The present work aims at establishing a mathematical model which describes the seasonal evolution of the most characteristic constituents of the pelagic phase of the ecosystem in the southern bight of the North Sea. After the notions of system and model, as applied to the analysis of the marine environment, have been defined, the different analytical forms that are required to describe the various ecological interactions are explained and the principal ecological models which were published in the litterature are reviewed. They are divided into five distinct categories. The experimental data which have enabled the establishment of the mean yearly cycles of nitrogen through the different compartments of this particular ecosystem are then examined. These yearly cycles are considered to be a necessary preliminary for any modeling attempt. The equations for the seasonal evolution of dissolved nitrogen, phytoplankton, zooplankton and dissolved organic matter in the Belgian coastal zone, supposed to be spatially homogenous, i.e. one box, are then established. The results of this model are in good agreement with the data collected in situ. Furthermore, to reveal the spatial variations of these variables in the eastern part of the southern bight of the North Sea this model has been extended to this entire marine area, which for this purpose has been divided into thirty boxes, the geographical delimitation of which is based on the distribution of the residual currents. Finally, the great number of parameters which need to be introduced in the evolution equations and whose values are not known very precisely, is identified as a major handicap for ecological modeling. This is why a sensitivity analysis of the parameters is made, to emphasize the ones which influence the dynamics of this ecosystem the most, on a yearly average
