Letter

Nature 447, 999-1002 (21 June 2007) | doi:10.1038/nature05885; Received 16 February 2007; Accepted 27 April 2007

The significance of nitrification for oceanic new production

Andrew Yool1, Adrian P. Martin1, Camila Fernández2,3 & Darren R. Clark4

  1. National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH, UK
  2. Laboratoire d'Océanographie et de Biogéochimie, Centre d'Océanologie de Marseille, 163 avenue de Luminy, Case 901, F-13288 Marseille, France
  3. Laboratorio de Procesos Oceanográficos y Clima, Departamento de Oceanografía y Centro de Investigación Oceanográfica en el Pacifico Sur Oriental, Universidad de Concepción, Casilla 160-C, Concepción, Chile
  4. Plymouth Marine Laboratory, Prospect Place, Plymouth PL1 3DH, UK

Correspondence to: Andrew Yool1 Correspondence and requests for materials should be addressed to A.Y. (Email: axy@noc.soton.ac.uk).

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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