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

Type Article
Acceptance Date 2018 IN PRESS
Language English
Author(s) Sauzède Raphaëlle1, Martinez Elodie1, Pasqueron De Fommervault Orens2, Poteau Antoine3, Mignot Alexandre3, Maes Christophe4, Claustre Hervé3, Uitz Julia3, Maamaatuaiahutapu Keitapu5, Rodier Martine1, Schmechtig Catherine3, Laurent Victoire6
Affiliation(s) 1 : Ecosystèmes Insulaires Océaniens (EIO, UMR-241), IRD, Ifremer, UPF and ILM, Tahiti, French Polynesia
2 : Departamento de Oceanografìa Fisica, Centro de Investigacion Cientìfica y de Educacion Superior de Ensenada, Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, BC 22860, Mexico
3 : Sorbonne 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
4 : Laboratoire d'Océanographie Physique et Spatiale (LOPS), IUEM, Univ. Brest, Ifremer, CNRS, IRD, F-29280, Brest, France
5 : Laboratoire de Géosciences du Pacifique Sud, Université de la Polynésie française, Tahiti, French Polynesia
6 : Direction Inter Régionale de Polynésie Française, Météo France, BP 6005 98702 Tahiti, French Polynesia
Source Biogeosciences Discussions (1810-6285) (Copernicus GmbH) In Press
DOI 10.5194/bg-2017-541
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.

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Sauzède Raphaëlle, Martinez Elodie, Pasqueron De Fommervault Orens, Poteau Antoine, Mignot Alexandre, Maes Christophe, Claustre Hervé, Uitz Julia, Maamaatuaiahutapu Keitapu, Rodier Martine, Schmechtig Catherine, Laurent Victoire Seasonal dynamics and disturbance of phytoplankton biomass in the wake of Tahiti as observed by Biogeochemical-Argo floats. Biogeosciences Discussions IN PRESS. Publisher's official version : https://doi.org/10.5194/bg-2017-541 , Open Access version : https://archimer.ifremer.fr/doc/00421/53295/