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Impact of the natural Fe-fertilization on the magnitude, stoichiometry and efficiency of particulate biogenic silica, nitrogen and iron export fluxes
Lemaitre, N.; Planquette, H.; Dehairs, F.; Van der Merwe, P.; Bowie, A.R.; Trull, T.W.; Laurenceau-Cornec, E.C.; Davies, D.; Bollinger, C.; Le Goff, M.; Grossteffan, E.; Planchon, F. (2016). Impact of the natural Fe-fertilization on the magnitude, stoichiometry and efficiency of particulate biogenic silica, nitrogen and iron export fluxes. Deep-Sea Res., Part 1, Oceanogr. Res. Pap. 117: 11-27.
In: Deep-Sea Research, Part I. Oceanographic Research Papers. Elsevier: Oxford. ISSN 0967-0637; e-ISSN 1879-0119, more
Peer reviewed article  

Available in  Authors 

Author keywords
    Particulate export fluxes; Thorium-234; Natural Fe-fertilization;Biological pump

Authors  Top 
  • Lemaitre, N., more
  • Planquette, H.
  • Dehairs, F., more
  • Van der Merwe, P.
  • Bowie, A.R.
  • Trull, T.W.
  • Laurenceau-Cornec, E.C.
  • Davies, D.
  • Bollinger, C.
  • Le Goff, M.
  • Grossteffan, E.
  • Planchon, F., more

    The Kerguelen Plateau is characterized by a naturally Fe-fertilized phytoplankton bloom that extends more than 1000 km downstream in the Antarctic Circumpolar Current. During the KEOPS2 study, in austral spring, we measured particulate nitrogen (PN), biogenic silica (BSi) and particulate iron (PFe) export fluxes in order to investigate how the natural fertilization impacts the stoichiometry and the magnitude of export fluxes and therefore the efficiency of the biological carbon pump. At 9 stations, we estimated elemental export fluxes based on element concentration to 234Th activity ratios for particulate material collected with in-situ pumps and 234Th export fluxes (Planchon et al., 2015). This study revealed that the natural Fe-fertilization increased export fluxes but to variable degrees. Export fluxes for the bloom impacted area were compared with those of a high-nutrient, low-chlorophyll (HNLC), low-productive reference site located to the south-west of Kerguelen and which had the lowest BSi and PFe export fluxes (2.55 mmol BSi m2 d−1 and 1.92 µmol PFem2 d−1) and amongst the lowest PN export flux (0.73 mmol PN m2 d−1). The impact of the Fe fertilization was the greatest within a meander of the polar front (PF), to the east of Kerguelen, with fluxes reaching 1.26 mmol PN m2 d−1; 20.4 mmol BSi m2 d−1 and 22.4 µmol PFe m2 d−1. A highly productive site above the Kerguelen Plateau, on the contrary, was less impacted by the fertilization with export fluxes reaching 0.72 mmol PN m2 d−1; 4.50 mmol BSi m2 d−1 and 21.4 µmol PFe m2 d−1. Our results suggest that ecosystem features (i.e. type of diatom community) could play an important role in setting the magnitude of export fluxes of these elements. Indeed, for the PF meander, the moderate productivity was sustained by the presence of large and strongly silicified diatom species while at the higher productivity sites, smaller and slightly silicified diatoms dominated. Interestingly, our results suggest that PFe export fluxes can be driven by the lithogenic pool of particles, especially over the Plateau where such inputs from the sediments are important. Finally, for the Plateau and the PF meander, the comparison between PFe export and the particulate PFe stock integrated over the mixed layer depth revealed an efficient PFe export out of the mixed layer at these sites. Export efficiencies (i.e. the ratio between export and uptake) exhibit a very efficient silica pump especially at the HNLC reference station where heavily silicified diatoms were present. On the contrary, the increase with depth of the C:N ratio and the low nitrogen export efficiencies support the idea of a strong remineralization and nitrification activity.

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