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Dissolved gaseous mercury dynamics in Lake Michigan
Citation:
Lepak, R., D. Krabbenhoft, J. Ogorek, J. DeWild, M. Tate, AND J. Hurley. Dissolved gaseous mercury dynamics in Lake Michigan. International Conference of Great Lakes Research, Brockport, NY, June 10 - 14, 2019.
Impact/Purpose:
Mercury is a neurotoxic metal that is rapidly distributed via the atmosphere to remote locations worldwide. However, we currently do not understand the reaction dynamics between speciated components of atmospheric mercury and the exchange of these Hg species with freshwater. Therefore, we measured these components over Lake Michigan and present the first continuous, whole ecosystem dynamic exchange model for a lake in the Great Lakes.
Description:
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 ng m-2 hr-1 and 2.96 ± 1.23 ng m-2 hr-1 for low and high wind conditions. 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 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 uptake of DGM by biota.