Planktonic ciliates in the Mediterranean Sea: longitudinal trends

We analysed samples taken through the euphotic zone from 18 stations between the Ligurian Sea (6°E) and the Levantin Basin (32°E) from 24 May to 25 June 1996. Both ciliate and chlorophyll concentrations ranged over a factor of about 7, but ciliate concentrations (0.4–2.8 mg C m³) varied irregularly compared to a longitudinal decline, west to east, in chlorophyll concentration (0.07–0.48 mg m³). The lower chlorophyll concentrations (0.1 mg m²) of the eastern basin stations corresponded with a relatively high stock of ciliates (0.5 mg C m²). Large mixotrophic ciliates were more abundant, in both absolute and relative terms, in the eastern Mediterranean stations with less chlorophyll. The species diversity of tintinnid ciliates appeared higher in the central and eastern basins compared to the west. Our results suggest a shift from the western to eastern Mediterranean in the planktonic food towards a microbially dominated system.