The subtropical gyres represent 40% of the total ocean. They have been considered, since a long time, as steady state biological desert due to low nutrients and phytoplankton biomass throughout the year. Primary production (carbon uptake by phytoplankton) is low too in these regions, but as these structures are huge, it makes the total contribution to ocean primary production important. The South Pacific gyre is considered to be the place where the phytoplankton biomass is the lowest in the global ocean. This area remains poorly sampled and studied, that's why the seasonal variability of phytoplankton, primary production and physical forcing remains unclear. By using in-situ bio-optical profiling float capable to measure physical parameters (temperature and salinity) and proxy of the phytoplankton biomass in the water column, we acquired one year of data near the eastern islands (in the middle of the South Pacific gyre). We found a seasonal variability of the phytoplankton biomass and the carbon flux. This variability is driven by the vertical structure density of the ocean, from very low phytoplankton biomass and carbon uptake in winter (due to the mixing of the water column) to low phytoplankton biomass and carbon uptake in summer (associated with strong stratification). Studying the interaction between the physical forcing and phytoplankton biomass in the subtropical gyres is essential. Any increase in sea water temperature will change the stratification of the water column and thus affect the annual carbon fluxes budget.