It has frequently been reported that concentrations of mercury in fish increase significantly, often by three to four times, in recently constructed hydroelectric reservoirs. In the central-western parts of Brazil, a hydroelectric reservoir, called Lago Manso, was completed in 1999. Two previous studies concerning mercury levels in fish have been performed in the reservoir; one in 1999, before the construction of the dam and one in 2002, two years after completion of the reservoir. The objectives of this study were to continue to monitor the concentrations of mercury in fish in the reservoir as well as downstream, to determine the degree of bioaccumulation of mercury, and to identify groups of primary producers contributing energy to the fish food web. The effects of abiotic, physiological and ecological factors on the accumulation of mercury by fish examined. In total, 41 specimens of seven species; Brycon hilarii, Piaractus
mesopotamicus, Serrasalmus marginatus, Serrasalmus spilopleura, Salminus brasiliensis, Pseudoplatystoma fasciatum, and Pseudoplatystoma corruscans, were analyzed for total
mercury content. The mercury levels were found to have increased drastically and were well above the dietary limits, recommended by WHO and FAO, for mercury in fish aimed at human consumption. Piscivorous Salminus brasiliensis, caught downstream, demonstrated the highest level of mercury, 1323 ng/g wet weight, followed by piscivorous Pseudoplatystoma fasciatum, 1204 ng/g wet weight, caught in the reservoir.
The concentration varied widely between species, which could be related to differences in diet and trophic position. The degree of mercury accumulation in different species, shown by means of a bioconcentration factor (concentration in biota/concentration in
water), ranged between 4.08 and 5.93 log units. The most important groups of primary producers, determined by stable carbon isotope analysis (δ13C), appeared to be terrestrial
C3 plants, aquatic macrophytes, and phytoplankton. A relationship was found between mercury concentration and carbon source but the mercury concentration in fish is
probably better explained by differences in trophic position. Dissolved organic carbon the water was shown to have increased slightly while dissolved oxygen had decreased, indicating enhanced transport, methylation, and bioavailability of mercury.