Within the framework of the Programme National
d?Océanographie Côtière (PNEC), an intensive study of the
biogeochemical pelagic cycles of carbon and nitrogen was
performed in connection with the hydrodynamic processes in
the Gulf of Lions (Northwestern Mediterranean Sea). Results,
exposed and interpreted in the part one of the manuscript
were obtained from three seasonal and spatial cruises
(MoOGLi cruises in 1998-99) and from an unidimensional site
(SOFi site) that was monthly sampled between September 1997
and June 1999. The numeric and graphical results (profiles)
are gathered in the part two of the manuscript. A seasonal
accumulation of dissolved organic carbon (40 g C m-2), that
is the predominant form for the exported carbon, has been
observed during the 1998 year over the whole continental
shelf. The budgets of primary production and of new
production show that the continental shelf would be in
balance towards CO2 uptake and release; however, this area
would be a sink for the organic carbon up to 700±400 103 T
a-1 due to the Rhone River supply. The second part of the
manuscript (in the part I) is devoted to the development and
validation of a biogeochemical model that is based on the
nitrogen and phosphorus cycles in the gulf of Lions. This
model is coupled to an hydrodynamic model of coastal ocean
(the SYMPHONIE model). The 3D modelling of the spring MoOGLi
cruise revealed that the Gulf of Lions is a privileged site
for the production of organic matter; the most part of the
organic matter is exported through the south-western
boundary of the modelled domain in the course of the study
period. This coupled modelling also showed that the
privileged site of export could be geographically changed
with the atmospheric conditions (wind regimes).