Influence of mesoscale physical processes on planktonic ecosystems in the regional ocean: application to the Eastern Boundary Upwelling Systems.
2010
The objectives were numerous but centered on a scientific overarching question: the influence of mesoscale physical processes on planktonic ecosystems in coastal upwellings. The use of several and complementary tools was necessary to assess the various mechanisms occurring at these scales. First, the variability of the physical and biogeochemical properties of the Iberian Peninsula Upwelling System was studied using in-situ data from the MOUTON 2007 survey. The coastal domain was divided into different biogeochemical provinces: high nutrient concentrations are found in the northern part due to upwelled waters of subpolar origin whereas the southern area was characterized by moderate nutrient concentrations (waters of subtropical origin). The intense cross-shore gradient revealed that the biological activity is associated with the successive upwelling pulses. Data from a filament allowed us to assess its impact in terms of cross-shore exchanges of biological material. Then, a secondary upwelling front located at the shelf break was observed in these data. We built a simplified 2D numerical model to explain it by the eastward advection from upwelling currents of deep water columns onto the shelf. It creates a negative anomaly of Potential Vorticity (PV) which generates the secondary front. A simulation revealed a filamentation process that was further investigated numerically. The interaction between the alongshore upwelling current and a promontory creates anomalies of PV in the lower layer that generates the development of a filament trapped in the vicinity of the promontory. Finally, we used satellite data associated with a lagrangian tool (Finite-Size Lyapunov Exponent) to study the horizontal mixing in relation to the biological activity over the four eastern boundary upwelling zones. We showed a negative relationship between horizontal surface mixing and chlorophyll content, which seems related to a diminution of vertical velocities of the coastal upwelling, as well as a lateral loss of nutrient due to moving eddies. Horizontal mixing induced by eddies thus has an inhibiting effect on the phytoplankton in coastal upwellings, whereas a positive effect stands for the open ocean. Using diverse and complementary tools allowed us to improve our knowledge of the large influence of physical processes on marine ecosystems.
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