Modélisation de la production phytoplanctonique dans la zone côtière atlantique enrichie par les apports fluviaux

In order to simulate the influence of river loadings in coastal area, a three-dimensional model of primary production was set up. It is based on a hydrodynamic model previously developed at Ifremer. The calculation of the temperature and salinity takes into account the advection and the diffusion induced by tide, wind and density currents. The coupling with the biological model was done by the intermediary of transport, diffusion and heat fluxes. The cycles of three limiting elements for the phytoplankton growth were modelised : nitrogen, phosphorus and silicon. The second phase of the study aimed at reproducing the summer dinoflagellate blooms. In this new approach a model of the specie Gymnodinium mikimotoï was coupled with the previous biomass model which provides the physical and chemical environments. The parametrisation was based on some physiological studies reported from laboratory cultures and on parameters existing in the literature. The seasonal biological cycles were in good agreement with data cruises, with data obtained from network of surveillance (REPHY and RNO) and with satellital measurements of ocean colour. The simulation of the physical structure and of the main biological processes provides a synoptic view of the interactions between mesoscale physical and biological phenomena on the continental shelf. The model thus highlights the influence of upwellings, fronts and shallow waters on the primary production. This model also provides a better understanding of the influence of the Loire and the Gironde as it allow to estimate the share of the primary production related to the anthropic contributions of these two rivers. This is otherwise impossible to estimate from in-situ measurements. Lastly, the analysis of simulations over 10 years underlines the determining role of wind on the vertical structure of the water column, on the spreading of the river plumes and consequently, on the primary production.