 |
| Titel |
Benthic mineralization and nutrient exchange over the inner continental shelf of western India |
| VerfasserIn |
A. K. Pratihary, S. W. A. Naqvi, G. Narvenkar, S. Kurian, H. Naik, R. Naik, B. R. Manjunatha |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1726-4170
|
| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 11, no. 10 ; Nr. 11, no. 10 (2014-05-27), S.2771-2791 |
| Datensatznummer |
250117423
|
| Publikation (Nr.) |
 copernicus.org/bg-11-2771-2014.pdf |
|
|
|
|
|
| Zusammenfassung |
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. NO3− + NO2− + NH4+), PO43− and SiO44− to the
overlying water column. The efflux of DIN increased from 1.4 to
3.74 mmol m−2 d−1 from April to October, of which NH4+
flux comprised 59–100%. During the oxic regime, ∼75% of
diffusing NH4+ appeared to be nitrified
(2.55 mmol m−2 d−1), of which ∼77% remained coupled
to benthic denitrification. Consequently, 58% of NH4+ flux was
lost in active coupled nitrification–denitrification, resulting in substantial
N loss (1.98 mmol m−2 d−1) in the sediments. The continental
shelf sediments switched over from being a NO3− source during the oxic
regime to a NO3− sink during the anoxic regime. During suboxia,
benthic denitrification that is fed by NO3- from the overlying water
caused N loss at the rate of 1.04 mmol m−2 d−1. Nitrogen loss
continued even under sulfidic conditions during October, possibly through the
chemolithoautotrophic denitrification, at a potential rate of
3.21 mmol m−2 d−1. Phosphate flux increased more than 4-fold
during October as compared to April, due to reductive dissolution of Fe- and
Mn oxides. The SiO44− flux increased during October apparently due to
the higher availability of siliceous ooze from diatom blooms commonly
occurring in the monsoon season.
Slow oxidation of organic carbon (Corg) under anoxia, lower
temperature and reduced benthic faunal activity appeared to decrease benthic
mineralization by 25% as suggested by the drop in the Corg
oxidation rate from 63.8 mmol C m−2 d−1 in April to 47.8 mmol
C m−2 d−1 in October. This indicated a higher preservation of
Corg during the late southwest monsoon. Sediment porosity,
Corg content and nutrients did not show significant variations
from April to October. Porewaters were found to be enriched with
NH4+, PO43− and SiO44− but depleted in NO3−
and NO2− in these organic-rich sediments. Significant DIN,
PO43− and SiO44− effluxes indicate the potential of benthic
input in meeting nutrient demand of the phytoplankton community in this
seasonally N-limited shelf system. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|