The French coasts exhibit two kiruJs of eutrophicati.on; the prolif erati.on of green macroa/gae (Ulva sp.) affects the very shallow Mediterranean /agoons as we/1 as a lot of widely open beaches in Brittany, whereas phytoplanktonic mass blooms are to be f ound in front of the large estuaries and in some Atlantic -embayments. Bath these types of eutrophicati.on seem to have increased during the fast twenty years, probably in relation with the increase of the nutrients /oadings from agriculture and sewage. However, the severity of eutrophicati.on is not on/y related to the absolute amount of /oadings. The local residence time of the waters, deduced from hydrodynamics, is a crucial parameter; numerica/ mode/s have highlighted the raie of tidal residua/ circulation. Reduction of eutrophicati.on requires a precise knowledge of the in-situ limiting nutrient. If green macroalgae are c/early nitrogen limited, phytoplankton may be either phosphorus or nitrogen limited. Ecological numerical mode/s may help to quanti/y the effects of various strategies of lowering the loadings of nutrients.