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Magnitude of oceanic nitrogen fixation influenced by the nutrient uptake ratio of phytoplankton

Nature Geoscience volume 3, pages 412416 (2010) | Download Citation

Abstract

The elemental stoichiometry of sea water and particulate organic matter is remarkably similar. This observation led Redfield to hypothesize that the oceanic ratio of nitrate to phosphate is controlled by the remineralization of phytoplankton biomass1. The Redfield ratio is used universally to quantitatively link the marine nitrogen and phosphorus cycles in numerous biogeochemical applications2,3,4. Yet, empirical and theoretical studies show that the ratio of nitrogen to phosphorus in phytoplankton varies greatly with taxa5,6 and growth conditions7,8,9. Here we present a dynamic five-box ecosystem model showing that non-Redfield utilization of dissolved nitrogen and phosphorus by non-nitrogen-fixing phytoplankton controls the magnitude and distribution of nitrogen fixation. In our simulations, systems dominated by rapidly growing phytoplankton with low nitrogen to phosphorus uptake ratios reduce the phosphorus available for nitrogen fixation. In contrast, in systems dominated by slow-growing phytoplankton with high nitrogen to phosphorus uptake ratios nitrogen deficits are enhanced, and nitrogen fixation is promoted. We show that estimates of nitrogen fixation are up to fourfold too high when non-Redfield uptake stoichiometries are ignored. We suggest that the relative abundance of fast- and slow-growing phytoplankton controls the amount of new nitrogen added to the ocean.

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Acknowledgements

We thank G. van Dijken and B. Saenz for help coding the model and L. Thomas for help parameterizing upwelling fluxes. We are also indebted to the members of the Arrigo laboratory and to C. M. Moore for fruitful discussions on nutrient utilization ratios of different phytoplankton and for comments on the manuscript. This work was supported by NSF grant ANT 0732535 and DOE grant DE-FG02-04ER63896 to K.R.A.

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

    • Matthew M. Mills
    •  & Kevin R. Arrigo

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Contributions

All authors contributed equally to this work.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Matthew M. Mills.

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DOI

https://doi.org/10.1038/ngeo856

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