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Nitrate supply from deep to near-surface waters of the North Pacific subtropical gyre

Nature volume 465, pages 1062–1065 (2010)Cite this article

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Abstract

Concentrations of dissolved inorganic carbon (DIC) decrease in the surface mixed layers during spring and summer in most of the oligotrophic ocean. Mass balance calculations require that the missing DIC is converted into particulate carbon by photosynthesis1,2,3. This DIC uptake represents one of the largest components of net community production in the world ocean2,4. However, mixed-layer waters in these regions of the ocean typically contain negligible concentrations of plant nutrients such as nitrate and phosphate3,5. Combined nutrient supply mechanisms including nitrogen fixation, diffusive transport and vertical entrainment are believed to be insufficient to supply the required nutrients for photosynthesis6,7. The basin-scale potential for episodic nutrient transport by eddy events is unresolved8,9. As a result, it is not understood how biologically mediated DIC uptake can be supported in the absence of nutrients. Here we report on high-resolution measurements of nitrate (NO3−) and oxygen (O2) concentration made over 21 months using a profiling float deployed near the Hawaii Ocean Time-series station in the North Pacific subtropical gyre. Our measurements demonstrate that as O2 was produced and DIC was consumed over two annual cycles, a corresponding seasonal deficit in dissolved NO3− appeared in water at depths from 100 to 250 m. The deep-water deficit in NO3− was in near-stoichiometric balance with the fixed nitrogen exported to depth. Thus, when the water column from the surface to 250 m is considered as a whole, there is near equivalence between nutrient supply and demand. Short-lived transport events (<10 days) that connect deep stocks of nitrate to nutrient-poor surface waters were clearly present in 12 of the 127 vertical profiles.

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Figure 1: Shipboard observations at HOT station ALOHA.
Figure 2: Profiling float observations in the upper 300 m.
Figure 3: Profiling float observations over 100 days from 19 December 2008 to 22 March 2009.

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Acknowledgements

This work was made possible by the engineering efforts of L. Coletti, H. Jannasch and D. Swift to integrate ISUS with the APEX float. C. Sakamoto performed ISUS sensor calibration. This work was supported by the David and Lucile Packard Foundation, the National Science Foundation, the National Oceanic and Atmospheric Administration and the US Office of Naval Research through the National Oceanographic Partnership Program. Funding was also provided by the Gordon and Betty Moore Foundation and the Center for Microbial Oceanography: Research and Education.

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Authors and Affiliations

  1. Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA,

    Kenneth S. Johnson

  2. School of Oceanography, University of Washington, Seattle, Washington 98195, USA,

    Stephen C. Riser

  3. School of Ocean and Earth Science and Technology, Center for Microbial Oceanography: Research and Education, University of Hawaii, Honolulu, Hawaii 96822, USA,

    David M. Karl

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  1. Kenneth S. Johnson
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Contributions

S.C.R. and K.S.J. wrote proposals to support incorporation of ISUS into the APEX float and co-lead the engineering effort to complete sensor integration. The system was deployed at the HOT station to take advantage of the background information and hypotheses generated by the HOT program, which was led by D.M.K. The initial data analysis was performed by K.S.J. and all authors contributed to writing the manuscript.

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Correspondence to Kenneth S. Johnson.

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Competing interests

Monterey Bay Aquarium Research Institute and K.S.J. receive small annual licence fees for commercial versions of the ISUS nitrate sensor used in this work.

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Johnson, K., Riser, S. & Karl, D. Nitrate supply from deep to near-surface waters of the North Pacific subtropical gyre. Nature 465, 1062–1065 (2010). https://doi.org/10.1038/nature09170

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