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Large-scale distribution of Atlantic nitrogen fixation controlled by iron availability

Nature Geoscience volume 2, pages 867–871 (2009)Cite this article

Abstract

Oceanic fixed-nitrogen concentrations are controlled by the balance between nitrogen fixation and denitrification1,2,3,4. A number of factors, including iron limitation5,6,7, can restrict nitrogen fixation, introducing the potential for decoupling of nitrogen inputs and losses2,5,8. Such decoupling could significantly affect the oceanic fixed-nitrogen inventory and consequently the biological component of ocean carbon storage and hence air–sea partitioning of carbon dioxide2,5,8,9. However, the extent to which nutrients limit nitrogen fixation in the global ocean is uncertain. Here, we examined rates of nitrogen fixation and nutrient concentrations in the surface waters of the Atlantic Ocean along a north–south 10,000 km transect during October and November 2005. We show that rates of nitrogen fixation were markedly higher in the North Atlantic compared with the South Atlantic Ocean. Across the two basins, nitrogen fixation was positively correlated with dissolved iron and negatively correlated with dissolved phosphorus concentrations. We conclude that inter-basin differences in nitrogen fixation are controlled by iron supply rather than phosphorus availability. Analysis of the nutrient content of deep waters suggests that the fixed nitrogen enters North Atlantic Deep Water. Our study thus supports the suggestion that iron significantly influences nitrogen fixation5, and that subsequent interactions with ocean circulation patterns contribute to the decoupling of nitrogen fixation and loss2,4,8.

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Figure 1: Cruise tracks and maps of annual mean excess nutrient distributions.
Figure 2: Transect data from the AMT-17 cruise.
Figure 3: Deep water N* distribution and fluxes in the Atlantic.

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Acknowledgements

We thank the scientific complement and crew of the RRS Discovery during AMT-17 for all of their assistance. P. Holligan, A. Martin, R. Sanders and T. Tyrrell are thanked for discussions and comments on the manuscript. This work was supported by EU Carboocean grants to R.J.G. and J.L.R. and the UK Natural Environment Research Council, through a fellowship to C.M.M. and the Atlantic Meridional Transect consortium. This is contribution number 172 of the AMT programme.

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Authors and Affiliations

  1. Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK

    C. Mark Moore, Richard J. Geider & David J. Suggett

  2. National Oceanography Centre, University of Southampton, European Way, Southampton SO14 3ZH, UK

    C. Mark Moore, Eric P. Achterberg, Elaine L. McDonagh, Xi Pan, Alex J. Poulton & Micha J. A. Rijkenberg

  3. Department of Environmental Earth System Science, Stanford University, Stanford, California 94305, USA

    Matthew M. Mills

  4. Marine Biogeochemistry, Leibniz-Institut für Meereswissenschaften, D-24105 Kiel, Germany

    Julie LaRoche

  5. Department of Zoology, University of Cape Town, Rondebosch 7701, South Africa

    Mike I. Lucas

  6. School of Earth, Ocean and Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, UK

    Simon J. Ussher

  7. Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK

    E. Malcolm S. Woodward

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Contributions

R.J.G., J.L.R., C.M.M., M.M.M. and E.P.A. designed the research. C.M.M., M.M.M., E.P.A., M.I.L., X.P., D.J.S., S.J.U. and E.M.S.W. collected samples and carried out the work at sea. M.J.A.R. analysed iron samples. A.J.P. enumerated Trichodesmium. C.M.M. and E.L.M. analysed nutrient distributions and calculated transport data. C.M.M. wrote the first draft of the paper. C.M.M., M.M.M., R.J.G., J.L.R. and E.P.A. discussed the results and extensively edited subsequent drafts. All of the authors commented on the manuscript.

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Correspondence to C. Mark Moore.

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Mark Moore, C., Mills, M., Achterberg, E. et al. Large-scale distribution of Atlantic nitrogen fixation controlled by iron availability. Nature Geosci 2, 867–871 (2009). https://doi.org/10.1038/ngeo667

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