The fertilization of an eutrophic ecosystem of Western Europe (the Bay of Brest, 180 km super(2)) is observed from 1979 to 1982. Rich inputs in nutrients and dissolved organic matter flow into this semi-enclosed basin which waters are well mixed by the intensive action of tides. Natural and waste waters inputs are variable on a seasonal and annual scale; this induces large variations on the phytoplanktonic communities. This ecosystem presents high levels of primary production and low degree of maturity. By means of a model, the nitrate uptake by phytoplankton photosynthetic activity is simulated during spring 1980.