Résumé
Biomass and activities of planktonic microorganisms (bacteria, nanoplankton and microplankton) were measured in the northwestern Black Sea during summer 1995. The method based on the uptake of fluorescently labeled prey was chosen to determine the ingestion rate of bacteria and nanoplankton by phagotrophic microorganisms. This method revealed the presence of mixotrophic organisms such as ‘plastid-retaining ciliates’ in the whole coastal area. Mixotrophic ciliates were dominated by micro-sized forms and maximum biomasses were recorded in the water masses characterised by low nutrient concentrations but high food particle concentrations. Mixotrophic nanoflagellates were absent and mixotrophic dinoflagellates were observed at one station only. Mixotrophic ciliates were shown to ingest preferably bacteria while mixotrophic dinoflagellates were grazing almost exclusively on nanoflagellates. Although the biomass of mixotrophic organisms were significantly lower than those of aplastidic protozoa, their feeding activity contributed to 14 and 24% of the ingestion of bacteria and nanoplankton, respectively. This is due to the high specific ingestion rate of mixotrophic micro-sized ciliates and dinoflagellates, which were two and three times higher, respectively, than the specific ingestion rate of bacteria and nanoplankton by aplastidic protozoa. This suggests a sigrifncant contribution of phagotrophic mixotrophs to the microbial network of the northwestern Black Sea.