Mercury Stable Isotopes in Seabird Eggs Reflect a Gradient from Terrestrial Geogenic to Oceanic Mercury Reservoirs
Russell D. Day, David G. Roseneau, Sylvain Berail, Keith A. Hobson, O.F.X Donard, Stacy S. Schuur, Rebecca S. Pugh, Amanda J. Moors, Stephen E. Long, Paul R. Becker
Elevated mercury concentrations ([Hg]) were found in Alaskan seabird eggs (Uria spp.) from the coastal embayment of Norton Sound relative to insular colonies in the northern Bering Sea. Stable isotopes of Hg, carbon (C), and nitrogen (N) were measured in these eggs to investigate the source of this enrichment. Lower δ13C values in eggs from within Norton Sound (-23.3 to -20.0 ) relative to the northern Bering Sea (-20.9 to -18.7) indicate a major terrestrial C source is associated with the elevated [Hg] in Norton Sound, implicating the Yukon River and other smaller watersheds as the Hg source. The gradient of increasing [Hg] extending inshore was accompanied by a strong gradient of decreasing δ202Hg and Δ199Hg in eggs, indicating a lower degree of mass dependent (MDF) and mass independent Hg fractionation (MDF) (respectively) in the Norton Sound food web. Negative or zero MDF and MIF signatures are typical of geological Hg sources, which further suggests a recent geological origin of Hg in Norton Sound biota that has experienced a relatively limited extent of aquatic fractionation. Oceanic colonies exhibited more highly fractionated positive MDF and MIF values, with transitional colonies having intermediate values suggesting mixing of Hg from terrestrial geogenic and oceanic reservoirs. The association of low δ202Hg and Δ199Hg with elevated [Hg] and terrestrial δ13C values suggests Hg stable isotopes in seabird eggs effectively differentiate Hg from terrestrial/geogenic sources from oceanic reservoirs.