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| Titel |
Production regime and associated N cycling in the vicinity of Kerguelen Island, Southern Ocean |
| VerfasserIn |
A. J. Cavagna, F. Fripiat, M. Elskens, P. Mangion, L. Chirurgien, I. Closset, M. Lasbleiz, L. Florez-Leiva, D. Cardinal, K. Leblanc, C. Fernández, D. Lefèvre, L. Oriol, S. Blain, B. Quéguiner, F. Dehairs |
| 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. 21 ; Nr. 12, no. 21 (2015-11-13), S.6515-6528 |
| Datensatznummer |
250118166
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| Publikation (Nr.) |
 copernicus.org/bg-12-6515-2015.pdf |
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| Zusammenfassung |
| Although the Southern Ocean is considered a high-nutrient, low-chlorophyll
(HNLC) area, massive and recurrent blooms are observed over and downstream
of the Kerguelen Plateau. This mosaic of blooms is triggered by a higher
iron supply resulting from the interaction between the Antarctic Circumpolar
Current and the local bathymetry. Net primary production, N uptake
(NO3− and NH4+), and nitrification rates were measured
at eight stations in austral spring 2011 (October–November) during the KEOPS 2
cruise in the Kerguelen Plateau area. Natural iron fertilization stimulated
primary production, with mixed layer integrated net primary production and
growth rates much higher in the fertilized areas (up to 315 mmol C
m−2 d−1 and up to 0.31 d−1 respectively) compared to the HNLC
reference site (12 mmol C m−2 d−1 and 0.06 d−1
respectively). Primary production was mainly sustained by nitrate uptake,
with f ratios (corresponding to NO3−-uptake / (NO3−-uptake + NH4+-uptake)) lying at the upper end of
the observations for the Southern Ocean (up to 0.9). We report high rates of
nitrification (up to ~ 3 μmol N L−1 d−1, with ~ 90 % of them < 1 μmol N L−1 d−1) typically
occurring below the euphotic zone, as classically observed in the global
ocean. The specificity of the studied area is that at most of the stations,
the euphotic layer was shallower than the mixed layer, implying that
nitrifiers can efficiently compete with phytoplankton for the ammonium
produced by remineralization at low-light intensities. Nitrate produced by
nitrification in the mixed layer below the euphotic zone is easily supplied
to the euphotic zone waters above, and nitrification sustained 70 ± 30 % of the nitrate uptake in the productive area above the Kerguelen
Plateau. This complicates estimations of new production as potentially
exportable production. We conclude that high productivity in deep mixing
system stimulates the N cycle by increasing both assimilation and
regeneration. |
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