Production of methymercury by solar radiation

Abstract
Methylmercury (MeHg) is photochemically degraded at the surface of freshwater lakes at around 18% day ^-1. Since MeHg is not thought to be formed in the water column, concentrations of MeHg at the lake surface should display a strong diurnal pattern due to the photodegradation of MeHg. However, previous investigators have found that MeHg concentrations peak at noon and microbial inhibitors do not prevent mercury methylation. These reports suggest that mercury methylation may mitigate the effect of photodegradation on surface MeHg concentrations. Lake water was fractionated by a tangential ultrafiltrator and exposed in Teflon bottles to sunlight for different periods of time. Concentrations of MeHg initially decreased by 50% but then increased once again. Since, this is a sealed system, we assume that dissolved organic carbon between 300 and 30 kDa was responsible for the abiotic photochemical production of MeHg. Using rate data collected on site, photomethylation rates are estimated at 20 pg L ^-1 (kW m ^-2)-'which corresponds to 35% of MeHg inputs in freshwater lakes. The observed dependence of photomethylation on dissolved organic carbon may provide a mechanistic explanation for why clear cutting which increases dissolved organic carbon, increases MeHg in biota.