Ciliated Protozoa and other Microorganisms from Two African Soda Lakes (Lake Nakuru and Lake Simbi, Kenya)

Abstract

Summary The protozoan fauna and associated microorganisms in two alkaline-saline (soda) lakes in tropical Africa (L. Nakuru and L. Simbi, Kenya) have been examined and quantified. L. Simbi is a relatively deep (23 m) volcanic crater lake. L. Nakuru is shallow (~ 2 m) with an extensive littoral region. In both lakes the principal ions were Na + and HCO 3 - -CO 3 2- and the p H was approximately 10. The biomass of plankton in both lakes was extremely high (206 mg particulate carbon l -1 in L. Nakuru and46 mg l -1 in L. Simbi). L. Nakuru supported dense populations of coiling cyanobacteria ( Anabaenopsis magna and Spirulina fusiformis ) with short filaments, and high densities of both bacteria (2.7 × 10 8 ml -1 ) and flagellates (8.4 × 10 4 ml -1 ). All of these served as food for an abundant (> 1,000 ml -1 ) and diverse (21 species) ciliate community. The high numbers of bacteria and Protozoa in L. Nakuru are probably partly due to the large quantities of excretory waste products deposited by vast numbers of flamingoes. Most ciliates were small ( Cyclidium ) suspension feeders but algivores (e.g. Frontonia ) and carnivores (e.g. Spathidium ) were also present. Other Protozoa included at least one heliozoan and three species of naked amoebae. The number of protozoan species recorded more than doubles the number of aquatic animal species so far recorded in L. Nakuru. L. Simbi supported a dense and almost unispecific bloom of coiling S. fusiformis . Filaments were significantly longer (modal length ~ 400 μm) than in L. Nakuru and both bacteria and flagellates were less abundant (1.2–2.3 × 10 7 and 1.4–7.5 × 10 3 ml -1 respectively). These factors probably contributed to a lower abundance ( -1 ) and diversity (6 spp.) in the ciliate community which was almost totally confined to the epilimnion. The diet of the ciliate Holophrya was varied and included the rotifers Brachionus and Hexarthra . The jaws of consumed rotifers remained visible within Holophrya after the soft parts had been digested. In some ciliates several sets of rotifer jaws were observed. The Protozoa in these soda lakes are more diverse and abundant than was previously known. Because of their abundance and their efficient and voracious grazing activities they must play a significant role in the relatively short and simple food chains in these lakes.