Planktonic ciliates in the oligotrophic Eastern Mediterranean: vertical, spatial distribution and mixotrophy

Abundance, biomass, vertical distribution, species composition and mixotrophy of plank tonic ciliates were investigated during March 1997 in the oligotrophic Eastern Mediterranean. Six depth layers were sampled in the euphotic zone (0 to 100 m) at 7 stations in the North and 4 in the South Aegean Sea, resulting in a total of 66 samples and 82 ciliate species. Abundance was in general very low (0 to 780 cells l(-1)) except from specific stations in the North Aegean stations (20 to 2040 cells l(-1)), influenced by the Black Sea Water. Aloricate species dominated the depth-integrated abundance and biomass in both the North and the South. South Aegean stations presented more diverse ciliate fauna, uniform distribution down to 100 m and higher values of integrated abundance and biomass than the North Aegean stations. Among North stations, those subjected to the influence of the Black Sea presented very pronounced stratification, with mixotrophs and heterotrophs decreasing sharply below 20 m; at these stations, mixotrophic species of 18 to 30 mu m dominated the ciliate community at the surface whereas at the South as well as at the rest of the North stations, nanociliates were the major mixotrophic size class. Mixotrophs (20% of abundance in the South, 38% in the North) and nanociliates (28% of abundance in the South, 44% in the North) were found to be very important components of the community, which indicates that these organisms play also an important role in oligotrophic systems.