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. Ciliate cell sizes were measured and converted into cell volumes using appropriate geometric formulae of Peuto-Moreau (Peuto-Moreau M., 1991. Symbiose plastidiale et mixotrophie des cilies planctoniques marins Oligotrichina (Ciliophora). These de Doctorat d' etat, Universite de Nice-Sophia Antipolis). For biomass estimation the conversion factor 140 fgC µm^-3 was used (Putt M., Stoecker D.K., 1989. An experimentally determined carbon: volume ratio for marine oligotrichous ciliates from estuarine and coastal waters. Limnology and Oceanography 34, 1097-1103).