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The grazing impact of protists on phytoplankton primary production in a highly oligo- trophic area of the subtropical north-east Atlantic, south-east of the Azores, was studied using the dilution method. The phytoplankton community in the experiments, analysed by flow-cytometry, was dominated by Prochlorococcus and Synechococcus although larger phototrophic groups, beyond flow-cytometry sensitivity, were also present. The microzooplankton community was dominated by small gymnodinoid dinoflagellates and aloricate ciliates, and its biomass was low, averaging 0.7 mg C m -3 . We measured average phytoplankton growth rates of 0.76 and 0.22 d -1 in surface waters and in deep chlorophyll maximum (DCM) waters respectively; mortality rates due to protists grazing were 0.58 and 0.22 d -1 , respectively. We did not find significant differences in growth and mortality rates of the different phytoplankton groups, i.e. Prochlorococcus, Synechococcus or nanoeukaryotes; thus no prey preference was apparent from the results. Protists consumed on average 79 and 109% of pri- mary production in surface and DCM waters respectively, controlling phytoplankton growth in the area. The net primary production closely oscillated around zero in the chlorophyll a analyses and was positive in the flow-cytometry analyses, suggesting a balanced or slightly autotrophic phytoplankton- protist compartment.

Protist control of phytoplankton growth in the subtropical north-east Atlantic