Invertebrates of East African Soda Lakes

Abstract

Soda lakes are among the world’s most productive natural ecosystems, but the fauna consists of only a few specialized groups which can attain high biomass, seasonally or throughout the year. The invertebrate fauna is generally poor in species diversity. Protozoa are restricted to a few ciliates dominated by Condylostoma and Frontonia spp. The zooplankton biomass is dominated by a calanoid, Paradiaptomus Africanus, with a maximum value of 1.17 g m-3 reported in Lake Nakuru, Kenya. Rotifers dominate in terms of species diversity, with six species/lake being the highest recorded number at six. Cladocerans are rare and include some euryhaline forms also common in other Rift Valley lakes, such as Alona sp., Macrothrix triseralis, Moina and Ceriodaphnia spp. Rotifera are represented by a few euryhaline and halobiontic species such as Brachionus dimidiatus, B. plicatilis and Hexarthra jenkinae. At times, B. dimidiatus can attain extremely high abundances (800 million m -3). The major food sources for zooplankton grazers are the large cyanoprokaryote Arthrospira fusiformis, Bacteria and detritus. Larger rotifers such as B. plicatilis obtain 48 % of their diet from fragments of A. fusiformis, whereas the smaller B. dimidiatus remove particles < 2 µm. Assimilation of this cyanoprokaryote was low. Despite this food constraint, secondary production of rotifers can exceed that of calanoids by a factor of 100x in soda lakes and over 600x in freshwater lakes. The macroinvertebrate community of soda lakes is dominated by insects of the order Heteroptera, Family Corixidae (water boatmen), genera Micronecta and Sigara and Family Notonectidae (backswimmers), genus Anisops. Other taxa include nematodes, oligochaetes, chironomids (non-biting midges), culicine mosquitoes, an anostracan (Branchinella spinosa) and ostracods (mussel shrimp). The chironomid community consists of halobiontic forms such as Microchironomus deribae, Kiefferulus disparilis and Tanytarsus minutipalpis. M. deribae can increase to enormous numbers seasonally and become a nuisance for lakeshore residents around soda lakes. In Lake Nakuru, Kenya, a high secondary production of 120 mg dry mass m-2 day-1 was recorded for M. deribae, but this is lower than the highest secondary production value of 182 mg dry mass m-2 day-1 for chironomids reported from the saline Australian Lake Werowrap. Secondary production can be high seasonally in soda lakes, but the biomass turnover rate (P/B ratio) of zooplankton and macroinvertebrates is low, probably because of the seasonal food limitations (e.g. Anisops is predatory on calanoids and fish) or the poor food quality of detritus and decomposing cyanoprokaryotes in the system. Soda lakes have simple food chains and are ideal as natural experimental models to study food web dynamics and energy flow in lakes.