For sample preservation borax-buffered formalin (final concentration 2% formaldehyde) has been used. Samples were stored at 4°C in the dark until observation. 100 ml samples have been transferred into Utermohl chambers for settling during 24 hours and finally examined by means of an inverted microscope. Ciliate cell sizes were measured and converted into cell volumes using appropriate geometric formulae (Peuto-Moreau, 1991). For biomass estimation the conversion factor 140 fgC µm^-3 (Putt and Stoecker, 1989) has been used.