The mercury experiment to assess atmospheric loading in Canada and the United States (METAALICUS) aims at establishing the link between atmospheric deposition of mercury (Hg) and Hg concentrations in fish. As part of this initiative, we conducted an enclosure experiment in Lake 239 (ON, Canada). Our goal was to follow over time dissolved gaseous mercury (DGM) concentrations, after the addition of 200HgCl2, to assess post-depositional Hg dynamics. DGM concentrations reached very high levels in surface waters (up to 6ngl−1) during the days following the spike. This increase in DGM levels coincided with a decrease in total Hg in the enclosure. Photoreduction rates of Hg were high after spiking (1ngl−1 h−1) and decreased by two orders of magnitude during the summer, with low rates observed in August (0.01ngl−1 h−1). These low rates may be caused by photobleaching of dissolved organic carbon. Water-to-air Hg fluxes (evasion) were measured with a flux chamber and modelled using DGM; both methods yielded similar fluxes when using time-averaged DGM values. Together, these results indicate that, under certain conditions, large amounts of newly deposited Hg(II) may be converted to DGM by photochemical processes and lost by evasion across the air/water interface.