Dissolved Gaseous Mercury Dynamics in Lake Michigan

Session: Poster Session

Michael Tate, U.S. Geological Survey, [email protected]
Ryan Lepak, USEPA - ORD, [email protected]
Jacob Ogorek, USGS, [email protected]
John Dewild, U.S. Geological Survey, [email protected]
David Krabbenhoft, USGS, [email protected]
James Hurley, Wisconsin Sea Grant, [email protected]

Abstract

While aboard the EPA R/V Lake Guardian, speciated atmospheric mercury (Hg) was measured in combination with continuous surface dissolved gaseous Hg (DGM) and at select points profiles of DGM, filterable and filter passing total Hg and methylmercury in Lake Michigan. Reactive and particle-associated fractions constituted on average 1.0 ± 0.5%, of the overall gaseous elemental Hg (1.34 ± 0.14 ng m^-3) and concentrations were increased in the urbanized southern basin. In the open lake, DGM was low and it was elevated two-fold in Green Bay likely due to microbial reduction.  Surface fluxes of Hg were estimated to range 0.60 ± 0.36 and 2.96 ± 1.23 ng m^-2 hr^-1 for low and high wind conditions respectively. While constantly supersaturated, surface DGM exhibited diel cycling, with lower DGM levels during the day, suggesting increased flux during the day, due to increased winds. During stratified periods, epilimnetic DGM is formed via photochemical reduction and does not penetrate hypolimnetic waters. Therefore, Hg reduction pathways in the hypolimnion must rely on biological reduction mechanisms. In vertical profiles, we observed an increase in DGM at the thermocline, however the concentration of DGM was negatively correlated to biomass, suggesting biologically-mediated DGM oxidation.