Abstract
We investigated the feeding behaviour of the dominant microzooplankton of saline lakes in the East African Rift Valley. A set of grazing experiments revealed high ingestion rates of the two euryhaline rotifers Brachionus dimidiatus and Brachionus plicatilis and of the large-sized omnivorous ciliates Frontonia sp. and Condylostoma magnum reflecting the unique nature of tropical saline systems. The size spectrum of ingested particles was broad and even included filamentous cyanobacteria such as the commonly dominating Arthrospira fusiformis. Feeding selectivity on cyanobacteria, however, was rather low showing higher values for cryptomonads and small ciliates. Bacterial biomass was favoured by the presence of grazers, as small bacterivorous predators were reduced at an average of 13.9%, showing the cascading effect of large zooplankton on the food web structure. Overall, based on this first-time study of the microzooplankton feeding behaviour in East African soda lakes, a strong structuring effect of rotifers and large ciliates on microbial plankton communities is assumed, especially in times of high consumer biomass.
Highlights
Grazing of zooplankton is a major factor structuring plankton communities (Gliwicz, 1975) and influencing biomass, competitiveness, growth rates and morphology of prey organisms (Arndt, 1993; Jurgens & Matz, 2002; Verschoor et al, 2007)
The omnivorous ciliate Euplotes and the bacterivorous ciliate Cyclidium dominated the biomass, heterotrophic nanoflagellates (HNF) played a considerable role with a biomass of 0.9 g C m-3, equivalent to 12.9% of the biomass of heterotrophic bacteria, their major food source
Even highest A. fusiformis densities did Discussion Size spectrum of food particles and selectivity High abundance of filamentous cyanobacteria is well documented for tropical soda lakes (Ballot et al, 2004; Schagerl & Oduor, 2008)
Summary
Grazing of zooplankton is a major factor structuring plankton communities (Gliwicz, 1975) and influencing biomass, competitiveness, growth rates and morphology of prey organisms (Arndt, 1993; Jurgens & Matz, 2002; Verschoor et al, 2007). Zooplankton in the size class 250–100 lm (hereafter referred to as microzooplankton) consists mainly of rotifers, naupliae and large ciliates and is able to ingest a wide range of food particles. Feeding behaviour and grazing rates are influenced by several characteristics of food particles. Factors such as food quantity and quality, temperature, taste, digestibility, catchability and toxicity can play important roles (Demott, 1986; Montagnes et al, 2001; Mitra & Flynn, 2007). Prey size is often considered most important (Rothhaupt, 1990; Hansen et al, 1997)
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