MODELING NITRATE FLUXES IN AN OPEN COASTAL ENVIRONMENT (GULF OF LIONS) - TRANSPORT VERSUS BIOGEOCHEMICAL PROCESSES

A 13-compartment model of primary production and degradation of dissol ved organic matter has been coupled with a general circulation model i r an open coastal environment (Gulf of Lions, Mediterranean) so as to quantify exchanges with the open sea. The biogeochemical model had bee n previously calibrated with a one-dimensional vertical version on a 1 -year data set, and this simulation provides boundary conditions for t he three-dimensional model. After a 1-year spin-up simulation, quasi-e quilibrium is obtained for all the compartments, and the results of an additional annual simulation are compared with coastal zone color sca nner images and other data collected in the area: the simulation of ch lorophyll a, nutrient concentrations, and primary production is quite satisfactory, although the spring bloom starts slightly earlier than u sual. Total gross community production is estimated at 76 g C m(-2) y( -1), and New Gross Community Production at 45 g Carbon m(-2) y(-1) (f = 0.37). The model was used to understand the nitrate annual cycle in the coastal zone of the Gulf of Lions. Nitrate input from the Rhone ri ver, from the sediment, and from marine advection are compared in term s of potential fertilization. Model results indicate that, regarding t he open sea, the margin acts most of the time as a sink for nitrate. H owever, during winter, when phytoplankton growth is reduced and cascad ing of dense waters is active along the shelf, the margin is shown to export nitrate toward the open sea. These results may qualitatively ap ply to other open Mediterranean margins, although the intensity of the fluxes would be modulated by local features.