The trophic position of planktonic ciliate populations in the food webs of some East African lakes
The trophic role of planktonic ciliates was investigated in the food webs of 17 East African lakes of different salinity and trophy. Experiments with 1m latex beads showed that Cyclidium, Halteria, Dysteria, Aspidisca and Vorticella are bacterivores. Examination of food vacuole contents showed that the ciliate communities also included herbivores, carnivores, omnivores and mixotrophs. While some herbivorous ciliates grazed picoplanktonic algae or flagellates, large omnivores such as Frontonia and Linostoma ingested cyanobacterial filaments or preyed on other ciliates. Most predaceous ciliates were haptorids, and consumed other ciliates or metazoa. Growth rate estimates for planktonic ciliate populations in East African lakes ranged from 0.18 to 6.56d−1. From these growth rates and biomass, the estimated production of ciliate populations ranged from 5.9 to 2 335g carbon l−1 d−1 and increased with lake trophic status and water electrical conductivity. Bacterivores, herbivores, carnivores and mixotrophs accounted for 36%, 39%, 21% and 4% of total production, respectively, over all the lakes. Zooplankton were the probable consumers of much of this ciliate production, although some taxa were consumed mostly within the microzooplankton community, presumably by other ciliates.