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| Titel |
Seasonal evolution of net and regenerated silica production around a natural Fe-fertilized area in the Southern Ocean estimated with Si isotopic approaches |
| VerfasserIn |
I. Closset, M. Lasbleiz, K. Leblanc, B. Quéguiner, A.-J. Cavagna, M. Elskens, J. Navez, D. Cardinal |
| 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 ; 11, no. 20 ; Nr. 11, no. 20 (2014-10-21), S.5827-5846 |
| Datensatznummer |
250117651
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| Publikation (Nr.) |
 copernicus.org/bg-11-5827-2014.pdf |
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| Zusammenfassung |
| A massive diatom bloom is observed each year in the surface waters of the
naturally Fe-fertilized Kerguelen Plateau (Southern Ocean). We measured
biogenic silica production and dissolution fluxes (ρSi and ρDiss,
respectively) in the mixed layer in the vicinity of the Kerguelen Plateau
during austral spring 2011 (KEOPS-2 cruise). We compare results from a
high-nutrient low-chlorophyll reference station and stations with different
degrees of iron enrichment and bloom conditions. Above the plateau biogenic
ρSi are among the highest reported so far in the Southern Ocean (up to
47.9 mmol m−2 d−1). Although significant (10.2 mmol m−2
d−1 on average), ρDiss were generally much lower than production
rates. Uptake ratios (ρSi : ρC and ρSi : ρN)
confirm that diatoms strongly dominate primary production in this area.
At the bloom onset, decreasing dissolution-to-production ratios (D : P)
indicate that the remineralization of silica could sustain most of the low
silicon uptake and that the system progressively shifts toward a silica
production regime which must be mainly supported by new source of silicic
acid. Moreover, by comparing results from the two KEOPS expeditions (spring
2011 and summer 2005), we suggest that there is a seasonal evolution of the
processes decoupling Si and N cycles in the area. Indeed, the consumption of
H4SiO4 standing stocks occurs only during the growing stage of the
bloom when strong net silica production is observed, contributing to higher
H4SiO4 depletion relative to NO3−. Then, the decoupling
of H4SiO4 and NO3− is mainly controlled by the more
efficient nitrogen recycling relative to Si. Gross Si : N uptake ratios
were higher in the Fe-rich regions compared to the high-nutrient low-chlorophyll (HNLC) area, likely due to
different diatom communities. This suggests that the diatom responses to
natural Fe fertilization are more complex than previously thought, and that
natural iron fertilization over long timescales does not necessarily
decrease Si : N uptake ratios as suggested by the silicic acid leakage
hypothesis. Finally, we propose the first seasonal estimate of the
Si biogeochemical budget above the Kerguelen Plateau based on direct
measurements. This study points out that naturally iron-fertilized areas of
the Southern Ocean could sustain very high regimes of biogenic silica
production, similar to those observed in highly productive upwelling systems. |
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