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Methylmercury production below the mixed layer in the North Pacific Ocean

Abstract

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.

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Acknowledgements

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). https://doi.org/10.1038/ngeo1918

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