Mercury Methylation Influenced by Eutrophic Conditions in Salmon Falls Creek Reservoir, Idaho

Salmon Falls Creek Reservoir (SFCR) in southern Idaho has been under a mercury (Hg) advisory since 2001 because fish in this reservoir contain methyl-Hg exceeding the State of Idaho advisory concentration of 0.5 mg/kg. Concentrations of total Hg and methyl-Hg were measured in reservoir water, bottom sediment, and porewater extracted from reservoir bottom sediment to examine processes of Hg methylation at the sediment:water interface in SFCR. Rates of Hg methylation and methyl-Hg demethylation were also measured in SFCR bottom sediment using isotopic tracer techniques to further evaluate methylation of Hg. The highest concentrations for total Hg and methyl-Hg in bottom sediment were consistently found at the sediment:water interface, and concentrations of both species declined with depth. Porewater extracted from bottom sediment contained highly elevated concentrations of total Hg ranging from 11-230 ng/L and methyl-Hg ranging from 0.68-8.5 ng/L. Methylation rate experiments carried out on sediment from the sediment:water interface show high rates of Hg methylation ranging from 2.3-17 %/day, which are significantly higher than Hg methylation rates reported in other Hg contaminant studies. Methanogenic bacteria were especially active at one site studied in SFCR, resulting in the highest methylation rates. Using porewater MeHg measured at sites studied throughout SFCR, we calculated a total diffusive methyl-Hg flux of 197 g/year for SFCR at the sediment:water interface. This methyl-Hg is delivered to the overlying SFCR water column, and eventually transferred to biota such as fish. Data indicate that methylation of Hg in SFCR is highest under the hypolimnetic and eutrophic conditions where dissolved plant nutrients were high, decaying organic matter and organic-rich bottom sediment were prevalent, numerous blue-green algae were present, and bottom water is thermo-chemically stratified and colder than the remainder of SFCR water and the bottom water is also low in dissolved oxygen with reduced redox conditions.