Abstract : In marine waters, several biogeochemical perturbations originate from land-based loadings. The eutrophication of coastal water bodies is one of the striking consequences of excessive nutrients inputs and is a major contributing factor to algal blooms. However, the data currently available mainly come from monitoring networks and are thus intermittent and punctual. They are therefore insufficient to accurately assess the contribution of any watershed to such pollutions as well as the origin of the pollutants, especially in the case of non-point source pollutions. In this context, watershed modelling approaches appear to be a complementary and relevant method in a management perspective, particularly regarding their abilities to implement scenarios. Through the example of a study site located in southern Brittany, which constitutes the main hydrosystem discharging in the Gulf of Morbihan, we implemented the Soil and Water Assessment Tool model over a six-year period. In an operational perspective, both parameter settings using common available datasets, and different computing applications were developed in order to be used by managers responsible for the monitoring and the management of littoral waters. This approach enabled to continuously quantify land-based nutrient discharges and to determine their origins. Identifying watersheds that contribute the most to these inputs may constitute a useful tool to design priority areas of intervention for any improvement of water quality in the hydrosystem of the Auray River.