Eutrophication of French Brittany coastal shelf is a recurrent problem, more and more worrying. This eutrophication exhibits two main forms :– “green tides”, which are constituted by local proliferations and strandings of green macro-algae (particularly the Ulva species) in nitrogen-enriched areas.– phytoplanktonic blooms, which are responsible of coloured water events. Toxic species can be present (Pseudonitzschia, Alexandrium, Dinophysis), and induce banning of shellfish consumption.Several programs and directives compel authorities to initiate actions in order to diminish these problems. (WFD, OSPAR). This implies a better understanding of these phenomena.This work deals with the influences of the plumes of Loire, Vilaine and bay of Brest rivers (Elorn and Aulne) on the nitrate enrichment of the French Brittany coastal shelf. A 3D hydrodynamic model, MARS 3D, is used. The nitrate is considered as a conservative tracer, which is advected and dispersed by the model. Thanks to the introduction of 4 different tracers, we can distinctively follow the nitrate from Loire, Vilaine, Bay of Brest rivers and other rivers.The study shows that the Loire river influences the southern and western parts of coastal shelf of Brittany. The plume propagates up to the north of Iroise sea, and contributes for a large part to its enrichment. On the other hand, the high enrichment of the bay of Brest remains confined and is smoothly dispersed in Iroise sea, except during flood events : in this case, nitrate jets are pulsed into the Iroise sea.