Seasonal evolution of net and regenerated silica production around a natural Fe-fertilized area in the Southern Ocean estimated with Si isotopic approaches

Type Article
Date 2014-10-21
Language English
Author(s) Closset I.1, Lasbleiz M.2, Leblanc K.2, Queguiner B.2, Cavagna A. -J.3, Elskens M.3, Navez J.4, Cardinal D.1, 4
Affiliation(s) 1 : Univ Paris 06, Sorbonne Univ, LOCEAN Lab, CNRS,IRD,MNHN, F-75005 Paris, France.
2 : Univ Toulon & Var, Aix Marseille Univ, MIO UM110, CNRS,IRD, F-13288 Marseille, France.
3 : Vrije Univ Brussel, B-1050 Brussels, Belgium.
4 : Royal Museum Cent Africa, Dept Earth Sci, B-3080 Tervuren, Belgium.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2014-10-21 , Vol. 11 , N. 20 , P. 5827-5846
DOI 10.5194/bg-11-5827-2014
WOS© Times Cited 27
Note Special issue KEOPS2: Kerguelen Ocean and Plateau Study 2
Abstract 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 (rho Si and rho Diss, 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 rho Si 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), rho Diss were generally much lower than production rates. Uptake ratios (rho Si : rho C and rho Si : rho N) 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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Closset I., Lasbleiz M., Leblanc K., Queguiner B., Cavagna A. -J., Elskens M., Navez J., Cardinal D. (2014). Seasonal evolution of net and regenerated silica production around a natural Fe-fertilized area in the Southern Ocean estimated with Si isotopic approaches. Biogeosciences, 11(20), 5827-5846. Publisher's official version : https://doi.org/10.5194/bg-11-5827-2014 , Open Access version : https://archimer.ifremer.fr/doc/00291/40220/