Temporal Changes in Photoreducible Mercury, Photoreduction Rates, and the Role of Dissolved Organic Matter in Freshwater Lakes.

Total photoreducible mercury [Hg(II) RED ] and photoreduction rates in the surface waters of four lakes in Kejimkujik National Park, Nova Scotia were measured monthly over a summer. The percent of THg that was photoreducible [%Hg(II) RED ] decreased significantly in two of the four lakes from early to late summer: North Cranberry (maximum 42% to minimum 14%) and Big Dam East (maximum 51% to minimum 6%). Hg(II) RED was found to have a linear relationship with THg for all combined site data. THg and Hg(II) RED were found to have positive linear relationships with DOC concentrations (R 2  = 0.97; n = 36; p < 0.01 and R 2  = 0.75; n = 36; p < 0.01, respectively). A smaller proportion of THg was found to be photoreducible with increasing DOC concentration.