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Seasonal variations of photosynthetic ciliates at a Mediterranean coastal site

Standing stocks of ciliated protozoa were analyzed in weekly samples over 3 yr at a Mediterranean coastal site (Gulf of Naples, Tyrrhennian Sea). Photosynthetic ciliates contributed 49% of total ciliate biomass; heterotrophic naked choreotrichs and tintinnids contributed 25 and 16%, respectively. The present paper focuses on the occurrence and succession of the autotrophic cil- iate Mesodinium rubrum and of aloricate mixotrophic and heterotrophic choreotrichs. A very diverse assemblage of Strombidium spp. dominated the mixotrophic choreotrich populations, while Tontonia spp. contributed 14.3% and Laboea strobila 4.7% to plastidic ciliate abundance. The annual pattern of occurrence of these 3 genera of mixotrophic ciliates was similar over the 3 years of this study. Autotrophic and mixotrophic ciliates differed in the periods of maximum relative contribution to the ciliate assemblage. Mixotrophic choreotrichs dominated the ciliate populations in spring and summer while the maximum contribution of autotrophic ciliates occurred in winter. M. rubrum occurred in 2 forms — the larger form was the only Mesodinium observed in winter, while it was absent during summer, when the smaller Mesodinium was at times observed. The contribution of M. rubrum to annual primary production in the Gulf of Naples was about 3%. Aloricate choreotrich populations were dominated by small cells; 92% of the aloricate heterotrophs were <30 µm equivalent spherical diameter (ESD), and for the mixotrophic ciliates 73% were <30 µm ESD and 24% were between 30 and 50 µm ESD. Notwithstanding a very wide range of variation in aloricate choreotrich densities, 135 to 48 800 cells l -1 , the relative contribution of different size classes was remarkably stable at any level of abundance. The calculated production for aloricate choreotrichs was between 2.3 and 3.3 g C m -3 yr -1 for the surface layer, about 50% of which was estimated to rely on ciliate grazing of bacterio- and picoplankton, as well as on photosynthesis covering, in part, the carbon demand of mixotrophic ciliates.