Changes in phytoplankton communities and primary production

Abstract

Lake Victoria is the second largest lake in the world(69000km2) by surface area, but it is the shallowest (69mmaximum depth) of the African Great Lakes. It is situatedacross the equator at an altitude of 1240m and lies in ashallow basin between two uplifted ridges of the easternand western rift valleys (Beadle 1974). Despite theirtropical locations, African lakes exhibit considerableseasonality related to the alteration of warm, wet andcool, dry seasons and the accompanying changes inlucustrine stratification and mixing (Tailing, 1965; 1966;Melack 1979; Hecky& Fee 1981; Hecky& Kling,1981;1987; Bootsma 1993; Mugidde 1992; 1993).Phytoplankton productivity, biomass and species composition change seasonally inresponse to variations in light environment and nutrient availability which accompanychanges in mixed layer depth and erosion or stabilization of the metalimnion /hypolimnion (Spigel & Coulter 1996; Hecky et al., 1991; Tailing 1987). Over longer,millennial time scales, the phytoplankton communities of the African Great Lakes haveresponded to variability in the EastAfrican climate (Johnson 1996; Haberyan& Hecky,1986) which also alters the same ecological factors (Kilham et al., 1986). Recently, overthe last few decades, changes in external and or internal factors in Lake Victoria and itsbasin have had a profound inlluence on the planktic community of this lake (Hecky,1993; Lipiatou et al., 1996). The lake has experienced 2-10x increases in chlorophylland 2x increase in primary productivity since Tailing's observations in the early 1960s(Mugidde 1992, 1993). In addition to observed changes in the lake nutrient chemistry(Hecky & Mungoma, 1990; Hecky & Bugenyi 1992; Hecky 1993; Bootsma & Hecky 1993), the deep waters previouslyoxygenated to the sediment surface through most ofthe year are now regularly anoxic(Hecky et al., 1994).