CILIATE GRAZING ON PICOPLANKTON IN A EUTROPHIC RESERVOIR DURING THE SUMMER PHYTOPLANKTON MAXIMUM - A STUDY AT THE SPECIES AND COMMUNITY-LEVEL

In late summer 1993 an intensive study was carried out on protozoan gr azing in the epilimnion and metalimnion of the eutrophic Rimov Reservo ir in south Bohemia. On average, similar to 70% of bacterial productio n was consumed by heterotrophic flagellates and similar to 20% by cili ates. Ciliate numbers increased from 5 to 70 cells ml(-1) over the 5-w eek study period. Ciliates <30 mu m in size were numerically dominant in both layers and included Halteria grandinella and Strobilidium hexa kinetum (Oligotrichida), Cyrtolophosis mucicola (Cyrtolophosida), Cine tochilum margaritaceum (Scuticociliatida), Urotricha spp., and Coleps sp. (Prestomatida). Ciliate species-specific grazing rates on bacteria and picocyanobacteria were determined. The highest individual cell gr azing rates, 4,200 bacteria and 560 picocyanobacteria cell(-1) h(-1), were observed in Vorticella aquadulcis-complex. Oligotrichs ingested o n average 360-2,130 bacteria and 76-210 picocyanobacteria cell(-1) h(- 1), with H. grandinella (1,560 bacteria cell(-1) h(-1)), due to its hi gh abundance, as the most important ciliate bacterivore within the sys tem. C. mucicola ingested on average 173 bacteria and 27 cyanobacteria cell(-1) h(-1); C. margaritaceum, 57 bacteria and 7 picocyanobacteria cell(-1) h(-1); and prostomatids, 23-100 bacteria and 2-14 picocyanob acteria cell(-1) h(-1). Although there was a tight relationship betwee n grazing rates on bacteria and picocyanobacteria (r(s) = 0.89, n = 12 , P < 0.001), most of the ciliate species preferred larger picoplankto n (i.e. picocyanobacteria), as indicated by their clearance rates. Acc ording to our data, several oligotrichous ciliate species and Cyclidiu m sp. can grow in pelagic conditions and exclusively on picoplankton f ood at rates of one doubling every 24-75 h.