IMPACT OF ACIDIFICATION ON THE METHYLMERCURY CYCLE OF REMOTE SEEPAGE LAKES

Concentrations of monomethylmercury [CH3Hg] were measured in the water and seston of five nearly pristine Wisconsin lakes, which span a range of pH from about 4.6 to 7.2. Previous studies had established a clear inverse relationship between [CH3Hg] in fish and the pH of lakes in this region. Here, we examined the pH dependency of [CH3Hg] in lake water and explored the partitioning of CH3Hg between water, seston, and fish as a function of pH. Results indicate that [CH3Hg] in lake water tends to increase as pH decreases, but that seasonal and spatial variability of [CH3Hg] in individual lakes confounds a simple analysis of the relationship. The partitioning of CH3Hg was related only weakly, if at all, to pH. Average partitioning coefficients (log k(d) = log (C(p)/C(w))) were higher for yearling yellow perch (6.0 to 6.5) than for seston (5.5 to 6.0) but did not vary significantly between lakes. This suggests that acidification has a stronger effect on the supply of CH3Hg to the ecosystem than on specific rates of uptake by the biota.