The residence time of the water in different parts of the Marennes-Oléron Bay is one important characteristic of the biological system (Raillard, 1990). For instance, the primary production potentiality of a given area depends on the local turbidity and the residence time of the waters in that area. Further, global assessments of the nitrogen flow between the biological compartments and inflows from the ocean or the Charente river derived from the ecosystem model by Raillard (1990), showed the predominance of the latter term onto the former. A new hydrological model of the Marennes-Oléron Bay yielded two dimensional velocity fields for sorne standard tidallevels: neap-tide, spring-tide, mean tide (Lazur, unpub.). These computations were the basis for the calculation of the transport of conservative substances throughout the Bay and were coupled to the resolution of resuspension and sedimentation equations. For this work, the instantaneous values of the bidimensional flows and the volume of each 1000 m x 1000 m cell were used to estimate the residence time ofwater inside the Bay. Eulerian residual values were computed for the salee of simplicity. Emphasis was given to the comparison of the linear resolution of transport under steady conditions of tidal level between the box model used by Raillard and the previous model. This simplification of the equations allowed to formulate the transport process in a probabilistic fashion .