Heterotrophic flagellates in samples collected over a period of 15 months were classified and enumerated by epifluorescence microscopy of DAPI-stained cells deposited on 1 μm polycarbonate filters. Identifications were confirmed using other microscopic techniques. The heterotrophic flagellate community was dominated by bacterivorous nanoflagellates, with chrysomonads being the most numerous, followed by acanthoecid choanoflagellates, bicosoecids, bodonids and non-loricate choanoflagellates. The proportions of the constituent groups of this assemblage remained similar throughout the year. Larger flagellates were assumed from observations to feed on small phytoplankton and other nanoflagellates rather than bacteria. They were present in lower and rather variable numbers, but the total biomass of these larger forms was greater than that of the bacterivorous forms. These larger flagellates included dinoflagellates, the kinetoplastid Hemistasia phaeocystidicola and forms whose taxonomic position is uncertain such as Telonema spp. and the kathablepharids Leucocryptos marina and Kathablepharis remigera . The composition of the heterotrophic flagellate community in Southampton Water is compared with that reported from other studies on marine plankton.
