Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean
<p>The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen d...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-07-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/16/2715/2019/bg-16-2715-2019.pdf |
| _version_ | 1811330746825048064 |
|---|---|
| author | N. C. Harms N. Lahajnar B. Gaye T. Rixen T. Rixen K. Dähnke M. Ankele U. Schwarz-Schampera K.-C. Emeis K.-C. Emeis |
| author_facet | N. C. Harms N. Lahajnar B. Gaye T. Rixen T. Rixen K. Dähnke M. Ankele U. Schwarz-Schampera K.-C. Emeis K.-C. Emeis |
| author_sort | N. C. Harms |
| collection | DOAJ |
| description | <p>The Indian Ocean subtropical gyre (IOSG) is one of five
extensive subtropical gyres in the world's ocean. In contrast to those of
the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We
investigate the water mass distributions based on temperature, salinity and oxygen data, and the
concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between <span class="inline-formula">∼30</span><span class="inline-formula"><sup>∘</sup></span> S and the Equator during two expeditions: MSM 59/2 in 2016 and
SO 259 in 2017. Our results are the first from this oceanic region and provide new
information on nitrogen sources and transformation processes. We identify
the thick layer of nutrient-depleted surface waters of the oligotrophic IOSG
with nitrate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a186e28964d6ae507e65dbc91f8b1f71"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2715-2019-ie00001.svg" width="25pt" height="16pt" src="bg-16-2715-2019-ie00001.png"/></svg:svg></span></span>) and phosphate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">PO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">3</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="2702fab0ee38cf232058ddc22a79d590"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2715-2019-ie00002.svg" width="29pt" height="17pt" src="bg-16-2715-2019-ie00002.png"/></svg:svg></span></span>) concentrations of < 3 and < 0.3 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span>, respectively (< 300 m; <span class="inline-formula"><i>σ</i></span> < 26.4 kg<span class="inline-formula"><sup>−1</sup></span> m<span class="inline-formula"><sup>−3</sup></span>). Increased nutrient concentrations towards the
Equator represent the northern limb of the gyre, which is characterized by typical
strong horizontal gradients of the outcropping nutriclines. The influx of
the Subantarctic Mode Water (SAMW) from the Southern Ocean injects oxygen-saturated waters with preformed nutrients, indicated by the increased N and O
isotope composition of nitrate (<span class="inline-formula"><i>δ</i><sup>15</sup>N</span> > 7 ‰; <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> > 4 ‰) at 400–500 m (26.6–26.7 kg<span class="inline-formula"><sup>−1</sup></span> m<span class="inline-formula"><sup>−3</sup></span>), into the subtropical thermocline. These values reflect partial
N assimilation in the Southern Ocean. Moreover, in the northern study area,
a residue of nitrate affected by denitrification in the Arabian Sea is
imported into intermediate and deep water masses (> 27.0 kg<span class="inline-formula"><sup>−1</sup></span> m<span class="inline-formula"><sup>−3</sup></span>) of the gyre, indicated by an N deficit (N<span class="inline-formula"><sup>*</sup></span>
<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mo>-</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="8pt" class="svg-formula" dspmath="mathimg" md5hash="34f0458549ddd11e5dc90877b78ee96c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2715-2019-ie00003.svg" width="22pt" height="8pt" src="bg-16-2715-2019-ie00003.png"/></svg:svg></span></span>1 to <span class="inline-formula">−</span>4 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span>) and by elevated isotopic ratios
of nitrate (<span class="inline-formula"><i>δ</i><sup>15</sup>N</span> > 7 ‰; <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> > 3 ‰). Remineralization of
partially assimilated organic matter, produced in the subantarctic, leads to a
decoupling of N and O isotopes in nitrate and results in a relatively low
<span class="inline-formula">Δ</span>(15–18) value of < 3 ‰ within the SAMW. In
contrast, remineralization of <span class="inline-formula"><sup>15</sup>N</span>-enriched organic matter from
the Arabian Sea indicates higher <span class="inline-formula">Δ</span>(15–18) values of > 4 ‰ within the Red Sea–Persian Gulf Intermediate Water
(RSPGIW). Thus, the subtropical southern Indian Ocean is supplied by preformed
nitrate from the lateral influx of water masses from regions exhibiting
distinctly different N-cycle processes documented in the dual isotope
composition of nitrate. Additionally, a significant contribution of
<span class="inline-formula">N<sub>2</sub></span> fixation between 20.36 and 23.91<span class="inline-formula"><sup>∘</sup></span> S is
inferred from reduced <span class="inline-formula"><i>δ</i><sup>15</sup>N</span>–<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M29" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="f406d9210c9988b6f1f99fbfd13290fc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2715-2019-ie00004.svg" width="25pt" height="16pt" src="bg-16-2715-2019-ie00004.png"/></svg:svg></span></span> values towards
surface waters (upward decrease of <span class="inline-formula"><i>δ</i><sup>15</sup>N</span> <span class="inline-formula">∼2.4</span> ‰), N<span class="inline-formula"><sup>*</sup></span> values of > 2 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> and
a relatively low <span class="inline-formula">Δ</span>(15–18) value of < 3 ‰. A
mass and isotope budget implies that at least 32 %–34 % of the nitrate in
the upper ocean between 20.36 and 23.91<span class="inline-formula"><sup>∘</sup></span> S is
provided from newly fixed nitrogen, whereas <span class="inline-formula">N<sub>2</sub></span> fixation appears to be
limited by iron or temperature south of 26<span class="inline-formula"><sup>∘</sup></span> S.</p> |
| first_indexed | 2024-04-13T16:07:23Z |
| format | Article |
| id | doaj.art-e4d24e316ea54143a4a9da85daa017c7 |
| institution | Directory Open Access Journal |
| issn | 1726-4170 1726-4189 |
| language | English |
| last_indexed | 2024-04-13T16:07:23Z |
| publishDate | 2019-07-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Biogeosciences |
| spelling | doaj.art-e4d24e316ea54143a4a9da85daa017c72022-12-22T02:40:21ZengCopernicus PublicationsBiogeosciences1726-41701726-41892019-07-01162715273210.5194/bg-16-2715-2019Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian OceanN. C. Harms0N. Lahajnar1B. Gaye2T. Rixen3T. Rixen4K. Dähnke5M. Ankele6U. Schwarz-Schampera7K.-C. Emeis8K.-C. Emeis9Institute for Geology, Universität Hamburg, Hamburg, 20146, GermanyInstitute for Geology, Universität Hamburg, Hamburg, 20146, GermanyInstitute for Geology, Universität Hamburg, Hamburg, 20146, GermanyInstitute for Geology, Universität Hamburg, Hamburg, 20146, GermanyLeibniz Centre for Tropical Marine Research, Bremen, 28359, GermanyHelmholtz-Zentrum Geesthacht (HZG), Institute for Coastal Research, Geesthacht, 21502, GermanyHelmholtz-Zentrum Geesthacht (HZG), Institute for Coastal Research, Geesthacht, 21502, GermanyFederal Institute for Geosciences and Natural Resources (BGR), Hannover, 30655, GermanyInstitute for Geology, Universität Hamburg, Hamburg, 20146, GermanyHelmholtz-Zentrum Geesthacht (HZG), Institute for Coastal Research, Geesthacht, 21502, Germany<p>The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between <span class="inline-formula">∼30</span><span class="inline-formula"><sup>∘</sup></span> S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrient-depleted surface waters of the oligotrophic IOSG with nitrate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a186e28964d6ae507e65dbc91f8b1f71"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2715-2019-ie00001.svg" width="25pt" height="16pt" src="bg-16-2715-2019-ie00001.png"/></svg:svg></span></span>) and phosphate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">PO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">3</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="2702fab0ee38cf232058ddc22a79d590"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2715-2019-ie00002.svg" width="29pt" height="17pt" src="bg-16-2715-2019-ie00002.png"/></svg:svg></span></span>) concentrations of < 3 and < 0.3 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span>, respectively (< 300 m; <span class="inline-formula"><i>σ</i></span> < 26.4 kg<span class="inline-formula"><sup>−1</sup></span> m<span class="inline-formula"><sup>−3</sup></span>). Increased nutrient concentrations towards the Equator represent the northern limb of the gyre, which is characterized by typical strong horizontal gradients of the outcropping nutriclines. The influx of the Subantarctic Mode Water (SAMW) from the Southern Ocean injects oxygen-saturated waters with preformed nutrients, indicated by the increased N and O isotope composition of nitrate (<span class="inline-formula"><i>δ</i><sup>15</sup>N</span> > 7 ‰; <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> > 4 ‰) at 400–500 m (26.6–26.7 kg<span class="inline-formula"><sup>−1</sup></span> m<span class="inline-formula"><sup>−3</sup></span>), into the subtropical thermocline. These values reflect partial N assimilation in the Southern Ocean. Moreover, in the northern study area, a residue of nitrate affected by denitrification in the Arabian Sea is imported into intermediate and deep water masses (> 27.0 kg<span class="inline-formula"><sup>−1</sup></span> m<span class="inline-formula"><sup>−3</sup></span>) of the gyre, indicated by an N deficit (N<span class="inline-formula"><sup>*</sup></span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mo>-</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="8pt" class="svg-formula" dspmath="mathimg" md5hash="34f0458549ddd11e5dc90877b78ee96c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2715-2019-ie00003.svg" width="22pt" height="8pt" src="bg-16-2715-2019-ie00003.png"/></svg:svg></span></span>1 to <span class="inline-formula">−</span>4 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span>) and by elevated isotopic ratios of nitrate (<span class="inline-formula"><i>δ</i><sup>15</sup>N</span> > 7 ‰; <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> > 3 ‰). Remineralization of partially assimilated organic matter, produced in the subantarctic, leads to a decoupling of N and O isotopes in nitrate and results in a relatively low <span class="inline-formula">Δ</span>(15–18) value of < 3 ‰ within the SAMW. In contrast, remineralization of <span class="inline-formula"><sup>15</sup>N</span>-enriched organic matter from the Arabian Sea indicates higher <span class="inline-formula">Δ</span>(15–18) values of > 4 ‰ within the Red Sea–Persian Gulf Intermediate Water (RSPGIW). Thus, the subtropical southern Indian Ocean is supplied by preformed nitrate from the lateral influx of water masses from regions exhibiting distinctly different N-cycle processes documented in the dual isotope composition of nitrate. Additionally, a significant contribution of <span class="inline-formula">N<sub>2</sub></span> fixation between 20.36 and 23.91<span class="inline-formula"><sup>∘</sup></span> S is inferred from reduced <span class="inline-formula"><i>δ</i><sup>15</sup>N</span>–<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M29" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="f406d9210c9988b6f1f99fbfd13290fc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2715-2019-ie00004.svg" width="25pt" height="16pt" src="bg-16-2715-2019-ie00004.png"/></svg:svg></span></span> values towards surface waters (upward decrease of <span class="inline-formula"><i>δ</i><sup>15</sup>N</span> <span class="inline-formula">∼2.4</span> ‰), N<span class="inline-formula"><sup>*</sup></span> values of > 2 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> and a relatively low <span class="inline-formula">Δ</span>(15–18) value of < 3 ‰. A mass and isotope budget implies that at least 32 %–34 % of the nitrate in the upper ocean between 20.36 and 23.91<span class="inline-formula"><sup>∘</sup></span> S is provided from newly fixed nitrogen, whereas <span class="inline-formula">N<sub>2</sub></span> fixation appears to be limited by iron or temperature south of 26<span class="inline-formula"><sup>∘</sup></span> S.</p>https://www.biogeosciences.net/16/2715/2019/bg-16-2715-2019.pdf |
| spellingShingle | N. C. Harms N. Lahajnar B. Gaye T. Rixen T. Rixen K. Dähnke M. Ankele U. Schwarz-Schampera K.-C. Emeis K.-C. Emeis Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean Biogeosciences |
| title | Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean |
| title_full | Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean |
| title_fullStr | Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean |
| title_full_unstemmed | Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean |
| title_short | Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean |
| title_sort | nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern indian ocean |
| url | https://www.biogeosciences.net/16/2715/2019/bg-16-2715-2019.pdf |
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