Dominance of ciliate grazing on bacteria during spring in a shallow eutrophic lake

Abstract

The objective of this study was to investigate the relationships between bacterioplankton biomass and activity and protistan grazing during the spring in shallow Lake Vörtsjarv, Estonia. Bacterial and heterotrophic protistan (flagellate and ciliate) abundance was determined by fluorescence direct counts, protistan grazing on planktonic bacteria was measured from fluorescently labeled bacteria uptake rates, and the estimate of bacterial heterotrophic activity was obtained from leucine and thymidine incorporation rates. The abundance of heterotrophic nanoflagellates (HNF) was low, ranging from 8.4 to 27.1 cells ml-1, while ciliate numbers dominated, ranging from 55 to 180 cells ml-1. The population of ciliated protozoans was dominated by scuticociliates (Uronema sp., Cyclidium spp., and 1 unidentified scuticociliate) and oligotrichs (Strobilidium spp. (diameter > 50 μm), Strobilidium spp. (diameter < 50 μm), and Strombidium sp.). Ciliates were predominant grazers of bacteria, showing higher clearance rates (14 to 65 nl ciliate-1 h-1) than HNF (1 to 11 nl HNF-1 h-1); the total grazing rate of ciliates (on average 2398 bacteria h-1 ml-1) was 28 times higher than total grazing of HNF (87 bacteria h-1 ml-1). In general terms protozoan grazing balanced with bacterial production but was not sufficient to support ciliate growth: on the basis of conversion factors the calculated generation time of bacterivorous ciliates was long, on average 121 d (ranging from 43 to 198 d).