Department of Biology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

The importance of atmospheric sources to the nutrient loading of Lake Victoria

Nutrient budgets are required to guide lake management and restoration of lakes degraded by eutrophication from nutrient enrichment. The foundation of the nutrient budget is the water budget. In the African Great Lakes, direct precipitation to and evaporation from the lakes' surfaces dominate the water budget. Consequently, the chemistry of the rain and gas exchange can have profound influences on these lakes. Our recent studies have evaluated, for the first time, the wet and dry deposition of N and P from the atmosphere and the importance of biological fixation of atmospheric nitrogen as sources of N and P to Lake Victoria. Wet and dry atmospheric deposition is the largest single source of P to Lake Victoria (14 kilotonnes per year) while biological fixation of nitrogen is the largest single source of N (480 kilotonnes per year). These atmospheric sources likely account for 60% of total P loading and over 80% of total N loading to the lake. The areal wet and dry deposition rates measured around Lake Victoria are similar to those measured around Lake Malawi indicating the potential importance of atmospheric sources to the large East African lakes. The high P deposition rates are the result of the semi-arid climate and the increasing prevalence of biomass burning in land management, which has increased along with population growth in the region. Nutrient management in Lake Victoria must address the atmospheric non-point source loading as well as more readily managed point sources to reduce the undesirable effects of eutrophication in Lake Victoria.