SPATIAL AND TEMPORAL VARIABILITY OF PHYTOPLANKTON BIOMASS IN UPWELLING AREAS OF THE NORTHWESTERN MEDITERRANEAN - A COUPLED PHYSICAL AND BIOGEOCHEMICAL MODELING APPROACH

The coupling of a 3-D hydrodynamic model and the biogeochemical algori thms of phytoplankton biomass production was performed at the meso sca le of the northwestern Mediterranean shelf. This study, which was part of the French Programme National d'Oceanographie Cotitere, represente d a theoretical approach to study the phytoplankton dynamics at mesosc ale, according to upwelling processes induced by both the general circ ulation (LPC) and the typical northwestern wind that prevail in the Gu lf of Lions. The coupling task consisted in writing the equations of n itrogen and carbon cycles from both the algorithms describing the biog eochemical gain and loss terms, and the advective-diffusive numerical scheme of the physical model, according to the same spatial grid. This approach allowed fine 3-D spatial descriptions of the biogeochemical processes during 20-day periods. The physical parameters used in the a dvective-diffusive numerical scheme, such as velocities and coefficien ts of vertical eddy diffusivity, were first calculated by the hydrodyn amic model, averaged over one inertial period (17.6 h), and then intro duced into the biogeochemical coupled model as constant forcing variab les. First results of the coupled model are obtained in the Gulf of Li ons under three typical forcing conditions such as the LPC current, wh ich corresponds to a geostrophic circulation, the LPC current with a m oderate northwestern wind of 7 m . s(-1) and the LPC current with a st rong northwestern wind of 14 m . s(-1). The results are discussed to e mphasize the discontinuous features of physical and biogeochemical pro cesses within the very contrasted environment of the Gulf of Lions. Th e most important result of the coupled model is to emphasize the devel opment of two different processes of phytoplankton biomass production that successively occur in the Gulf of Lions during a 20-day period. T he first type of process appears during the first period of the simula tion and consists in inducing a biomass production from the autochtono us nutrient supply according to the initial conditions considered in t he Gulf. The second type of process appears during the following perio d and consists in inducing a delayed biomass production from allochton ous nutrient supply, due to vertical transport in upwelling area. In a ddition, these two types of production seem respectively controlled by the two major physical processes involved in the water column: the ve rtical diffusion and the vertical advection. The biogeochemical couple d model, in its present form, represents the first stage in the develo pment of a tool, which should be able to determine the time and space scales of the non linear phytoplankton dynamics in the Gulf of Lions, and the locations of the most sensitive areas for phytoplankton growth under realistic forcing conditions.