Résumé : This chapter illustrates the role of watersheds and estuaries in the biogeochemical functioning of coastal zones, specifically in terms ofeutrophication, which is a widespread phenomenon. Along the land–ocean aquatic continuums, estuaries are receptacles of thepollution generated by human activities (e.g., domestic wastewater, agriculture) in watersheds; they can further modify the riverinenutrient delivery before it reaches the marine zone. We present several applications of a chain of modeling tools able to simulate waterquality transfers and transformations for a reference situation in a variety of systems to be compared with scenarios involvingimprovement of domestic wastewater and structural changes in agriculture.