Distribution and Food-web Transfer of Mercury in Napoleon and Winam Gulfs, Lake Victoria, East Africa

Abstract

Mercury (Hg) concentrations were measured for the food webs and water of Napoleon Gulf (Uganda) and Winam Gulf (Kenya) in northern Lake Victoria. Water total mercury (THg) concentrations in Lake Victoria range from 1.7 to 5.8 ng/L, while methylmercury (MeHg) concentrations range from 0.2 to 1 ng/L. Water Hg concentrations in Lake Victoria are higher than in temperate great lakes, including Lakes Baikal, Michigan, and Ontario, but the top predator Nile perch have 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, which may be due to biogeochemical differences in each gulf. THg concentrations in Nile perch and Nile tilapia consistently increase with total length in both gulfs and the rates of increase are similar. The rates of THg bioaccumulation, as indicated by the regression slopes of log-THg vs. stable nitrogen isotope values for each food web (slopes of 0.163 and 0.165 for Napoleon and Winam gulfs, respectively), are within the ranges of bioaccumulation rates observed in temperate and tropical lakes elsewhere which suggests that Hg bioaccumulates at a similar rate in diverse aquatic food webs, regardless of latitude or species composition.