Presenter: Elodie Martinez (LOPS , IRD)

Description:
The Marquesas archipelago is located southward of the Pacific equatorial upwelling and benefits from its high-nutrient low-Chlorophyll (HNLC) plume which is nevertheless micronutrient (iron) limited. These Islands are characterized by a strong phytoplanktonic enhancement (island mass effect-IME) and rich marine biodiversity, whose origin is still to be clarified. The interdisciplinary MOANA-MATY project has combined physical, biogeochemical and biological approaches through a high-resolution numerical model, satellite and in situ observations from 2 oceanographic cruises in 2018 and 2019 to 1) understand the physical and biogeochemical coastal processes at the origin of the Marquesas IME, 2) characterize its spatio-temporal variability and 3) assess the influence of the large-scale dynamics. The different results support the hypothesis of a macro and micro (iron) nutrient contribution from the islands themselves to fertilize coastal waters allowing phytoplankton growth. These nutrients are suspected to be supplied to the nearshore upper-sunlit layer from the bathymetry (i.e. internal waves breaking) and/or induced by land drainage. Both processes appear to be strongly influenced by larger-scale dynamics. First, internal waves are generated offshore before dissipating their energy along the island shelves. Second, large scale atmospheric conditions (especially during ENSO events) influence the high island induced rain, and consequently the land drainage. In addition, potential iron advection from the equatorial under current toward the archipelago, through tropical instability waves during La Nina, may also have been at play during the 1998 and 2010 events. Finally, large-scale dynamics also imprint the local offshore phytoplankton plume, through the seasonal variability of the South Equatorial Current and the pathway of mesoscale eddies.