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| Titel |
Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions |
| VerfasserIn |
F. Dehairs, F. Fripiat, A.-J. Cavagna, T. W. Trull, C. Fernández, D. Davies, A. Roukaerts, D. Fonseca Batista, F. Planchon, M. Elskens |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 5 ; Nr. 12, no. 5 (2015-03-05), S.1459-1482 |
| Datensatznummer |
250117848
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| Publikation (Nr.) |
 copernicus.org/bg-12-1459-2015.pdf |
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| Zusammenfassung |
| 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. |
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