Total mercury cycling and the food web structure are investigated in Lake Victoria, East Africa. Lake Victoria is not only the largest tropical lake in the world by area, but also supports the largest freshwater fishery in the world. This lake is threatened by several environmental pressures. Methylmercury, a serious neurotoxic chemical, has the potential to be a major environmental concern in Lake Victoria as it is easily bioaccumulated in aquatic biota. Any potential negative impact on the valuable fisheries can have serious ramifications.

Because food web structure is important in terms of total mercury (THg) bioaccumulation, the food web structures in Napoleon and Winam Gulfs were characterized using stable nitrogen and carbon isotope analyses. Similar biota in Napoleon Gulf had significantly lighter delta-15 N values and heavier delta-13 C values than similar biota in Winam Gulf, indicating different basal isotopic values but the fractionation rates remained consistent between trophic transfers. Stable isotope data are consistent with Nile perch feeding on a restricted set of fish and macroinvertebrate species, including its own young, and indicate that Nile perch populations may be self-limited by cannibalism.

THg concentrations in the food-webs and water of Napoleon Gulf (Uganda) and Winam Gulf (Kenya) in northern Lake Victoria were determined. Water Hg concentrations in Lake Victoria are higher than in temperate great lakes, but the top predator Nile perch has relatively low THg concentrations compared to temperate piscivorous fish. While the water Hg concentrations are similar between Napoleon and Winam Gulfs, the THg concentrations in biota are significantly higher in Napoleon Gulf than in the same species from Winam Gulf. The rates of THg bioaccumulation, as indicated by the regression slopes of log-THg vs delta-15 N values for each food-web are within average bioaccumulation rates observed elsewhere which suggests that methylmercury (MeHg) bioaccumulates at a similar rate in diverse aquatic food webs, regardless of latitude or species composition. The amount of MeHg entering the food web and the length of the food-web are more important for determining the final THg concentrations in top trophic predators.

Patterns in THg in water, soil and 210Pb-dated sediment cores from northern and central Lake Victoria were determined. There were no apparent differences in THg water concentrations between Napoleon and Winam Gulfs in northern L. Victoria. THg in surface agricultural soil samples are associated with the oxyhydroxides in clay components. A near-shore core and an offshore core have similar THg concentrations and profiles. A preliminary abiotic THg budget indicated that atmospheric sources and erosion represent potentially important sources of THg to the lake, with biomass burnings very likely being a major source of THg to Lake Victoria.

Data were compiled from published and unpublished studies in the three countries sharing Lake Victoria, namely Tanzania, Uganda and Kenya. THg concentrations in urine and hair from human volunteers have indicated that most of the population, including fishermen, have sufficiently "safe" concentrations of THg in their body. Only gold miners and frequent skin-bleaching cream users are at risk of inorganic mercury poisoning. Human exposure assessments have demonstrated that fish intake and soil geophagy (soil consumption) constitute major sources of THg to humans, but the total estimated daily intake are below the Health Canada Tolerable Daily Intake limits. However, if there are no interventions to reduce anthropogenic nutrient run-offs and biomass burnings in the L. Victoria basin, then MeHg production may increase in the lake, resulting in elevated MeHg concentrations in fish and humans.