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The significance of nitrification for oceanic new production


The flux of organic material sinking to depth is a major control on the inventory of carbon in the ocean1. To first order, the oceanic system is at equilibrium such that what goes down must come up2. Because the export flux is difficult to measure directly, it is routinely estimated indirectly by quantifying the amount of phytoplankton growth, or primary production, fuelled by the upward flux of nitrate3. To do so it is necessary to take into account other sources of biologically available nitrogen. However, the generation of nitrate by nitrification in surface waters has only recently received attention. Here we perform the first synthesis of open-ocean measurements of the specific rate of surface nitrification4,5,6,7,8,9,10,11,12 and use these to configure a global biogeochemical model13,14 to quantify the global role of nitrification. We show that for much of the world ocean a substantial fraction of the nitrate taken up is generated through recent nitrification near the surface. At the global scale, nitrification accounts for about half of the nitrate consumed by growing phytoplankton. A consequence is that many previous attempts to quantify marine carbon export, particularly those based on inappropriate use of the f-ratio (a measure of the efficiency of the ‘biological pump’), are significant overestimates.

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Figure 1: Global distribution of nitrification observations and the relationship with depth.
Figure 2: Modelled primary production and f -ratio.


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We thank B. Sinha, B. de Cuevas, G. Nurser and A. Coward for technical assistance with OCCAM; A. Rees for discussions of nitrification; A. Oschlies for comments on the manuscript; and P. Raimbault and R. Sutka for providing nitrification data. PATOM/PROOF (France) provided funding for the POMME experimental programme, the largest source of nitrification observations so far. A.Y. is funded by a UK National Environment Research Council (NERC) standard grant; A.P.M. is funded by a NERC Advanced Research fellowship; C.F. is funded by Fundación Andes, Chile; and D.R.C. was supported by a NERC grant and through the Atlantic Meridional Transect consortium.

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Correspondence to Andrew Yool.

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Supplementary Information

This file contains Supplementary Notes divided into following parts: Nitrification data which describes the different nitrification measurement protocols used in the papers from which this manuscript draws its data. It also outlines the procedures by which the observational data were processed to specific nitrification rates; Physical model which describes the physical ocean model, OCCAM, used in the modelling section of this manuscript; Biogeochemical model which describes the biogeochemical / ecological model used in the modelling section of this manuscript. This includes a full description of the changes made to include ammonium, “new” and “regenerated” nitrate, and an explanation of the “permanent thermocline” involved in the transfer of nitrate from the “regenerated” to “new” pool. Important nitrogen cycle processes that are omitted in this model are also discussed; Simulations which describe the initial conditions, spin-up and forcing period of the model simulation. It also describes the sensitivity experiments undertaken, and outlines a “correction procedure” used to try to correct “new” production for the effects of nitrification; Supplementary Results with some additional results not included within the main text for reasons of brevity. These include Supplementary Figures 3-7 with Legends and Supplementary Tables 2-4. (PDF 24171 kb)

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Yool, A., Martin, A., Fernández, C. et al. The significance of nitrification for oceanic new production. Nature 447, 999–1002 (2007).

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