Nonpoint source loading of phosphorus to Lake Victoria from the atmosphere and rural catchments in Tanzania, East Africa

The ecosystem of Lake Victoria has undergone many changes during the last three decades.; In an attempt to reduce data deficit on nonpoint source P yield in the region, field data were collected as an input to the AGNPS model to evaluate tributary nonpoint P and sediment loading to the lake from two predominantly agricultural sub-watersheds in the south and southwestern parts of Lake Victoria in Tanzania. The results shows that approximately 1.1 million metric tons of sediment and 1,100 metric tons of P were transported annually to Lake Victoria from the study catchments. The total P yield from rivers 0.25 kg ha −2 yr−1.; The estimate of atmospheric P deposition determined by measuring wet and dry atmospheric fluxes at four different sites on the south and southwestern shores of Lake Victoria showed that, on average, 13,000 metric tons of P yr −1 was deposited into the lake from this source. This represents a lake-wide loading rate of 1.9 kg ha−2 yr−1 . By combining the lakewide runoff estimates determined in other studies conducted in the lake's watershed with the results of this study the atmosphere represents approximately 55% of the total P input to the lake.; Data derived from P sorption studies indicated that P sorption capacity of soils and river sediment in the two catchments could be estimated using a few routinely measured soil properties. A regression equation combining Al, Fe, and Ca best predicted P sorption capacity of soils and sediment in the catchments (p < 0.05; r2 = 0.87). Simulations conducted on the transport and potential release of sediment-bound P showed between 50 and 130 metric tons of SRP yr−1 are potentially released into Lake Victoria from the sediment transported by rivers draining the two study catchments.; Because much of the tributary and atmospheric P loading apparently derives from the watershed disturbances, a basin wide conservation approach remains the key to reduction of nonpoint source P inputs to the lake. (Abstract shortened by UMI.)...