Spatial and temporal variability of phytoplankton biomass in upwelling areas of the northwestern mediterranean: a coupled physical and biogeochemical modelling approach

Abstract

Abstract The coupling of a 3-D hydrodynamic model and the biogeochemical algorithms of phytoplankton biomass production was performed at the meso scale of the northwestern Mediterranean shelf. This study, which was part of the French Programme National d'Oceanographie Cotiere, represented a theoretical approach to study the phytoplankton dynamics at mesoscale, according to upwelling processes induced by both the general circulation (LPC) and the typical northwestern wind that prevail in the Gulf of Lions. The coupling task consisted in writing the equations of nitrogen and carbon cycles from both the algorithms describing the biogeochemical 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 advective-diffusive numerical scheme, such as velocities and coefficients of vertical eddy diffusivity, were first calculated by the hydrodynamic model, averaged over one inertial period (17.6 h), and then introduced into the biogeochemical coupled model as constant forcing variables. First results of the coupled model are obtained in the Gulf of Lions under three typical forcing conditions such as the LPC current, which corresponds to a geostrophic circulation, the LPC current with a moderate northwestern wind of 7 m · s−1 and the LPC current with a strong northwestern wind of 14 m · s−1. The results are discussed to emphasize the discontinuous features of physical and biogeochemical processes within the very contrasted environment of the Gulf of Lions. The most important result of the coupled model is to emphasize the development of two different processes of phytoplankton biomass production that successively occur in the Gulf of Lions during a 20-day period. The first type of process appears during the first period of the simulation and consists in inducing a biomass production from the autochtonous nutrient supply according to the initial conditions considered in the Gulf. The second type of process appears during the following period and consists in inducing a delayed biomass production from allochtonous nutrient supply, due to vertical transport in upwelling area. In addition, these two types of production seem respectively controlled by the two major physical processes involved in the water column: the vertical diffusion and the vertical advection. The biogeochemical coupled model, in its present form, represents the first stage in the development 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.