Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions

• Published in: Biogeosciences
• Main Authors: Dehairs, F., Fripiat, F., Cavagna, A.-J., Trull, T. W., Fernandez, C., Davies, D., Roukaerts, A., Fonseca Batista, D., Planchon, Frédéric, Elskens, M.
• Other Authors: Analytical and Environmental Chemistry and Earth System Sciences, 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), Institute for Marine and Antarctic Studies Horbat (IMAS), University of Tasmania Hobart, Australia (UTAS), Centro de Investigación Oceanográfica en el Pacífico Sur Oriental (COPAS), Universidad de Concepción Chile, 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), ANR-10-BLAN-0614,KEOPS 2,Kerguelen : Comparaison plateau Ocean2(2010)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2015
• Subjects: NATURAL IRON FERTILIZATION; MARINE NITRIFYING BACTERIA; OXYGEN ISOTOPES; FRACTIONATION; NITRITE; AMMONIUM; CARBON; REMINERALIZATION; ASSIMILATION; CIRCULATION; ACL; envir; geo; Heard Island; Kerguelen; Southern Ocean
• Online Access: https://doi.org/10.5194/bg-12-1459-2015; https://hal.sorbonne-universite.fr/hal-01207322/file/bg-12-1459-2015.pdf; https://hal.sorbonne-universite.fr/hal-01207322

International audience This paper presents whole water column data for nitrate N, O isotopic composition for the Kerguelen Plateau area and the basin extending east of Heard Island, aiming at understanding the N-cycling in this naturally iron fertilized area that is characterized by large re-current phytoplankton blooms. The KEOPS 2 expedition (October–November 2011) took place in spring season and complements knowledge gathered during an earlier summer expedition to the same area (KEOPS 1, February–March 2005). As noted by others a remarkable condition of the system is the moderate consumption of nitrate over the season (nitrate remains >20 μM) while silicic acid becomes depleted, suggesting significant recycling of nitrogen. Nitrate isotopic signatures in the upper water column do mimic this condition, with surprising overlap of spring and summer regressions of δ18ONO3 vs. δ15NNO3 isotopic compositions. These regressions obey rather closely the 18ϵ/15ϵ discrimination expected for nitrate uptake (18ϵ/15ϵ = 1), but regression slopes as large as 1.6 were observed for the mixed layer above the Kerguelen Plateau. A preliminarily mass balance calculation for the early bloom period points toward significant nitrification occurring in the mixed layer and which may be equivalent to up to 47% of nitrate uptake above the Kerguelen Plateau. A further finding concerns deep ocean low δ18ONO3 values (<2‰) underlying high chlorophyll waters at the Polar Front Zone and which cannot be explained by remineralization and nitrification of the local particulate nitrogen flux, which is too small in magnitude. However, the studied area is characterized by a complex recirculation pattern that would keep deep waters in the area and could impose a seasonally integrated signature of surface water processes on the deep waters.