Biogeosciences, 12, 6515-6528, 2015
https://doi.org/10.5194/bg-12-6515-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
13 Nov 2015
Production regime and associated N cycling in the vicinity of Kerguelen Island, Southern Ocean
A. J. Cavagna1, F. Fripiat1, M. Elskens1, P. Mangion2, L. Chirurgien3, I. Closset4, M. Lasbleiz3, L. Florez-Leiva5,6, D. Cardinal4, K. Leblanc3, C. Fernandez7,8, D. Lefèvre3, L. Oriol8, S. Blain8, B. Quéguiner3, and F. Dehairs1 1Analytical, Environmental and Geo-Chemistry dept. (AMGC), Earth System Sciences Research Group, Vrije Universiteit Brussel, Brussels, Belgium
2Centre for Coastal Biogeochemistry Research, Southern Cross University, Lismore, Australia
3Aix-Marseille Université, Université de Toulon, CNRS/INSU, IRD, MIO, UM110, 13288, Marseille, CEDEX 09, France
4Sorbonne Universités, UPMC, LOCEAN Laboratory, 4 place Jussieu, 75005 Paris, France
5Program of Oceanography, University of Antioquia, Medellin, Colombia
6Program of Biology, University of Magdalena, Santa Marta, Colombia
7Department of Oceanography, COPAS SurAustral program and Interdisciplinary Center for Aquaculture Research (INCAR), University of Concepción, Chile
8Sorbonne Universités, UPMC, UMR7621, Laboratoire d'Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls/mer, France
Abstract. Although the Southern Ocean is considered a high-nutrient, low-chlorophyll (HNLC) area, massive and recurrent blooms are observed over and downstream of 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 eight stations in austral spring 2011 (October–November) during the KEOPS 2 cruise in the Kerguelen Plateau area. Natural iron fertilization stimulated primary production, with mixed layer 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 was mainly sustained by nitrate uptake, with f ratios (corresponding to NO3-uptake / (NO3-uptake + NH4+-uptake)) lying at the upper end of the observations for the Southern Ocean (up to 0.9). We report high rates of nitrification (up to ~ 3 μmol N L−1 d−1, with ~ 90 % of them < 1 μmol N L−1 d−1) typically occurring below the euphotic zone, as classically observed in the global ocean. The specificity of the studied area is that at most of the stations, the euphotic layer was shallower than the mixed layer, implying that nitrifiers can efficiently compete with phytoplankton for the ammonium produced by remineralization at low-light intensities. Nitrate produced by nitrification in the mixed layer below the euphotic zone is easily supplied to the euphotic zone waters above, and nitrification sustained 70 ± 30 % of the nitrate uptake in the productive area above the Kerguelen Plateau. This complicates estimations of new production as potentially exportable production. We conclude that high productivity in deep mixing system stimulates the N cycle by increasing both assimilation and regeneration.

Citation: Cavagna, A. J., Fripiat, F., Elskens, M., Mangion, P., Chirurgien, L., Closset, I., Lasbleiz, M., Florez-Leiva, L., Cardinal, D., Leblanc, K., Fernandez, C., Lefèvre, D., Oriol, L., Blain, S., Quéguiner, B., and Dehairs, F.: Production regime and associated N cycling in the vicinity of Kerguelen Island, Southern Ocean, Biogeosciences, 12, 6515-6528, https://doi.org/10.5194/bg-12-6515-2015, 2015.
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Short summary
Primary production, NO3- and NH4+ uptake, and nitrification rates were measured during the KEOPS 2 cruise (austral spring 2011) in the Kerguelen Plateau area. Natural iron fertilization stimulated primary production which is much higher in the fertilized areas compared to the HNLC site. We report high rates of nitrification in the mixed layer below the euphotic zone. We conclude that high productivity in deep mixing system stimulates the N cycle by increasing both assimilation and regeneration.
Primary production, NO3- and NH4+ uptake, and nitrification rates were measured during the KEOPS...
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