Methylmercury (MeHg) bioaccumulation is a growing conc in ecosystems worldwide.The absorption of solar radiation by dissolved organic matter (DOM) and other photoreactive ligands can convert MeHg into less toxic forms of mercury through photodemethylation.In this study,spectral changes and photoreactivity of DOM were measured to assess the potential to control photoreactions and predict in situ MeHg concentration.Water samples collected from a series of lakes in southwest Nova Scotia in June,August,and September were exposed to controlled ultraviolet-A (W-A) radiation for up to 24 hr.Dissolved organic matter photoreactivity,measured as the loss of absorbance at 350 nm at constant UV-A irradiation,was positively dependent on the initial DOM concentration in lake waters (r2 =0.94).This relationship was consistent over time with both DOM concentration and photoreactivity increasing from summer into fall across lakes.Lake in situ MeHg concentration was positively correlated with DOM concentration and likely catchment transport in June (r =0.77) but not the other sampling months.Despite a consistent seasonal variation in both DOM and Fe,and their respective correlations with MeHg,no discable seasonal trend in MeHg was observed.However,a 3-year dataset from the 6 study lakes revealed a positive correlation between DOM concentration and both Fe (r =0.91) and MeHg concentrations (r =0.51) suggesting a more dominant landscape mobility control on MeHg.The DOM-MeHg relationships observed in these lakes highlights the need to examine DOM photoreactivity controls on MeHg transport and availability in natural waters particularly given future climate perturbations.