Biological productivity regime and associated N cycling in the vicinity of Kerguelen Island area, Southern Ocean

Type Article
Date 2014-12-19
Language English
Author(s) Cavagna A. J.1, Fripiat F.1, Elskens M.1, Dehairs F.1, Mangion P.2, Chirurgien L.3, Closset I.4, Lasbleiz M.3, Flores–leiva L.5, 6, 7, Cardinal D.4, Leblanc K.3, Fernandez C.5, 6, 7, 8, Lefèvre D.3, Oriol L.8, Blain S.8, Quéguiner B.3
Affiliation(s) 1 : Analytical, Environmental and Geo-Chemistry dept. (AMGC), Earth System Sciences Research Group, Vrije Universiteit Brussel, Brussels, Belgium
2 : Centre for Coastal Biogeochemistry Research, Southern Cross University, Lismore, Australia
3 : Aix-Marseille Université Université de Toulon, CNRS/INSU, IRD, MIO, UM 110, 13288, Marseille, CEDEX 09, France
4 : Sorbonne Universités (UPMC, Univ Paris 06)-CNRS-IRD-MNHN, LOCEAN Laboratory, 4 place Jussieu, 75005 Paris, France
5 : Department of Oceanography, COPAS SurAustral program and Interdisciplinary Center for Aquaculture Research (INCAR), University of Concepción, Chile
6 : Program of Oceanography, University of Antioquia, Medellin, Columbia
7 : Program of Biology, University of Magdalena, Santa Marta, Columbia
8 : Sorbonne Universités, UPMC Université Paris 06, UMR 7621, Laboratoire d'Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls/mer, France
Source Biogeosciences Discussions (1810-6277) (Copernicus GmbH), 2014-12-19 , Vol. 11 , N. 12 , P. 18073-18104
DOI 10.5194/bgd-11-18073-2014
Note Special issue KEOPS2: Kerguelen Ocean and Plateau Study 2
Abstract Although the Southern Ocean is considered a High Nutrient Low Chlorophyll area (HNLC), massive and recurrent blooms are observed over and downstream the Kerguelen Plateau. This mosaic of blooms is triggered by a higher iron supply resulting from the interaction between the Antarctic Circumpolar Current and the local bathymetry. Net primary production, N-uptake (NO3− and NH4+), and nitrification rates were measured at 8 stations in austral spring 2011 (October–November) during the KEOPS2 cruise in the Kerguelen area. Iron fertilization stimulates primary production, with integrated net primary production and growth rates much higher in the fertilized areas (up to 315 mmol C m−2 d−1 and up to 0.31 d−1, respectively) compared to the HNLC reference site (12 mmol C m−2 d−1 and 0.06 d−1, respectively). Primary production is mainly sustained by nitrate uptake, with f ratio (corresponding to NO3− uptake/(NO3− uptake + NH4+ uptake)) lying in the upper end of the observations for the Southern Ocean (up to 0.9). Unexpectedly, we report unprecedented rates of nitrification (up to ~3 mmol C m−2 d−1, with ~90% of them <1 mmol C m−2 d−1). It appears that nitrate is assimilated in the upper part of the mixed layer (coinciding with the euphotic layer) and regenerated in the lower parts. We suggest that such high contribution of nitrification to nitrate assimilation is driven by (i) a deep mixed layer, extending well below the euphotic layer, allowing nitrifiers to compete with phytoplankton for the assimilation of ammonium, (ii) extremely high rates of primary production for the Southern Ocean, stimulating the release of dissolved organic matter, and (iii) an efficient food web, allowing the reprocessing of organic N and the retention of nitrogen into the dissolved phase through ammonium, the substrate for nitrification.
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Cavagna A. J., Fripiat F., Elskens M., Dehairs F., Mangion P., Chirurgien L., Closset I., Lasbleiz M., Flores–leiva L., Cardinal D., Leblanc K., Fernandez C., Lefèvre D., Oriol L., Blain S., Quéguiner B. (2014). Biological productivity regime and associated N cycling in the vicinity of Kerguelen Island area, Southern Ocean. Biogeosciences Discussions, 11(12), 18073-18104. Publisher's official version : https://doi.org/10.5194/bgd-11-18073-2014 , Open Access version : https://archimer.ifremer.fr/doc/00294/40558/