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The abundance of pico- and nanophytoplankton, bacteria and heterotrophic nanoflagellates, and grazing rates on phototrophic pico- and nanoplankton and bacterioplankton were assessed along a salinity gradient (0.2–34.4) in the Mississippi River plume in May 2000. Grazing rates were established by serial dilution experiments, and analysis by flow cytometry allowed differentiation of grazing rates for different phytoplankton subpopulations (eukaryotes, Synechococcus spp., Prochlorococcus spp.). Grazing rates on phytoplankton tended to increase along the salinity gradient and often approached or exceeded 1 day −1 . Phytoplankton net growth rates (growth – grazing) were mostly negative, except for positive values for eukaryotic nanoplankton in the low-salinity, high-chlorophyll region. Grazing pressure on bacteria was moderate (∼0.5 day −1 ) and bacteria gained positive net growth rates of ∼0.3 day −1 . Eukaryotic nanophytoplankton were the major phototrophic biomass and protozoan food source, contributing 30–80% of the total consumed carbon. Bacteria were the second most important food source at 9–48% of the total consumed carbon. Synechococcus spp. and Prochlorococcus spp. remained an insignificant portion of protozoan carbon consumption, probably due to their low contribution to the total pico- and nanoplankton biomass. Group-specific grazing losses relative to standing stocks suggest protozoan prey preference for eukaryotes over bacteria. Protozoan grazers exerted a major grazing pressure on pico- and nanophytoplankton, but less so on bacteria.

Photo- and heterotrophic pico- and nanoplankton in the Mississippi River plume: distribution and grazing activity