Seasonal Dynamics and Interannual Variability in Mercury Concentrations and Loads through a Three-Reservoir Complex

The Hells Canyon
Complex (HCC) along the Snake River (Idaho–Oregon
border, U.S.A.) encompasses three successive reservoirs that seasonally
stratify, creating anoxic conditions in the hypolimnion that promote
methylmercury (MeHg) production. This study quantified seasonal dynamics
and interannual variability in mercury concentrations (inorganic divalent
mercury (IHg) and MeHg) and loads at four reservoir inflow and outflow
locations through the HCC (2014–2017). We observed (1) that
the HCC is a net sink for both IHg and MeHg, (2) interannual variability
in IHg and MeHg loads largely reflecting streamflow conditions, and
(3) seasonal variability in particulate IHg loading at the inflow
(greatest from February to April) and MeHg export from the outflow
(greatest from September to December) of the HCC. Seasonal export
of MeHg was evidenced by increases in monthly mean concentrations
of unfiltered MeHg (approximately 2-fold) and the percentage of total
mercury (THg) as MeHg (≥4-fold) coincident with reservoir destratification.
Despite evidence of seasonal export of MeHg from the HCC, annual loads
indicate a 42% decrease in unfiltered MeHg from HCC inflow to outflow.
Results from this study improve the understanding of seasonal variability
in mercury transport through and transformation within a reservoir
complex.