predicted from TdR minus F LP for samples collected in the deep euphotic zone during both cruises. The se discrepancies might be explained by several factors, including the inclu sion of prochlorophytes in the bacterial counts and/or the stimulation of b acterial growth during containment. The TdR conversion factor also was an i mportant 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 mu m) were responsible for mo st of the picoplanktonic grazing activity. Doubling times of 0.9-18.3 d for the heterotrophic nanoplankton were estimated based on the removal rates o f picoplankton. The complexity of the microbial food web of this oligotroph ic ecosystem is such that relatively little carbon may be recovered from no nliving organic material and passed on to metazoa.