Simulation de l'effet sur l’eutrophisation côtière bretonne de 3 scénarios de réduction des teneurs en nitrate et phosphate de chaque bassin versant breton et de la Loire

This study commissioned by the Région Bretagne and the Agence de l'Eau Loire Bretagne aimed at assessing on a mathematical modelling basis the marine influence of most of watersheds in Brittany, and the impact of 4 scenarios of reduction of the loadings of each of these watersheds on the phytoplanctonic biomass and ulva mass proliferation. The ecological 3D model ECOMARS3D has been applied to a domain containing Brittany (which is also used in an operational mode for putting everyday the 2dayforecasts on the website: http://www.previmer.org/previsions/production_primaire/modele_eco_mars3d_bretagne) and to 6 zooms which enabled a finer representation of coastal waters. This study highlighted the fact that: •a nitrate reduction for any river has no effect on diatom biomass. •a nitrate reduction has a more sensible effect on dinoflagellate biomass but only in case of a reduction of at least 75% of nitrate loading. •the plume of Loire river extends widely along the coasts of southand western Brittany, and can enter the English Channel after the high flows • a nitrate and phosphate reduction for Vilaine would have few effect on phytoplanctonic biomass if it is not coupled with a nitrate and phosphate reduction for Loire. •some rivers are mainly responsible for some green tides. It is the case for the rivers Yar and Douron which are respectively responsible for the ulva blooms of SaintEfflam beach and Douron's estuary. However, some green tides like these of Bay of SaintBrieuc or Moulin Blanc cowe in the Bay of Brest cannot be imputed to one river particularly.