The burning question: does burning before flooding lower methyl mercury production and bioaccumulation?

Production of methyl mercury (MeHg) is elevated in new hydroelectric reservoirs because organic carbon stimulates methylation of inorganic mercury (Hg) stored in the terrestrial system. This can cause adverse health in fish and in organisms that eat fish. We expected that burning vegetation before flooding would decrease the amount of Hg and organic carbon and thereby lower MeHg production. We conducted a replicated field experiment to investigate the effects of burning vegetation and soil before flooding on MeHg production and bioaccumulation. Vegetation and soil were added to mesocosms in the following combinations: unburned vegetation and unburned soil (Fresh treatments), burned vegetation and unburned soil (Partial Burn treatments), and burned vegetation and burned soil (Complete Burn treatments). Controls had no added vegetation or soil. During combustion with propane torches, a large percentage of the total Hg (THg) and MeHg was lost from vegetation and soil. THg and MeHg concentrations were highest in the surface water of Fresh treatments, lower in Partial Burn treatments and lowest in Complete Burn treatments and controls. Differences in concentrations of MeHg in biota were consistent among treatments, but did not follow aqueous concentrations. On the final sample date, MeHg concentrations in biota of Controls and Partial Burn treatments were greater than in Complete Burn and Fresh treatments. The lack of relationship between MeHg in biota and MeHg in water may have been due to modification of the bioavailability of MeHg by dissolved organic matter as the ratios of MeHg in biota to water were inversely correlated with concentrations of dissolved organic carbon. Although burning before flooding decreased MeHg concentrations in the water, it did not lower MeHg accumulation in the lower food web.