Zooplankton dynamics of two alkaline-saline lakes in the Kenyan Rift Valley.

Abstract

Underling influences on mesozooplankton dynamics of shallow, saline lakes have widely been undiscovered in spite of the ability of such communities to form blooms of exceptional high density and biomass. In one of our studies we investigated therefore environmental and resource based biotic factors to disclose fundamental patterns of such blooms, which are mainly formed of branchionid rotifers and large omnivorous ciliates. Particulate matter seemed to be a key influencing factor, showing the same dynamics of zooplankton, with both variables only separated by a time lag of two weeks. Phytoplankton also played a role in sustaining high rotifer abundances, but was too low in biomass to sustain high consumer densities on its own. Contrariwise, mesozooplankton had a high impact on algae composition by facilitating the transition between a mixed community of small unicellular algae and the filamentous cyanobacterium Anabaenopsis abijatae to a phase of high biomass of solely Arthrospira fusiformis. Further a strong top down controlling force of rotifer populations on small ciliates was observed, whereas the influence of bacterial densities on zooplankton dynamics was rather limited. Further, the ecological niches of the two frequently dominating rotifers of Lake Nakuru (Kenya), Brachionus dimidiatus and Brachionus plicatilis, were investigated using stable isotope analysis. The larger-sized B. pliciatilis showed a reliance on the filamentous cyanobacteria Arthrospira fusiformis and Anabaenopsis abijatae, which indicates substantial differences of feeding behaviour within the cryptic species complex of this taxon. B. dimidiatus fed to a large extend on components of the microbial loop and bacterial contribution of over 40% to total rotifer nutrition belongs to the highest ever recorded in-situ values. Further, the comparatively high δ15N value of B. dimidiatus implies an enrichment of δ15N caused by a reliance on the microbial loop within an ecosystem which is strongly depended on recycling of nutrients originating from higher trophic levels. This underlines the necessity of a careful application of δ15N values for interpreting trophic levels in such communities. Finally, we investigated the feeding behaviour of dominating pelagic zooplankton of saline lakes in the East African Rift Valley. A set of grazing experiments revealed extraordinary high ingestion rates of the two euryhaline rotifers Brachionus dimidiatus and Brachionus plicatilis and of the large omnivorous ciliates Frontonia and Condylostoma magnum. Increased feeding rates reflected the unique nature of tropical saline systems showing high water temperatures coupled with high food quantities but low food quality. The size spectrum of ingested particles was broad, and even included filamentous cyanobacteria like the commonly dominating Arthrospira fusiformis. Selectivity, however, showed higher values for cryptomonads and small protozoans. Bacterial biomass was positively influenced by presence of grazers, as small bacterivorous predators were significantly reduced in numbers, showing the cascading effect of large zooplankton on the food web structure. Overall, based on this first time study of the feeding behaviour of the mesozooplankton in East African saline lakes, a strong structuring influence of rotifers and large ciliates can be expected in times of high consumer biomass.