Significant mixed layer nitrification in a natural iron-fertilized bloom of the Southern Ocean

• Published in: Global Biogeochemical Cycles
• Main Authors: Fripiat, F., Elskens, M., Trull, T. W., Blain, S., Cavagna, A.-J., Fernandez, C., Fonseca-Batista, D., Planchon, F., Raimbault, Patrick, Roukaerts, A., Dehairs, F.
• Other Authors: Analytical, Environmental and Geo- Chemistry, Vrije Universiteit Brussel (VUB), CSIRO Marine and Atmospheric Research (CSIRO-MAR), Commonwealth Scientific and Industrial Research Organisation Canberra (CSIRO), Antarctic Climate and Ecosystems Cooperative Research Centre (ACE-CRC), Laboratoire d'Océanographie Microbienne (LOMIC), Centre National de la Recherche Scientifique (CNRS)-Sorbonne Université (SU)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire océanologique de Banyuls (OOB), Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), Laboratoire des Sciences de l'Environnement Marin (LEMAR) (LEMAR), Institut Universitaire Européen de la Mer (IUEM), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Centre National de la Recherche Scientifique (CNRS)-Université de Brest (UBO), Institut méditerranéen d'océanologie (MIO), Institut de Recherche pour le Développement (IRD)-Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Toulon (UTLN)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2015
• Subjects: organic-carbon; north pacific; ACL; stable-isotopes; fresh-water; biogenic silica; kerguelen plateau; nitrous-oxide production; oxygen isotopic composition; phytoplankton bloom; sea-ice; envir; socio; Kerguelen; Pacific; Southern Ocean; Sea ice
• Online Access: https://doi.org/10.1002/2014GB005051; https://hal.archives-ouvertes.fr/hal-02556217/file/2014GB005051.pdf; https://hal.archives-ouvertes.fr/hal-02556217

WOS:000368907500005 International audience Nitrification, the microbially mediated oxidation of ammonium into nitrate, is generally expected to be low in the Southern Ocean mixed layer. This paradigm assumes that nitrate is mainly provided through vertical mixing and assimilated during the vegetative season, supporting the concept that nitrate uptake is equivalent to the new primary production (i.e., primary production which is potentially available for export). Here we show that nitrification is significant (similar to 40-80% of the seasonal nitrate uptake) in the naturally iron-fertilized bloom over the southeast Kerguelen Plateau. Hence, a large fraction of the nitrate-based primary production is regenerated, instead of being exported. It appears that nitrate assimilation (light dependent) and nitrification (partly light inhibited) are spatially separated between the upper and lower parts, respectively, of the deep surface mixed layers. These deep mixed layers, extending well below the euphotic layer, allow nitrifiers to compete with phytoplankton for the assimilation of ammonium. The high contributions of nitrification to nitrate uptake are in agreement with both low export efficiency (i.e., the percentage of primary production that is exported) and low seasonal nitrate drawdown despite high nitrate assimilation.