Experimental evidence of a linear relationship between inorganic mercury loading and methylmercury accumulation by aquatic biota

Developing effective regulations on mercury (Hg) emissions requires a better understanding of how atmospheric Hg deposition affects methylmercury (MeHg) levels in aquatic biota. This study tested the hypothesis that MeHg accumulation in aquatic food webs is related to atmospheric Hg deposition. We simulated a range of inorganic Hg deposition rates by adding isotopically enriched Hg(II) (90.9% 202Hg) to 10-m diameter mesocosms in a boreal lake. Concentrations of experimentally added ("spike") Hg were monitored in zooplankton, benthic invertebrates, and fish. Some Hg(II) added to the mesocosms was methylated and incorporated into the food web within weeks, demonstrating that Hg(II) deposited directly to aquatic ecosystems can become quickly available to biota. Relationships between Hg(II) loading rates and spike MeHg concentrations in zooplankton, benthic invertebrates, and fish were linear and significant. Furthermore, spike MeHg concentrations in the food web were directly proportional to Hg(II) loading rates (i.e., a percent change in Hg(II) loading rate resulted in, statistically, the same percent change in MeHg concentration). This is the first experimental determination of the relationship between Hg(II) loading and MeHg bioaccumulation in aquatic biota. We conclude that changes in atmospheric Hg deposition caused by increases or decreases in Hg emissions will ultimately affect MeHg levels in aquatic food webs.