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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.

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.

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.

A three dimensional (3D) ecological model is developed and applied to the "baie de Seine" area. It consists of a 3D model for hydrodynamics and port (SiAM3D) coupled to an ecological model simulating nutrient cycles (nitrogen, phosphorus, silicon) and two phytoplanktonic classes (diatoms and flagellates). The model is validated by confronting simulations with nine year long series of measurements and then used to better understand the ecosystem functioning. It especially allows to show the role of the proportion of silicon with regard to nitrogen and phosphorus in the summer loadings of the Seine river on the more or less strong presence of flagellates in the river plume. The model was also used to test various hypotheses of reduction of nitrogen loading from the Seine river and their consequences on the phytoplanktonic production in the plume., Une modélisation écologique tri-dimensionelle (3D) a été développée en baie de Seine. Elle est constituée d'un modèle 3D hydrosédimentaire (SiAM3D) couplé à un modèle écologique simulant le cycle des éléments nutritifs (azote, phosphore, silicium) et deux classes phytoplanctoniques (diatomées et flagellés). Après validation par confrontation des simulations avec neuf années de mesures, le modèle a été utilisé afin de mieux comprendre le fonctionnement de l'écosystème. II a notamment permis de montrer le rôle de la proportion de silice par rapport à l'azote et au phosphore dans les apports estivaux de la Seine sur la présence plus ou moins forte de flagellés dans le panache du fleuve. Le modèle a également été utilisé pour tester différentes hypothèses de réduction des apports d'azote par la Seine et leurs conséquences sur la production phytoplanctonique dans le panache.

A three dimensional (3D) ecological model is developed and applied to the "baie de Seine" area. It consists of a 3D model for hydrodynamics and port (SiAM3D) coupled to an ecological model simulating nutrient cycles (nitrogen, phosphorus, silicon) and two phytoplanktonic classes (diatoms and flagellates). The model is validated by confronting simulations with nine year long series of measurements and then used to better understand the ecosystem functioning. It especially allows to show the role of the proportion of silicon with regard to nitrogen and phosphorus in the summer loadings of the Seine river on the more or less strong presence of flagellates in the river plume. The model was also used to test various hypotheses of reduction of nitrogen loading from the Seine river and their consequences on the phytoplanktonic production in the plume., Une modélisation écologique tri-dimensionelle (3D) a été développée en baie de Seine. Elle est constituée d'un modèle 3D hydrosédimentaire (SiAM3D) couplé à un modèle écologique simulant le cycle des éléments nutritifs (azote, phosphore, silicium) et deux classes phytoplanctoniques (diatomées et flagellés). Après validation par confrontation des simulations avec neuf années de mesures, le modèle a été utilisé afin de mieux comprendre le fonctionnement de l'écosystème. II a notamment permis de montrer le rôle de la proportion de silice par rapport à l'azote et au phosphore dans les apports estivaux de la Seine sur la présence plus ou moins forte de flagellés dans le panache du fleuve. Le modèle a également été utilisé pour tester différentes hypothèses de réduction des apports d'azote par la Seine et leurs conséquences sur la production phytoplanctonique dans le panache.

A three dimensional (3D) ecological model is developed and applied to the "baie de Seine" area. It consists of a 3D model for hydrodynamics and port (SiAM3D) coupled to an ecological model simulating nutrient cycles (nitrogen, phosphorus, silicon) and two phytoplanktonic classes (diatoms and flagellates). The model is validated by confronting simulations with nine year long series of measurements and then used to better understand the ecosystem functioning. It especially allows to show the role of the proportion of silicon with regard to nitrogen and phosphorus in the summer loadings of the Seine river on the more or less strong presence of flagellates in the river plume. The model was also used to test various hypotheses of reduction of nitrogen loading from the Seine river and their consequences on the phytoplanktonic production in the plume., Une modélisation écologique tri-dimensionelle (3D) a été développée en baie de Seine. Elle est constituée d'un modèle 3D hydrosédimentaire (SiAM3D) couplé à un modèle écologique simulant le cycle des éléments nutritifs (azote, phosphore, silicium) et deux classes phytoplanctoniques (diatomées et flagellés). Après validation par confrontation des simulations avec neuf années de mesures, le modèle a été utilisé afin de mieux comprendre le fonctionnement de l'écosystème. II a notamment permis de montrer le rôle de la proportion de silice par rapport à l'azote et au phosphore dans les apports estivaux de la Seine sur la présence plus ou moins forte de flagellés dans le panache du fleuve. Le modèle a également été utilisé pour tester différentes hypothèses de réduction des apports d'azote par la Seine et leurs conséquences sur la production phytoplanctonique dans le panache.