Biomagnification of mercury in fish from Thruston Bay, Napoleon Gulf, Lake Victoria (East Africa)

Total mercury concentrations (THg) were measured in fish from Thruston Bay, Napoleon Gulf in northern Lake Victoria between 1998 and 2000. Total Hg concentrations in Lates niloticus (Nile perch) and Oreochromis niloticus (Nile tilapia) ranged from 10.6 to 77.5ng g–1 and from 15.0 to 44.5ng g–1 wet weight respectively. These concentrations are lower than in the same fish species from elsewhere in Napoleon Gulf and are in the middle of the range of THg concentrations from across Lake Victoria. The rate of THg biomagnification, as indicated by the regression slope of log-THg vs stable nitrogen isotope values (0.28), is within the ranges of biomagnification rates observed in temperate and tropical lakes, suggesting that THg is biomagnifying at a similar rate in Thruston Bay as elsewhere. The low THg concentrations in fish were attributed to the storage capacity, high oxygen concentrations and high organic matter content of the wetlands surrounding Thruston Bay. However, caution is required because the storage capacity and the methylation rates present in the wetlands of Thruston Bay are unknown and the gradual accumulation of THg contamination from other sources (e.g. atmospheric THg) may result in unexpected THg increases in the biota of this system.

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