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Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 12, 1459-1482, 2015
https://doi.org/10.5194/bg-12-1459-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
05 Mar 2015
Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions
F. Dehairs1, F. Fripiat1, A.-J. Cavagna1, T. W. Trull2,3,4, C. Fernandez5,6, D. Davies2,3, A. Roukaerts1, D. Fonseca Batista1, F. Planchon7, and M. Elskens1 1Analytical, Environmental and Geo – Chemistry; Earth System Sciences Research Group, Vrije Universiteit Brussel, Belgium
2CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia
3Antarctic Climate and Ecosystems Cooperative Research Centre, Hobart, Tasmania, Australia
4Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia
5Sorbonne Universités, UPMC Univ Paris 06, UMR7621, Laboratoire d'Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls/mer, France
6Department of Oceanography, COPAS SurAustral program and Interdisciplinary center for Aquaculture Research (INCAR), University of Concepción, Chile
7Laboratoire des Sciences de L'Environnement Marin (LEMAR) Université de Bretagne Occidentale, Institut Européen de la Mer, Plouzané, Brest, France
Abstract. 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.

Citation: Dehairs, F., Fripiat, F., Cavagna, A.-J., Trull, T. W., Fernandez, C., Davies, D., Roukaerts, A., Fonseca Batista, D., Planchon, F., and Elskens, M.: Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions, Biogeosciences, 12, 1459-1482, https://doi.org/10.5194/bg-12-1459-2015, 2015.
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We investigated the nitrate N, O isotopic composition for the Southern Ocean Kerguelen Plateau area, aiming at understanding the N-cycling in this naturally iron fertilized area characterized by large recurrent phytoplankton blooms. The system is characterized by moderate consumption of nitrate over the season while silicic acid, on the contrary, becomes depleted, suggesting significant recycling of N. Mixed layer nitrate isotopic signatures corroborate this.
We investigated the nitrate N, O isotopic composition for the Southern Ocean Kerguelen Plateau...
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