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Methylmercury production below the mixed layer in the North Pacific Ocean


Mercury enters marine food webs in the form of microbially generated monomethylmercury. Microbial methylation of inorganic mercury, generating monomethylmercury, is widespread in low-oxygen coastal sediments. The degree to which microbes also methylate mercury in the open ocean has remained uncertain, however. Here, we present measurements of the stable isotopic composition of mercury in nine species of marine fish that feed at different depths in the central North Pacific Subtropical Gyre. We document a systematic decline in δ202Hg, Δ199Hg and Δ201Hg values with the depth at which fish feed. We show that these mercury isotope trends can be explained only if monomethylmercury is produced below the surface mixed layer, including in the underlying oxygen minimum zone, that is, between 50 and more than 400 m depth. Specifically, we estimate that about 20–40% of the monomethylmercury detected below the surface mixed layer originates from the surface and enters deeper waters either attached to sinking particles, or in zooplankton and micronekton that migrate to depth. We suggest that the remaining monomethylmercury found at depth is produced below the surface mixed layer by methylating microbes that live on sinking particles. We suggest that microbial production of monomethylmercury below the surface mixed later contributes significantly to anthropogenic mercury uptake into marine food webs.

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Figure 1: Mercury isotope diagrams illustrating MDF and MIF variation in fish with depth.
Figure 2: Mercury isotope diagrams illustrating correlations between MIF and MDF and between MIF of Δ199Hg and MIF of Δ201Hg.
Figure 3: Schematic representation of a simplified model for Hg cycling between atmosphere, SML, pycnocline and marine food web.


  1. 1

    Sunderland, E. M. Mercury exposure from domestic and imported estuarine and marine fish in the US seafood market. Environ. Health Perspect. 115, 235–242 (2007).

    Article  Google Scholar 

  2. 2

    Fitzgerald, W. F., Lamborg, C. H. & Hammerschmidt, C. R. Marine biogeochemical cycling of mercury. Chem. Rev. 107, 641–662 (2007).

    Article  Google Scholar 

  3. 3

    Sunderland, E. M., Krabbenhoft, D. P., Moreau, J. W., Strode, S. A. & Landing, W. M. Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models. Glob. Biogeochem. Cycles 23, GB2010 (2009).

    Article  Google Scholar 

  4. 4

    Hammerschmidt, C. R. & Bowman, K. L. Vertical methylmercury distribution in the subtropical North Pacific Ocean. Mar. Chem. 132–133, 77–82 (2012).

    Article  Google Scholar 

  5. 5

    Cossa, D. et al. Mercury in the Southern Ocean. Geochim. Cosmochim. Acta 75, 4037–4052 (2011).

    Article  Google Scholar 

  6. 6

    Cossa, D., Averty, B. & Pirrone, N. The origin of methylmercury in open Mediterranean waters. Limnol. Oceanogr. 54, 837–844 (2009).

    Article  Google Scholar 

  7. 7

    Mason, R. P. & Fitzgerald, W. F. Alkylmercury species in the equatorial Pacific. Nature 347, 457–459 (1990).

    Article  Google Scholar 

  8. 8

    Choy, C. A., Popp, B. N., Kaneko, J. J. & Drazen, J. C. The influence of depth on mercury levels in pelagic fishes and their prey. Proc. Natl Acad. Sci. USA 106, 13865–13869 (2009).

    Article  Google Scholar 

  9. 9

    Cossa, D. et al. Influences of bioavailability, trophic position, and growth on methylmercury in Hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic. Environ. Sci. Technol. 74, 4885–4893 (2012).

    Article  Google Scholar 

  10. 10

    Mason, R. P. et al. Mercury biogeochemical cycling in the ocean and policy implications. Environ. Res. 119, 101–117 (2012).

    Article  Google Scholar 

  11. 11

    Bloom, N. S. On the chemical form of mercury in edible fish and marine invertebrate tissue. Can. J. Fish. Aquat. Sci. 49, 1010–1017 (1992).

    Article  Google Scholar 

  12. 12

    Van der Velden, S., Dempson, J. B., Evans, M. S., Muir, D. C. G. & Power, M. Basal mercury concentrations and biomagnification rates in freshwater and marine food webs: Effects on Arctic charr (Salvelinus alpinus) from eastern Canada. Sci. Total Environ. 444, 531–542 (2013).

    Article  Google Scholar 

  13. 13

    Mason, R. P. & Fitzgerald, W. F. The distribution and biogeochemical cycling of mercury in the equatorial Pacific Ocean. 40, 1897–1924 (1993).

  14. 14

    Cossa, D., Martin, J. M., Takayanagi, K. & Sanjuan, J. The distribution and cycling of mercury species in the western Mediterranean. Deep Sea Res. II 44, 721–740 (1997).

    Article  Google Scholar 

  15. 15

    Monperrus, M. et al. Mercury methylation, demethylation and reduction rates in coastal and marine surface waters of the Mediterranean Sea. Mar. Chem. 107, 49–63 (2007).

    Article  Google Scholar 

  16. 16

    Lehnherr, I. St, Louis, V. L., Hintelmann, H. & Kirk, J. L. Methylation of inorganic mercury in polar marine waters. Nature Geosci. 4, 298–302 (2011).

    Article  Google Scholar 

  17. 17

    Heimbürger, L. E. et al. Methyl mercury distributions in relation to the presence of nano and picophytoplankton in an oceanic water column (Ligurian Sea, North-western Mediterranean). Geochim. Cosmochim. Acta 74, 5549–5559 (2010).

    Article  Google Scholar 

  18. 18

    Kraepiel, A. M., Keller, K., Chin, H. B., Malcolm, E. G. & Morel, F. M. Sources and variations of mercury in tuna. Environ. Sci. Technol. 37, 5551–5558 (2003).

    Article  Google Scholar 

  19. 19

    Blum, J. D. in Handbook of Environmental Isotope Geochemistry (ed. Baskaran, M.) Ch. 15, 229–246 (Springer, 2011).

    Google Scholar 

  20. 20

    Bergquist, B. A. & Blum, J. D. Mass-dependent and -independent fractionation of Hg isotopes by photo-reduction in aquatic systems. Science 318, 417–420 (2007).

    Article  Google Scholar 

  21. 21

    Senn, D. B. et al. Stable isotope (N, C, Hg) study of methylmercury sources and trophic transfer in the northern Gulf of Mexico. Environ. Sci. Technol. 44, 1630–1637 (2010).

    Article  Google Scholar 

  22. 22

    Point, D. et al. Methylmercury photodegradation influenced by sea-ice cover in Arctic marine ecosystems. Nature Geosci. 4, 188–194 (2011).

    Article  Google Scholar 

  23. 23

    McClelland, J. W. & Montoya, J. P. Trophic relationships and the nitrogen isotopic composition of amino acids in plankton. Ecology 83, 2173–2180 (2002).

    Article  Google Scholar 

  24. 24

    Rodriguez-Gonzalez, P. Species-specific stable isotope fractionation of mercury during Hg(II) methylation by an anaerobic bacteria (Desulfobulbus propionicus) under dark conditions. Environ. Sci. Technol. 43, 9183–9188 (2009).

    Article  Google Scholar 

  25. 25

    Kritee, K., Barkay, T. & Blum, J.D. Mass dependent stable isotope fractionation of mercury during microbial degradation of methylmercury. Geochim. Cosmochim. Acta 73, 1285–1296 (2009).

    Article  Google Scholar 

  26. 26

    Gantner, N., Hintelmann, H., Zheng, W. & Muir, D. C. Variations in stable isotope fractionation of Hg in food webs of Arctic lakes. Environ. Sci. Technol. 43, 9148–9154 (2009).

    Article  Google Scholar 

  27. 27

    Laffont, L. et al. Anomalous mercury isotopic compositions of fish and human hair in the Bolivian Amazon. Environ. Sci. Technol. 43, 8985–8990 (2009).

    Article  Google Scholar 

  28. 28

    Perrot, V. et al. Higher mass-independent isotope fractionation of methylmercury in the pelagic food web of Lake Baikal (Russia). Environ. Sci. Technol 46, 5902–5911 (2012).

    Article  Google Scholar 

  29. 29

    Sherman, L. S. & Blum, J. D. Mercury stable isotopes in sediments and largemouth bass from Florida lakes, USA. Sci. Total Environ. 448, 163–175 (2013).

    Article  Google Scholar 

  30. 30

    Gehrke, G. E., Blum, J. D., Slotton, D. G. & Greenfield, B. K. Mercury isotopes link mercury in San Francisco Bay forage fish to surface sediments. Environ. Sci. Technol. 4, 1264–1270 (2011).

    Article  Google Scholar 

  31. 31

    Kritee, K., Blum, J. D., Johnson, M. W., Bergquist, B. & Barkay, T. Mercury stable isotope fractionation during reduction of Hg(II) to Hg(0) by mercury resistant bacteria. Environ. Sci. Technol. 4, 1889–1895 (2007).

    Article  Google Scholar 

  32. 32

    Donovan, P. M., Blum, J. D., Yee, D., Gehrke, G. E. & Singer, M. B. An isotopic record of mercury in San Francisco Bay sediment. Chem. Geol. 349–350, 87–98 (2013).

    Article  Google Scholar 

  33. 33

    Sherman, L. S., Blum, J. D., Keeler, G. J., Demers, J. D. & Dvonch, J. T. Investigation of mercury pollution from a coal fired power plant using mercury isotopes. Environ. Sci. Technol. 46, 382–390 (2012).

    Article  Google Scholar 

  34. 34

    Zheng, W. & Hintelmann, H. Isotope fractionation of mercury during its photochemical reduction by low-molecular-weight organic compounds. J. Phys. Chem. A 114, 4246–4253 (2010).

    Article  Google Scholar 

  35. 35

    Kwon, S. Y. et al. Absence of fractionation of mercury isotopes during trophic transfer of methylmercury to freshwater fish in captivity. Environ. Sci. Technol. 46, 7527–7534 (2012).

    Article  Google Scholar 

  36. 36

    Stramma, L. et al. Expansion of oxygen minimum zones may reduce available habitat for tropical pelagic fishes. Nature Clim. Change 2, 33–37 (2012).

    Article  Google Scholar 

  37. 37

    Stramma, L., Johnson, G. C., Sprintall, J. & Mohrholz, V. Expanding oxygen-minimum zones in the tropical oceans. Science 320, 655–658 (2008).

    Article  Google Scholar 

  38. 38

    Choy, C. A. et al. Global trophic position comparison of two dominant mesopelagic fish families (Myctophidae, Stomiidae) using amino acid nitrogen isotopic analyses. PLoS ONE 7, e50133 (2012).

    Article  Google Scholar 

  39. 39

    Chikaraishi, Y. et al. Determination of aquatic food-web structure based on compound-specific nitrogen isotopic composition of amino acids. Limnol. Oceanogr. Methods 7, 740–750 (2009).

    Article  Google Scholar 

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We thank J. Pitz, A. Asato and S. Bailey for assistance with THg analyses; and K. Busscher and observers of the PIRO Longline Observer Program for sample collection. Financial support was provided to J.D.B. by the John D MacArthur Professorship and National Science Foundation (NSF) grant EAR-0952108. Additional financial support was provided by NSF grant OCE-1041329 (to B.N.P and J.C.D.), the Pelagic Fisheries Research Program (to J.C.D. and B.N.P.) and University of Hawaii Sea Grant Award RFM-27PD. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF or NOAA. This is SOEST contribution number 8947.

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J.D.B. supervised Hg isotope measurements and co-wrote the manuscript, B.N.P. supervised determinations of trophic position and co-wrote the manuscript, J.C.D. supervised sample collection and contributed to data interpretation, C.A.C. carried out Hg concentration and N isotope measurements and contributed to data interpretation, and M.W.J. carried out Hg isotope measurements and contributed to data interpretation.

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Correspondence to Joel D. Blum.

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Blum, J., Popp, B., Drazen, J. et al. Methylmercury production below the mixed layer in the North Pacific Ocean. Nature Geosci 6, 879–884 (2013).

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