Using empirical methods to assess the risks of mercury accumulation in fish from lakes receiving acid rain

It has been hypothesized that concentrations of mercury (Hg]) in fish increase with increasing acidic deposition. Our investigation suggests that this hypothesis is false for three species of fish from Ontario lakes. Across Ontario, Canada, slightly lower average Hg] in Lake Trout, Walleye and Northern Pike (controlled for fish length) were associated with regions receiving progressively greater rates of sulphate deposition. We also observed that lake concentration of dissolved organic carbon (DOC]), not pH, alkalinity or sulphate ion concentration, was the strongest correlate of this residual fish Hg contamination. Path analysis also determined a net reduction in fish Hg] at higher rates of sulphate deposition. However, this analysis indicated a far more complex web of interactions between sulphate deposition, water chemistry and fish Hg] than simple correlation analysis could define. Path analysis provided empirical evidence in support of several previously described mechanisms interacting simultaneously to influence fish Hg accumulation. Based on these empirical observations, an alternate hypothesis is proposed whereby increasing sulphate deposition results in lower DOC] levels in lakes which, in turn, cause a decline in fish Hg contamination.