Diatom evidence for the timing and causes of eutrophication in Lake Victoria, East Africa

The determination of the history and causes of recent eutrophication and intensified thermal stratification in Lake Victoria is still hampered by the sparsity of paleolimnologic coverage of the enormous lake. Five new diatom records from Ugandan waters now show that a transition from Aulacoseira-dominated planktonic assemblages to those dominated by long Nitzschia spp., occurred in northern coastal sites from the mid-1970s to mid-1980s. Similar transitions developed from the late 1960s to early 1970s offshore and from the 1940s to early 1950s along the Kenyan coast, suggesting a time-transgressive process. These changes are not readily attributable to the trophic effects of Nile perch population growth during the early 1980s, but more likely reflect responses to long-term nutrient enrichment and climatic instability in the region. The diversity of planktonic diatom communities has declined dramatically, and a namesake variety of Aulacoseira nyassensis may now be nearly extirpated. Although local phytoplankton communities varied considerably in the past, the current domination of diatom assemblages by Nitzschia is apparently unprecedented in the 15,000-year history of Lake Victoria.