Benthic mineralization and nutrient exchange over the inner continental shelf of western India
The western Indian continental shelf is one of the most productive coastal systems of the world ocean. This system experiences extreme changes in its oxygen regime, being normoxic from November to May and suboxic (denitrifying)/anoxic from June to October, owing to the biogeochemical response to cyc...
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2014-05-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/11/2771/2014/bg-11-2771-2014.pdf |
| _version_ | 1811225276756000768 |
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| author | A. K. Pratihary S. W. A. Naqvi G. Narvenkar S. Kurian H. Naik R. Naik B. R. Manjunatha |
| author_facet | A. K. Pratihary S. W. A. Naqvi G. Narvenkar S. Kurian H. Naik R. Naik B. R. Manjunatha |
| author_sort | A. K. Pratihary |
| collection | DOAJ |
| description | The western Indian continental shelf is one of the most productive coastal
systems of the world ocean. This system experiences extreme changes in its
oxygen regime, being normoxic from November to May and suboxic
(denitrifying)/anoxic from June to October, owing to the biogeochemical
response to cyclical monsoonal influence. In order to understand the impact
of the seasonally varying oxygen regime on benthic mineralization, nutrient
exchange and, in turn, on the shelf ecosystem, we carried out the first ever
intact-core incubations during two contrasting seasons – spring intermonsoon
and fall intermonsoon (late southwest monsoon) at a 28 m-deep fixed site on
the inner shelf off Goa, dominated by fine-grained cohesive sediments. The
results showed that incomplete sediment oxygen consumption (SOC) occurred
during April as opposed to the complete SOC and subsequent sulfide flux
observed in the fall intermonsoon incubations. The sediments acted as a
perennial net source of DIN (dissolved inorganic nitrogen i.e. NO<sub>3</sub><sup>−</sup> + NO<sub>2</sub><sup>−</sup> + NH<sub>4</sub><sup>+</sup>), PO<sub>4</sub><sup>3−</sup> and SiO<sub>4</sub><sup>4−</sup> to the
overlying water column. The efflux of DIN increased from 1.4 to
3.74 mmol m<sup>−2</sup> d<sup>−1</sup> from April to October, of which NH<sub>4</sub><sup>+</sup>
flux comprised 59–100%. During the oxic regime, ∼75% of
diffusing NH<sub>4</sub><sup>+</sup> appeared to be nitrified
(2.55 mmol m<sup>−2</sup> d<sup>−1</sup>), of which ∼77% remained coupled
to benthic denitrification. Consequently, 58% of NH<sub>4</sub><sup>+</sup> flux was
lost in active coupled nitrification–denitrification, resulting in substantial
N loss (1.98 mmol m<sup>−2</sup> d<sup>−1</sup>) in the sediments. The continental
shelf sediments switched over from being a NO<sub>3</sub><sup>−</sup> source during the oxic
regime to a NO<sub>3</sub><sup>−</sup> sink during the anoxic regime. During suboxia,
benthic denitrification that is fed by NO<sub>3</sub><sup>-</sup> from the overlying water
caused N loss at the rate of 1.04 mmol m<sup>−2</sup> d<sup>−1</sup>. Nitrogen loss
continued even under sulfidic conditions during October, possibly through the
chemolithoautotrophic denitrification, at a potential rate of
3.21 mmol m<sup>−2</sup> d<sup>−1</sup>. Phosphate flux increased more than 4-fold
during October as compared to April, due to reductive dissolution of Fe- and
Mn oxides. The SiO<sub>4</sub><sup>4−</sup> flux increased during October apparently due to
the higher availability of siliceous ooze from diatom blooms commonly
occurring in the monsoon season.
<br><br>
Slow oxidation of organic carbon (C<sub>org</sub>) under anoxia, lower
temperature and reduced benthic faunal activity appeared to decrease benthic
mineralization by 25% as suggested by the drop in the C<sub>org</sub>
oxidation rate from 63.8 mmol C m<sup>−2</sup> d<sup>−1</sup> in April to 47.8 mmol
C m<sup>−2</sup> d<sup>−1</sup> in October. This indicated a higher preservation of
C<sub>org</sub> during the late southwest monsoon. Sediment porosity,
C<sub>org</sub> content and nutrients did not show significant variations
from April to October. Porewaters were found to be enriched with
NH<sub>4</sub><sup>+</sup>, PO<sub>4</sub><sup>3−</sup> and SiO<sub>4</sub><sup>4−</sup> but depleted in NO<sub>3</sub><sup>−</sup>
and NO<sub>2</sub><sup>−</sup> in these organic-rich sediments. Significant DIN,
PO<sub>4</sub><sup>3−</sup> and SiO<sub>4</sub><sup>4−</sup> effluxes indicate the potential of benthic
input in meeting nutrient demand of the phytoplankton community in this
seasonally N-limited shelf system. |
| first_indexed | 2024-04-12T09:03:07Z |
| format | Article |
| id | doaj.art-f2c3622bd14447bf8928e18cd6e9ee84 |
| institution | Directory Open Access Journal |
| issn | 1726-4170 1726-4189 |
| language | English |
| last_indexed | 2024-04-12T09:03:07Z |
| publishDate | 2014-05-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Biogeosciences |
| spelling | doaj.art-f2c3622bd14447bf8928e18cd6e9ee842022-12-22T03:39:10ZengCopernicus PublicationsBiogeosciences1726-41701726-41892014-05-0111102771279110.5194/bg-11-2771-2014Benthic mineralization and nutrient exchange over the inner continental shelf of western IndiaA. K. Pratihary0S. W. A. Naqvi1G. Narvenkar2S. Kurian3H. Naik4R. Naik5B. R. Manjunatha6CSIR – Centre for Excellence in Aquatic Biogeochemistry, Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa 403 004, IndiaCSIR – Centre for Excellence in Aquatic Biogeochemistry, Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa 403 004, IndiaCSIR – Centre for Excellence in Aquatic Biogeochemistry, Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa 403 004, IndiaCSIR – Centre for Excellence in Aquatic Biogeochemistry, Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa 403 004, IndiaCSIR – Centre for Excellence in Aquatic Biogeochemistry, Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa 403 004, IndiaCSIR – Centre for Excellence in Aquatic Biogeochemistry, Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa 403 004, IndiaDepartment of Marine Geology, Mangalore University, Mangalagangothri, Karnataka 574 199, IndiaThe western Indian continental shelf is one of the most productive coastal systems of the world ocean. This system experiences extreme changes in its oxygen regime, being normoxic from November to May and suboxic (denitrifying)/anoxic from June to October, owing to the biogeochemical response to cyclical monsoonal influence. In order to understand the impact of the seasonally varying oxygen regime on benthic mineralization, nutrient exchange and, in turn, on the shelf ecosystem, we carried out the first ever intact-core incubations during two contrasting seasons – spring intermonsoon and fall intermonsoon (late southwest monsoon) at a 28 m-deep fixed site on the inner shelf off Goa, dominated by fine-grained cohesive sediments. The results showed that incomplete sediment oxygen consumption (SOC) occurred during April as opposed to the complete SOC and subsequent sulfide flux observed in the fall intermonsoon incubations. The sediments acted as a perennial net source of DIN (dissolved inorganic nitrogen i.e. NO<sub>3</sub><sup>−</sup> + NO<sub>2</sub><sup>−</sup> + NH<sub>4</sub><sup>+</sup>), PO<sub>4</sub><sup>3−</sup> and SiO<sub>4</sub><sup>4−</sup> to the overlying water column. The efflux of DIN increased from 1.4 to 3.74 mmol m<sup>−2</sup> d<sup>−1</sup> from April to October, of which NH<sub>4</sub><sup>+</sup> flux comprised 59–100%. During the oxic regime, ∼75% of diffusing NH<sub>4</sub><sup>+</sup> appeared to be nitrified (2.55 mmol m<sup>−2</sup> d<sup>−1</sup>), of which ∼77% remained coupled to benthic denitrification. Consequently, 58% of NH<sub>4</sub><sup>+</sup> flux was lost in active coupled nitrification–denitrification, resulting in substantial N loss (1.98 mmol m<sup>−2</sup> d<sup>−1</sup>) in the sediments. The continental shelf sediments switched over from being a NO<sub>3</sub><sup>−</sup> source during the oxic regime to a NO<sub>3</sub><sup>−</sup> sink during the anoxic regime. During suboxia, benthic denitrification that is fed by NO<sub>3</sub><sup>-</sup> from the overlying water caused N loss at the rate of 1.04 mmol m<sup>−2</sup> d<sup>−1</sup>. Nitrogen loss continued even under sulfidic conditions during October, possibly through the chemolithoautotrophic denitrification, at a potential rate of 3.21 mmol m<sup>−2</sup> d<sup>−1</sup>. Phosphate flux increased more than 4-fold during October as compared to April, due to reductive dissolution of Fe- and Mn oxides. The SiO<sub>4</sub><sup>4−</sup> flux increased during October apparently due to the higher availability of siliceous ooze from diatom blooms commonly occurring in the monsoon season. <br><br> Slow oxidation of organic carbon (C<sub>org</sub>) under anoxia, lower temperature and reduced benthic faunal activity appeared to decrease benthic mineralization by 25% as suggested by the drop in the C<sub>org</sub> oxidation rate from 63.8 mmol C m<sup>−2</sup> d<sup>−1</sup> in April to 47.8 mmol C m<sup>−2</sup> d<sup>−1</sup> in October. This indicated a higher preservation of C<sub>org</sub> during the late southwest monsoon. Sediment porosity, C<sub>org</sub> content and nutrients did not show significant variations from April to October. Porewaters were found to be enriched with NH<sub>4</sub><sup>+</sup>, PO<sub>4</sub><sup>3−</sup> and SiO<sub>4</sub><sup>4−</sup> but depleted in NO<sub>3</sub><sup>−</sup> and NO<sub>2</sub><sup>−</sup> in these organic-rich sediments. Significant DIN, PO<sub>4</sub><sup>3−</sup> and SiO<sub>4</sub><sup>4−</sup> effluxes indicate the potential of benthic input in meeting nutrient demand of the phytoplankton community in this seasonally N-limited shelf system.http://www.biogeosciences.net/11/2771/2014/bg-11-2771-2014.pdf |
| spellingShingle | A. K. Pratihary S. W. A. Naqvi G. Narvenkar S. Kurian H. Naik R. Naik B. R. Manjunatha Benthic mineralization and nutrient exchange over the inner continental shelf of western India Biogeosciences |
| title | Benthic mineralization and nutrient exchange over the inner continental shelf of western India |
| title_full | Benthic mineralization and nutrient exchange over the inner continental shelf of western India |
| title_fullStr | Benthic mineralization and nutrient exchange over the inner continental shelf of western India |
| title_full_unstemmed | Benthic mineralization and nutrient exchange over the inner continental shelf of western India |
| title_short | Benthic mineralization and nutrient exchange over the inner continental shelf of western India |
| title_sort | benthic mineralization and nutrient exchange over the inner continental shelf of western india |
| url | http://www.biogeosciences.net/11/2771/2014/bg-11-2771-2014.pdf |
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