Enhancement of phytoplankton biomass leeward of Tahiti as observed by Biogeochemical-Argo floats

Type Article
Date 2020-04
Language English
Author(s) Sauzède Raphaëlle1, 2, Martinez Elodie1, 3, Maes Christophe3, de Fommervault Orens Pasqueron4, 5, Poteau Antoine6, Mignot Alexandre6, 7, Claustre Hervé6, Uitz Julia6, Oziel Laurent6, Maamaatuaiahutapu Keitapu2, Rodier Martine1, Schmechtig Catherine6, Laurent Victoire8
Affiliation(s) 1 : Ecosystèmes Insulaires Océaniens (EIO, UMR-241), IRD, Ifremer, UPF and ILM, Tahiti, French Polynesia
2 : Laboratoire de Géosciences du Pacifique Sud, Université de la Polynésie française, Tahiti, French Polynesia
3 : Laboratoire d'Océanographie Physique et Spatiale (LOPS), IUEM, Univ. Brest, Ifremer, CNRS, IRD, F-29280 Brest, France
4 : Departamento de Oceanografìa Fisica, Centro de Investigacion Cientìfica y de Educacion Superior de Ensenada, Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, BC 22860, Mexico
5 : ALSEAMAR-ALCEN company, 60 avenue Olivier Perroy, 13790 Rousset, France
6 : 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
7 : Mercator Océan, 31520 Ramonville Saint-Agne, France
8 : Direction Inter Régionale de Polynésie Française, Météo France, BP 6005 98702, Tahiti, French Polynesia
Source Journal Of Marine Systems (0924-7963) (Elsevier BV), 2020-04 , Vol. 204 , P. 103284 (16p.)
DOI 10.1016/j.jmarsys.2019.103284
WOS© Times Cited 3
Keyword(s) Phytoplankton biomass, Biogeochemical-Argo floats, Island mass effect, South Pacific Subtropical Gyre

The South Pacific Subtropical Gyre (SPSG) is a vast and remote oceanic system where the variability in phytoplankton biomass and production is still largely uncertain due to the lack of in situ biogeochemical observations. The SPSG is an oligotrophic environment where the ecosystem is controlled predominantly by nutrient depletion in surface waters. However, this dynamic is altered in the vicinity of islands where increased biological activity occurs (i.e. the island mass effect, IME). This study mainly focuses on in situ observations which show evidence of an IME leeward of Tahiti (17.7°S - 149.5°W), French Polynesia. Physical and biogeochemical observations collected with two Biogeochemical-Argo profiling floats are used to investigate the dynamics of phytoplankton biomass. Data from the first float, drifting from April 2015 to November 2016 over >1000 km westward of Tahiti, describe the open ocean conditions. The second float, deployed leeward of Tahiti in October 2015, stayed within 45 km off Tahiti for three months before it stopped communicating. In the oligotrophic central SPSG, our observations show that the deepening of the deep chlorophyll maximum (DCM) from winter to summer is light-driven and that the wintertime increase in chlorophyll a concentration in the upper layer is likely to be due to the process of photoacclimation, consistent with previous observations in oligotrophic environments. In contrast, leeward of Tahiti, the DCM widens toward the surface during late spring in association with a biological enhancement in the upper layer. Using Biogeochemical-Argo data, meteorological data from Tahiti, Hybrid Coordinate Ocean Model outputs and satellite-derived products (i.e., horizontal currents and associated fronts), the physical mechanisms involved in producing this biological enhancement leeward of Tahiti have been investigated. The IME occurs during a period of strong precipitation and in a zone of weak currents downstream of the island. 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 allows an accumulation zone to form, hence increasing phytoplankton growth up to 20 km away from the coastlines. A bio-optical-based community index suggests that the composition of the phytoplankton community differs leeward of Tahiti from that in the open ocean area, with more microphytoplankton within the IME, which is associated with an increase in the carbon export to the deeper ocean.

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Sauzède Raphaëlle, Martinez Elodie, Maes Christophe, de Fommervault Orens Pasqueron, Poteau Antoine, Mignot Alexandre, Claustre Hervé, Uitz Julia, Oziel Laurent, Maamaatuaiahutapu Keitapu, Rodier Martine, Schmechtig Catherine, Laurent Victoire (2020). Enhancement of phytoplankton biomass leeward of Tahiti as observed by Biogeochemical-Argo floats. Journal Of Marine Systems, 204, 103284 (16p.). Publisher's official version : https://doi.org/10.1016/j.jmarsys.2019.103284 , Open Access version : https://archimer.ifremer.fr/doc/00599/71130/