Methylmercury photodegradation influenced by sea-ice cover in Arctic marine ecosystems

Journal name:
Nature Geoscience
Volume:
4,
Pages:
188–194
Year published:
DOI:
doi:10.1038/ngeo1049
Received
Accepted
Published online
Corrected online

Abstract

Atmospheric deposition of mercury to remote areas has increased threefold since pre-industrial times. Mercury deposition is particularly pronounced in the Arctic. Following deposition to surface oceans and sea ice, mercury can be converted into methylmercury, a biologically accessible form of the toxin, which biomagnifies along the marine food chain. Mass-independent fractionation of mercury isotopes accompanies the photochemical breakdown of methylmercury to less bioavailable forms in surface waters. Here we examine the isotopic composition of mercury in seabird eggs collected from colonies in the North Pacific Ocean, the Bering Sea and the western Arctic Ocean, to determine geographical variations in methylmercury breakdown at northern latitudes. We find evidence for mass-independent fractionation of mercury isotopes. The degree of mass-independent fractionation declines with latitude. Foraging behaviour and geographic variations in mercury sources and solar radiation fluxes were unable to explain the latitudinal gradient. However, mass-independent fractionation was negatively correlated with sea-ice cover. We conclude that sea-ice cover impedes the photochemical breakdown of methylmercury in surface waters, and suggest that further loss of Arctic sea ice this century will accelerate sunlight-induced breakdown of methylmercury in northern surface waters.

At a glance

Figures

  1. Common and thick-billed murre colony locations and sea-ice conditions at the beginning of the 2002 breeding season.
    Figure 1: Common and thick-billed murre colony locations and sea-ice conditions at the beginning of the 2002 breeding season.

    a, The colonies include Cape Lisburne (CLIS) in the Chukchi Sea; St Lawrence (STLW), St George (STGE) and Bogoslof islands (BOGO) in the Bering Sea; and East Amatuli (EAAM) and St Lazaria islands (STLA) in the Gulf of Alaska (see Supplementary Table S1). Numbers of samples analysed from the colonies are shown in parentheses, and the dashed circles show the maximum 170km foraging range around them. Sea-ice concentrations (%) show the relative amounts of ice in 25km by 25km blocks in late March 2002. b, Murres breeding at northern latitudes stage and forage in the open lead systems shown in the inset pictures.

  2. Mercury three-isotope diagrams illustrating variations in mercury MDF and MIF in murre eggs.
    Figure 2: Mercury three-isotope diagrams illustrating variations in mercury MDF and MIF in murre eggs.

    ac, δ202Hg plotted against δ200Hg (a), δ201Hg (b) and Δ201Hg (c). See Fig. 1 for legend. The dashed lines in a and b represent the theoretically predicted MDF based on the δ202Hg values35. Mercury δ199Hg and Δ199Hg information is shown in Supplementary Fig. S1. The dotted lines in b and c show the regression trends for the birds breeding at northern latitudes (eastern Chukchi Sea) and southern latitudes (southern Bering Sea and Gulf of Alaska). The uncertainties reported for the samples are the external reproducibility (n = 7) of the method used for NIST Murre Egg Control Material QC04-ERM1 (±0.26‰, 2σ, s.d. for δ202Hg, and ±0.10‰, 2σ, s.d. for Δ201Hg).

  3. Ecological effects on mercury MDF and MIF in murre eggs.
    Figure 3: Ecological effects on mercury MDF and MIF in murre eggs.

    ac, Influence of trophic levels (colony mean ± σ, s.d.) on Hg concentrations (colony mean ± σ, s.d.) (a), Hg MDF (δ202Hg, colony mean ± σ, s.d.) (b) and Hg MIF (Δ201Hg, colony mean ± σ, s.d.) (c) for murre egg sampling events (see Supplementary Table S1 for details and Fig. 1 for legend). The trophic levels of the eggs were estimated from δ15N measurements made on the samples using the method reported by Hobson and co-workers39 and corrected for δ15N baseline shifts (see Supplementary Method S1). Solid lines show the regression trends for the southern Bering Sea and Gulf of Alaska eggs, and dashed lines show the regression trends for all colonies.

  4. Effect of latitude on mercury MIF.
    Figure 4: Effect of latitude on mercury MIF.

    Colony mean Δ201Hg (±σ, s.d.) for common and thick-billed murre eggs collected at different latitudes in the Gulf of Alaska and Bering and Chukchi seas (see Fig. 1 for legend).

  5. Influence of sea ice on egg mercury MIF.
    Figure 5: Influence of sea ice on egg mercury MIF.

    Influence of 2002 sea-ice concentrations on Hg MIF Δ201Hg (colony mean ± σ, s.d.) in common and thick-billed murre eggs (see Fig. 1 for legend). Seasonal ice concentration values represent the three-month mean condition (±σ, s.d.) from the arrival at the breeding grounds in early April until most eggs were laid in late June. Sea-ice values for the colonies integrate the 170km foraging distance around the nesting locations (that is, the maximum foraging range shown by the dashed circles in Fig. 1a).

  6. Odd-isotope anomalies in [Delta]201Hg versus [Delta]199Hg space.
    Figure 6: Odd-isotope anomalies in Δ201Hg versus Δ199Hg space.

    The solid and dashed lines show the Δ199/201Hg slopes associated with natural iHg and MeHg photoreduction respectively, based on experiments18 and a literature survey14 (see Fig. 1 for legend). The dash–dot line represents the York regression trend for all samples. The uncertainties reported for the samples are the external reproducibility (n = 7) of the method used for NIST Murre Egg Control Material QC04-ERM1 (±0.11‰, 2σ, s.d. for Δ199Hg, and ±0.10‰, 2σ, s.d. for Δ201Hg; see Supplementary Table S2b).

Change history

Corrected online 21 January 2011
In the version of this Article originally published online, 'Arctic sea' should have read 'Arctic sea ice' in the last sentence of the abstract. This error has now been corrected in all versions of the text.

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Author information

Affiliations

  1. National Institute of Standards and Technology (NIST), Hollings Marine Laboratory, Charleston, South Carolina 29412, USA

    • D. Point,
    • R. D. Day,
    • S. S. Vander Pol,
    • A. J. Moors,
    • R. S. Pugh &
    • P. R. Becker
  2. Laboratoire des Mécanismes et Transferts en Géologie (LMTG), Observatoire Midi-Pyrénées, UMR CNRS 5563, UMR IRD 154, Université Paul Sabatier, 31400 Toulouse, France

    • D. Point &
    • J. E. Sonke
  3. US Fish and Wildlife Service, Alaska Maritime National Wildlife Refuge, Homer, Alaska 99603, USA

    • D. G. Roseneau
  4. Environment Canada, Saskatoon, Saskatchewan, S7N 0X4, Canada

    • K. A. Hobson
  5. Institut Pluridisciplinaire de Recherche sur l’Environnement et les Materiaux, Equipe de Chimie Analytique BioInorganique et Environnement, UMR CNRS 5254, 64053 Pau, France

    • O. F. X. Donard

Contributions

P.R.B., O.F.X.D., D.P. and R.D.D. designed the study; D.G.R. obtained the scientific collecting permits, made arrangements to collect the eggs and coordinated field logistics; and S.S.V. managed sample processing and banking. A.J.M. and R.S.P. were responsible for specimen processing, cryogenic banking and cryogenic homogenizations. Mercury isotopes were measured by D.P., R.D.D. and J.E.S. R.D.D. measured total mercury and K.H.H. measured nitrogen stable isotopes. D.P. and J.E.S. prepared the manuscript and all of the authors reviewed it.

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