CLIMATE change, acid deposition and increasing solar ultraviolet irradiance(1) have all combined to produce marked effects on lake waters and their ecosystems, Schindler et al.(2) have shown that both climate warming and lake acidification have led to decreases in dissolved organic carbon concentrations in North American boreal lakes, resulting in a markedly increased exposure of the upper water column to solar ultraviolet radiation, Here we report ten years of observations of rainfall and lake chemistry at Swan Lake, Canada, that suggest that a fall in dissolved organic carbon concentrations can also occur by a different combination of climate change and acidification. In this boreal fresh water, a drought decreased lakewater levels, thus exposing to the atmosphere littoral sediments containing reduced sulphur from the previous atmospheric deposition of industrial sulphur dioxide into the lake and its catchment. This exposure caused a reoxidation of sediment sulphur, resulting in a remobilization of acid into the lake water, and thus a decrease in dissolved organic carbon concentrations sufficient to increase by three-fold the depth to which ultraviolet radiation can penetrate.