MEPS 221:29-38 (2001)  -  doi:10.3354/meps221029

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

Mario Quevedo*, Ricardo Anadón

Departamento de Biología de Organismos y Sistemas, Area de Ecología, Universidad de Oviedo, Catedrático Rodrigo Uría s/n, 33071 Oviedo Asturias, Spain

ABSTRACT: The grazing impact of protists on phytoplankton primary production in a highly oligotrophic 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 primary 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.

KEY WORDS: Protists grazing · Trophic coupling · DCM · Subtropical NE Atlantic · Phytoplankton groups

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