Séminaire

Primary productivity and oxygen availability are key drivers for fish communities and the biological carbon pump in the ocean. First results of current projects which explore the organization, from regional to mesoscale, of primary productivity and the oxygen minimum zone (OMZ) in the Peru-Chile upwelling system (PCUS) are presented. ROMS-PISCES physical-biogeochemical modeling (now CROCO platform) is used to explore the mechanisms behind the observed ocean variability. At regional scale, the model brings a new hypothesis to explain the meridional variability of the coastal upwelling productivity and its offshore extension in the coastal transition zone (CTZ). Iron limitation of the phytoplankton growth is indeed suggested to play a key role in the coastal upwelling of the PCUS. In the model, the iron limitation of coastal productivity (0-70 km from the coast) mostly relies on the iron inputs from the bottom sediments. These inputs likely depend on the contact time of upwelled waters with the sediments, and then on the continental shelf width. The offshore phytoplankton biomass (70-200 km from the coast) is mosty driven by the injection of living organisms from the coast, which itself relies on coastal productivity and cross-shore transport processes such as mesoscale eddies and filaments. The coastal productivity and offshore extension in the CTZ then depends on the continental shelf width and cross-shore exchange processes. The interaction between iron limitation and the level of eddy/filament activity may actually drive the regional distribution of plankton biomass in the PCUS. In recent years, quasi-zonal mesoscale jet-like features or striations have been  biquitously detected in the time-mean circulation of the world ocean. Off central Chile (southern PCUS), striations were attributed to the organization of the mesoscale eddy field in preferred eddy tracks. Yet, their impact on biogeochemistry has not been assessed yet. In the CTZ, the model reproduces the surface chlorophyll striations detected in satellite records of ocean color. The interaction between preferred eddy tracks and sharp background gradients of biogeochemical properties seems to structure at mesoscale the mean horizontal distribution of the plankton biomass in the CTZ and the OMZ. The respective  contributions of eddy trapping and eddy advection (stirring) to these striations are examined.

vech@locean-ipsl.upmc.fr