A simple, mechanistic model of mercury (Hg) dynamics in a lake has been developed, based on the fugacity/aquivalence approach of Mackay (1991) and Mackay and Diamond (1989) and its extension to treat several interconverting chemical species (Diamond et al., 1992). The model considers the distribution of inorganic (HgII), elemental (Hg°) and methyl (MeHg) mercury species between dissolved and particle-sorbed phases, and fate and transport in a system consisting of a well-mixed water column and an active sediment layer. Hg can enter the lake from watershed runoff and by atmospheric deposition directly to the lake surface. Once in the lake, Hg exchanges between water and air, and water and sediments, and exits by sediment burial, advective flow and volatilization. The model was applied to a hypothetical drainage lake on the Canadian Shield. Model estimates of water and sediment concentrations compare well with measured values. The results suggest that the three Hg species experience significantly different fates and persistence, with overall Hg dynamics dominated by the fate of HgII (the predominant species). A sensitivity analysis illustrates the importance of physical/chemical properties and lake characteristics on the total amount and behavior of Hg in the lake.