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 was used (Putt and Stoecker, 1989).Borax-buffered formalin (final concentration 2% formaldehyde). Samples for ciliate counting were stored at 4°C in the dark until observation. For ciliate identification and enumeration 100 ml samples were left for 24 h in sedimentation cylinders and then observed under an inverted epifluorescence microscope.