Algae

Algae types and primary production in natural lakes are dependent on several factors; the major ones are water levels, nutrient recruitment, subterranean rocks, and temperature. Primary production is relatively low in lakes Malawi and Tanganyika with records of 0.73 g.C.m2 and 1.4 g. C m2 per day, respectively, recorded in surface waters only. Of the Great Lakes, Lake Victoria is more productive with chloro-phyll-a values of 8.8-51.4 mg per m3. Many species of diatoms (Nitzschia, Stephanodiscus, Navicula, Rhopa-lodia, Cymbella + Encyonema, Ggyrosigma), green algae (Chlorella + Chlorocystis, Coelostrum, Chroo-coccus, Mougeotia, Euastrum) and blue green algae (Anabaena, Calothrix, Lyngbya, Tychonema, Schzo-thrix, Planktolyngbya, Chroococciods) commonly occur in lakes Tanganyika and Malawi. Melosira is prevalent in Lake Malawi, where it is a keystone species of the pelagic zone. Apart from forming the base of the food chain, deposited lake-bed forms of dead algae provide reliable material for seismic and carbon dating studies.

Epilithic periphyton is significant to Lakes Tanganyika and Malawi, where it is grazed upon by the fast evolving rock-dwelling haplochromis species. In Lake Malawi, heterocystous Cyanobacteria and diatoms

Figure 2 Situation of the major African wetlands.

are the dominant forms. They are endosymbionts that undertake nitrogen fixation. In some locations, there is a shift towards diatoms and chlorophytes owing to sediments and nutrient loading. Calothrix species dominate the littoral periphyton zone. It is stated that heterocytes decrease with depth in Cyanobac-teria. The dominance of nitrogen fixing algae is indicative of N-limitation in rocky littoral zones. The mixture of algal species found in sediment is grouped under the common name 'Awfuchs' and are rather poorly taxonomically defined. Eutrophication has resulted in algal shifts dominated by Anabaena and Microcystis in most lakes.

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