Using Mercury Stable Isotope Ratios to Trace Hg Bioaccumulation into the Lower Food Web of Lake Erie

Session: Chemical Monitoring and Surveillance in the Great Lakes: Multimedia (2)

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

Abstract

In aquatic ecosystems, the use of mercury (Hg) stable isotopes to identify Hg sources has typically focused on identification of point and non-point sources in sediments, with the assumption that sediments were primary sources of bioaccumulative methylmercury (MeHg). We measure Hg stable isotopes in the suspended particulate matter (SPM), water, and planktonic material of Lake Erie, and from surrounding watershed inputs to better understand the relative availability of contrasting Hg sources for bioaccumulation. Hg mass dependent and independent isotopic fractionation suggest contrasting Hg source portfolios for SPM and filtered water in both inflowing rivers and open water. Riverine SPM resembled terrestrial soils, and western Lake Erie sediments, while open lake SPM, primarily comprised of planktonic material resembled atmospheric Hg with isotopic signatures linked to the fraction of MeHg in planktonic material. This allowed calculation of ?²⁰²Hg and ?¹⁹⁹Hg for the source inorganic Hg and epilimnetic MeHg, useful for determining dynamics associated with recently deposited Hg. Large ?²⁰⁰Hg in planktonic material highlights the importance of recently deposited Hg from atmospheric precipitation. These results decouple Hg sources to the lower trophic epilimnetic food web from Hg to the sediments, highlighting the relative importance of atmospheric and watershed-sourced Hg to biota.