Eastern African Soda Lakes

Saline lakes rich in bicarbonate and carbonate, usually called soda lakes, are widespread in eastern Africa and are among the world's most productive, natural ecosystems. A conspicuous feature of these lakes is often the presence of enormous numbers of lesser flamingos (Phoeniconaias minor) (Figure 2) grazing on thick suspensions of the phytoplank-ter, Arthrospira fusiformis (previously called Spirulina platensis), but species diversity is low. Heterotrophic bacteria attain very high numbers, but have not been characterized with molecular methods. Phytoplankton and benthic algae include several species of green algae, diatoms, and cyano-bacteria. Of the few species of aquatic invertebrates, protozoa are the most diverse with 21 species reported from Lake Nakuru (Kenya). Consumers in Lake Nakuru at salinities of around 20 g l- include one species of fish, Sarotherodon alcalicus grahami (introduced from springs near Lake Magadi, a neighboring salt pan), one copepod (Paradiaptomus africanus), and two rotifers (Brachionus dimi-datus and B. plicatilis), and several aquatic insects including corixids, a notonectid, and chironomids. Modest changes in the salinity and in the vertical distribution of salinity can have major impacts on trophic structure and nutritional status of these lakes.

Figure 2 Flamingos (Lake Bogoria, Kenya).
Figure 3 Lake Elmenteita, Kenya.

Biological communities in shallow, tropical saline lakes are susceptible to slight variations in water balances and salinities. For example, intensive sampling during a period of low rainfall and abrupt increase in salinity in Lake Elmenteita (Kenya) (Figure 3) and Lake Nakuru (Kenya) revealed a precipitous drop in the abundance of phytoplankton and major shift in the zooplankton. As species of phytoplanktons, such as Arthrospira fusiformis, were replaced by much smaller phytoplanktons, the abundance of lesser flamingos decreased markedly.

Scattered across eastern Africa are numerous saline lakes inside volcanic craters. Several of these lakes have been studied in Ethiopia, Kenya, Uganda, and Tanzania. One common feature in the saline, crater lakes of eastern Africa is persistent chemical stratification, that is, they are meromictic, which has significant biological consequences. For example, Lake Sonachi (Kenya), a meromictic crater lake, had much lower algal biomass and rates of photosynthesis than the neighboring soda lakes that mixed more often. Moreover, studies of phosphorus uptake indicated that the lake was deficient in phosphorus, although a large reservoir of phosphorus was trapped below the chemocline.

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