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 grazing in the epilimnion and metalimnion of the eutrophic Rimov Reservoir in south Bohemia. On average, similar to 70% of bacterial production was consumed by heterotrophic flagellates and similar to 20% by ciliates. Ciliate numbers increased from 5 to 70 cells ml(-1) over the 5-week study period. Ciliates <30 mu m in size were numerically dominant in both layers and included Halteria grandinella and Strobilidium hexakinetum (Oligotrichida), Cyrtolophosis mucicola (Cyrtolophosida), Cinetochilum margaritaceum (Scuticociliatida), Urotricha spp., and Coleps sp. (Prestomatida). Ciliate species-specific grazing rates on bacteria and picocyanobacteria were determined. The highest individual cell grazing rates, 4,200 bacteria and 560 picocyanobacteria cell(-1) h(-1), were observed in Vorticella aquadulcis-complex. Oligotrichs ingested on 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 high abundance, as the most important ciliate bacterivore within the system. 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 picocyanobacteria cell(-1) h(-1). Although there was a tight relationship between grazing rates on bacteria and picocyanobacteria (r(s) = 0.89, n = 12, P < 0.001), most of the ciliate species preferred larger picoplankton (i.e. picocyanobacteria), as indicated by their clearance rates. According to our data, several oligotrichous ciliate species and Cyclidium sp. can grow in pelagic conditions and exclusively on picoplankton food at rates of one doubling every 24-75 h.