NUTRIENT DYNAMICS AND TURNOVER RATES OF PHOSPHATE AND SULFATE IN LAKEVICTORIA, EAST-AFRICA

The dominant process removing sulfate from the water of Lake Victoria is biological uptake by plankton > 1 mum in size. Uptake of sulfate in the water column is substantially greater than uptake at the sediment surface. Despite low ambient concentrations (3-4 muM), rates of uptak e of sulfate by phytoplankton do not increase when additional sulfate is added to the water, and ambient concentrations are adequate to supp ort maximum uptake rates. Uptake rates for phosphate are similarly nea r-maximal in offshore waters. Furthermore, experimental addition of su lfate or phosphate fails to increase biomass production over control l evels, but addition of N alone or in combination with P and S does inc rease algal biomass. Although sulfate concentrations in Lake Victoria are lower than values reported for any large lake worldwide, our resul ts indicate that sulfate behaves as a nonlimiting nutrient in Lake Vic toria and that it does not limit the primary productivity of the lake.