During the cruise EURHOGLI (Eutrophication by the Rhone of the Gulf of Lion) of the R.V. Le Noroit , a grid of stations covering the dilution area of the Rhone River has been carried out two times with a time interval of about two weeks. An exceptional growth during the two months preceding the cruise, has produced an important dilution effect over a wide spread areas. The absence of nutrients demonstrates their general consumption by biological activity. The tracer properties of the nutrient can be observed only in a small area near the mouth of the river. Nutrients are becoming the tracer of this dilution, and a great development of chlorophyll associated to this same area, show that nearly all the biological activity of nutrient assimilation is concentrated in the narrow surface layer. Such a situation proves the absence of downward mixing effect due to the wind. Determinations of nutrients in the waters show a strong increase of the concentration during the last 20 years; this increase has an anthropogenic origin.