Nitrogen cycling driven by organic matter export in the South Pacific oxygen minimum zone

Abstract

Oxygen minimum zones are expanding globally, and at present account for around 20–40% of oceanic nitrogen loss. Heterotrophic denitrification and anammox—anaerobic ammonium oxidation with nitrite—are responsible for most nitrogen loss in these low-oxygen waters. Anammox is particularly significant in the eastern tropical South Pacific, one of the largest oxygen minimum zones globally. However, the factors that regulate anammox-driven nitrogen loss have remained unclear. Here, we present a comprehensive nitrogen budget for the eastern tropical South Pacific oxygen minimum zone, using measurements of nutrient concentrations, experimentally determined rates of nitrogen transformation and a numerical model of export production. Anammox was the dominant mode of nitrogen loss at the time of sampling. Rates of anammox, and related nitrogen transformations, were greatest in the productive shelf waters, and tailed off with distance from the coast. Within the shelf region, anammox activity peaked in both upper and bottom waters. Overall, rates of nitrogen transformation, including anammox, were strongly correlated with the export of organic matter. We suggest that the sinking of organic matter, and thus the release of ammonium into the water column, together with benthic ammonium release, fuel nitrogen loss from oxygen minimum zones.

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Figure 1: Maps of sampling locations and nutrient distributions in the ETSP OMZ.
Figure 2: Depth-integrated N-cycling rates in the ETSP OMZ.
Figure 3: N fluxes and nutrient inventory of the ETSP OMZ.

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Acknowledgements

We wish to thank the Peruvian government for access to their territorial waters. We sincerely thank the cruise leader M. Frank as well as the crews of the cruises M77-3 and 4 onboard R/V Meteor for their support at sea. We are grateful for the technical and analytical assistance of G. Klockgether, I. Boosmann, V. Leon, I. Grefe and A. Ellrott. We thank N. P. Revsbech, who generously provided the STOX sensors, and T. Ferdelmann for offering valuable comments to improve the article. This study was supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 754 ‘Climate-Biogeochemistry Interactions in the Tropical Ocean’ and the Max Planck Society.

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T.K., G.L. and M.M.M.K. designed the study. T.K., G.L., S.C. and A.P. performed 15N-labelling experiments. T.K., G.L. and P.L. analysed the data. C.R.L. carried out functional gene analyses. L.A. and A.O. modelled export production rates. L.S. provided CTD and ADCP data. T.K., G.L., P.L. and M.M.M.K. wrote the manuscript with input from all co-authors..

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Correspondence to Sergio Contreras or Aurélien Paulmier or Marcel M. M. Kuypers.

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The authors declare no competing financial interests.

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Kalvelage, T., Lavik, G., Lam, P. et al. Nitrogen cycling driven by organic matter export in the South Pacific oxygen minimum zone. Nature Geosci 6, 228–234 (2013). https://doi.org/10.1038/ngeo1739

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