PHOTODEGRADATION OF METHYLMERCURY IN LAKES

METHYLMERCURY can accumulate in fish to concentrations that threaten h uman health(1). Fish methylmercury concentrations are high in many res ervoirs(2) and acidic lakes(3), and also in many remote lakes(4,5)-a f act that may be related to increased atmospheric deposition of anthrop ogenically mobilized mercury during the past few decades(6). Although sources of methylmercury to lakes and reservoirs are known(7), in-lake destruction has not been demonstrated to occur at the low concentrati ons found in most water bodies. Here we report in situ incubations of lake water that show that methylmercury is decomposed by photo-degrada tion in surface waters. This process is abiotic and the rate is first- order with respect to methylmercury concentration and the intensity of solar radiation. In our study lake, the calculated annual rates of me thylmercury photodegradation are almost double the estimated external inputs of methylmercury from rain, snow, streamflow and land runoff, i mplying the existence of a large source of methylmercury from bottom s ediments. Photodegradation could also be an important process in the m ercury cycle of other aquatic systems. This discovery fundamentally ch anges our understanding of aquatic mercury cycling, and challenges the long-accepted view that microbial demethylation dominates methylmercu ry degradation in natural fresh waters.