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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2017-541
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2017-541
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 10 Jan 2018

Submitted as: research article | 10 Jan 2018

Review status
This discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The revised manuscript was not accepted.

Seasonal dynamics and disturbance of phytoplankton biomass in the wake of Tahiti as observed by Biogeochemical-Argo floats

Raphaëlle Sauzède1, Elodie Martinez1, Orens Pasqueron de Fommervault2, Antoine Poteau3, Alexandre Mignot3, Christophe Maes4, Hervé Claustre3, Julia Uitz3, Keitapu Maamaatuaiahutapu5, Martine Rodier1, Catherine Schmechtig3, and Victoire Laurent6 Raphaëlle Sauzède et al.
  • 1Ecosystèmes Insulaires Océaniens (EIO, UMR-241), IRD, Ifremer, UPF and ILM, Tahiti, French Polynesia
  • 2Departamento de Oceanografìa Fisica, Centro de Investigacion Cientìfica y de Educacion Superior de Ensenada, Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, BC 22860, Mexico
  • 3Sorbonne Universités, UPMC Univ Paris 06, CNRS-INSU, Observatoire Océanologique de Villefranche, Laboratoire d'Océanographie de Villefranche, 181 Chemin du Lazaret, 06230 Villefranche-Sur-Mer, France
  • 4Laboratoire d'Océanographie Physique et Spatiale (LOPS), IUEM, Univ. Brest, Ifremer, CNRS, IRD, F-29280, Brest, France
  • 5Laboratoire de Géosciences du Pacifique Sud, Université de la Polynésie française, Tahiti, French Polynesia
  • 6Direction Inter Régionale de Polynésie Française, Météo France, BP 6005 98702 Tahiti, French Polynesia

Abstract. The South Pacific Subtropical Gyre (SPSG) is a vast and remote area where large uncertainties on variability in phytoplankton biomass and production remain due to the lack of biogeochemical in situ observations. In such oligotrophic environments, ecosystems are predominantly controlled by nutrients depletion in surface waters. However, this oligotrophic character can be disturbed in the vicinity of islands where enhancement of biological activity is known to occur (i.e. the island mass effect, IME). This study mainly focuses on in situ observations showing that an IME can be evidenced leeward of Tahiti (17.7° S–149.5° W), French Polynesia. Concomitant physical and biogeochemical observations collected with two Biogeochemical-Argo (BGC-Argo) profiling floats from April 2015 to November 2016 are used to investigate the dynamics of phytoplankton biomass. The first float has a transit of more than 1000 km westward of Tahiti (open ocean conditions) while the second one remained in the Tahitian wake (around 45 km from the island coasts). In the oligotrophic central SPSG, the wintertime increase in upper layer chlorophyll a concentration is likely due to photoacclimation process. Vertical observations show a light-driven deepening of the deep chlorophyll maximum (DCM) from winter to summer, consistently with previous descriptions. At the opposite, within the Tahitian wake, the DCM temporary widens during late spring in association with a biological enhancement in the upper layer. Combining in situ measurements with meteorological data along the Tahiti coasts, Hybrid Coordinate Ocean Model outputs and satellite-derived products (i.e., horizontal currents and associated fronts), the physical mechanisms involved in the disturbance of phytoplankton seasonal cycle in the Tahitian wake have been investigated. This disturbance results from the concomitant occurrence of strong precipitations and a zone of weak currents leeward Tahiti. We conjecture that the land drainage induces a significant supply of nitrate in the ocean upper layer (down to ~ 100 m) while a zone of weak currents in the southwestern zone behind Tahiti forms an accumulation zone, hence allowing phytoplankton growth up to 20 km away from the coastlines. Moreover, bio-optical measurements suggest that the composition of phytoplankton community could differ in the Tahitian wake vs. the open ocean area. Finally, in addition to extending information to the water column, only BGC-Argo floats could provide biogeochemical measurements in the SPSG region when clouds prevent the use of remote sensing.

Raphaëlle Sauzède et al.
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Interactive discussion
Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Raphaëlle Sauzède et al.
Raphaëlle Sauzède et al.
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