Predation on planktonic ciliates in Kenyan soda lakes

Ciliated protozoa are important components of the plankton of aquatic systems with ability to recycle nutrients and to link microbial food webs to metazoans. This is because they are numerous and have higher growth rates compared to other unicellular eukaryotes of similar size. Predation and competition are some of the factors that negatively affect their abundance and growth. Studies documenting the impact of predation by zooplankton on ciliates have been conducted in marine and freshwaters waters but are lacking in tropical waters especially soda lakes in Kenya. In this study, growth rates of planktonic ciliates were estimated in four soda lakes, Lakes Bogoria, Elmenteita, Simbi and Sonachi, using predator exclusion experiments. In the experiments, ciliate populations increased faster in incubations of lake water with the >40 mm fraction removed than in untreated controls. The difference in these rates was taken as an estimate of predation by zooplankton >40 mm ( M z), and ranged from 0.091 to 3.171 d(-1). Our best estimates of growth rates for particular ciliates abundant enough to derive estimates for ranged from 0.18 to 4.78 d(-1). When removing the >40 mm fraction did not result in increased numbers of ciliates relative to controls, we hypothesized that this was due to predators within the <40 mm fraction (M-c) and their release from zooplankton predators. We assessed and supported this hypothesis by looking at the production of ciliates in different feeding guilds. The significance of predation on ciliates to the food webs of soda lakes is discussed.