HIGH ABUNDANCE OF PICOPLANKTON-INGESTING CILIATES DURING LATE FALL IN LAKE KINNERET, ISRAEL

Small, aloricate ciliates dominated the biomass of heterotrophic protists throughout the water column at the end of the period of stratification in Lake Kinneret, Israel. The integrated biomass of cilates was 5-20 times that of heterotrophic flagellates. During incubation experiments, ciliate growth rates in epilimnetic water corresponded to population doubling times of 9.6-19.4 h, while flagellate populations showed no growth. Most of the ciliates were small forms (10-30-mu-m long), including scuticociliates, choreotrichs, Coleps spp. and Colpoda spp., and appeared to be consuming bacteria, coccoid cyanobacteria, and <5-mu-m eukaryotic algae. Grazing rates of ciliate assemblages on picoplankton in the epilimnion, as determined by the uptake of fluorescently labeled bacteria and cyanobacteria, ranged from 62 to 86 nl cell-1 h-1. Colpoda steini, isolated from lakewater, grew on a cultured freshwater Synechococcus sp. with a doubling time of 4.5 h, and a gross growth efficiency of 48%. The estimated daily requirements of ciliates for growth approximately equalled total phytoplankton production. We calculated that ciliates in the epilimnion were clearing approximately 4-10% of the bacterioplankton and cyanobacterial standing stocks per day. Since this would not be sufficient food consumption to meet daily carbon requirements of the ciliates, it is likely that these organisms were also grazing a significant amount of autotrophic and heterotrophic eukaryotic cells in Lake Kinneret.