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https://doi.org/10.5194/bg-2018-511
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2018-511
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Jan 2019

Research article | 03 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical South Indian Ocean

Natalie C. Harms1, Niko Lahajnar1, Birgit Gaye1, Tim Rixen1,2, Kirstin Dähnke3, Markus Ankele3, Ulrich Schwarz-Schampera4, and Kay-Christian Emeis1,3 Natalie C. Harms et al.
  • 1Institute of Geology, Universität Hamburg, Hamburg, 20146, Germany
  • 2Leibniz-Centre for Tropical Marine Research, Bremen, 28359, Germany
  • 3Helmholtz-Zentrum Geesthacht (HZG), Institute for Coastal Research, Geesthacht, 21502, Germany
  • 4Federal Institute for Geosciences and Natural Resources (BGR), Hannover, 30655, Germany

Abstract. Vast subtropical gyres are important areas for the exchange of carbon between atmosphere and ocean in spite of low nutrient concentrations, and supposedly for the influx of reactive nitrogen to the ocean by dinitrogen fixation. To identify sources and transformation processes in the nitrogen cycle of the southern Indian Ocean subtropical gyre, we investigated concentrations of water column nutrients and stable isotope composition of nitrate of samples from two expeditions in 2016 (MSM 59) and 2017 (SO 259) in the subtropical gyre between ~30°S and the equator. Low nitrate and phosphate concentrations mark the thick mixed layer of the oligotrophic gyre with values of <5.9µMNO3 and <0.5µM PO43− (<310m; σ<26.4kg/m³). Increased nutrient concentrations towards the equator represent the northern end of the gyre, characterized by typical strong horizontal gradients of the outcropping nutriclines. Measurements of stable isotopes of nitrate (δ15N and δ18O) indicate isotopic maxima of δ15N (>7‰) and δ18O (>4‰) centred at 400–500m, representing the preformed nitrate exported from the Southern Ocean with mode water and induced by partial N-assimilation there. Additionally, a residue of nitrate affected by denitrification in the Arabian Sea is imported into the sub-thermocline of the gyre, indicated by a strong N deficit (N*<−1µM) within the northern study area, accompanied by elevated isotopic ratios of nitrate (δ15N>7‰; δ18O>3‰). The subtropical South Indian Ocean is thus supplied by nitrate from lateral influx of water masses that have similar isotopic character, but antagonistic origin (preformed versus regenerated). A significant contribution of N2-fixation within the Indian Ocean subtropical gyre (17°S–25°S) is promoted by low nitrate to phosphate ratios in the surface layer, where approximately one-third of the nitrate in the upper ocean is derived by newly fixed N.

Natalie C. Harms et al.
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Natalie C. Harms et al.
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Short summary
The Indian Ocean subtropical gyre is a large oligotrophic area that is likely to adjust to continued warming by increasing stratification, reduced nutrient supply, and decreasing biological production. In this study, we investigated concentrations of nutrients and stable isotopes of nitrate. We determine the lateral influence of water masses entering the gyre from the northern Indian Ocean and from the Southern Ocean and quantify the input of nitrogen by N2-fixation into the surface layer.
The Indian Ocean subtropical gyre is a large oligotrophic area that is likely to adjust to...
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