CAN FRESH-WATER PLANKTONIC CILIATES SURVIVE ON A DIET OF PICOPLANKTON

Ciliate picoplanktivory was studied in two different freshwater system s with abundant autotrophic picoplankton (APP): the eutrophic Rimov Re servoir (South Bohemia) during the late summer of 1993 and the oligo- to mesotrophic lake Piburger See (Tyrol) during May 1994. Picoplankton were sized by an image-analysis system and species-specific grazing r ates of ciliates on bacteria and APP were determined using fluorescent ly labelled prey. According to the grazing data (as the mean uptake ra te of picoplankton ind.(-1) h(-1)), ciliates were split into three eco logical groups in order of their decreasing picoplanktivory and increa sing significance of nanophytoplankton in their diet. (i) Highly effic ient fine suspension feeders consisting of vorticellids (4200 bacteria , 560 APP), four oligotrichs and Cyclidium sp. (380-1580 bacteria, 57- 210 APP), with <7% of individuals ingesting algae >2 mu m. (ii) Less e fficient fine suspension feeders, coarse filter feeders and detritopha ges consisting of Cinetochilum margaritaceum, Pelagostrombidium fallax , Cyrtolophosis mucicola and Coleps spp. (60-173 bacteria, 2-27 APP), with 7-32% of individuals ingesting algae >2 mu m. (iii) Raptorial fee ders consisting of four prostomatids belonging to the genera Urotricha and Balanion, with a negligible importance of picoplanktivory (8-61 b acteria, 0.2-14 APP). With the exception of one prostomatid, >58% of i ndividuals ingested algae >2 mu m. Grazing data for the six species fr om the first group were converted into organic carbon. Assuming a 35% gross growth efficiency, the calculated potential doubling times were between 29 (Halteria grandinella) and 43 h (an unidentified oligotrich ), except for the mixotrophic Pelagohalteria viridis (118 h). The doub ling times estimated from changes in ciliate abundance for four out of the six species were very close to the calculated values, indicating that this group of heterotrophic ciliates can meet all of its carbon r equirements by feeding exclusively on picoplankton.