The Commission of the European Communities had brought an action for a declaration that, by having failed: - to identify certain areas as sensitive areas with respect to eutrophication as regards the Seine-Normandy, Loire-Brittany, Artois-Picardy and Rhône Mediterranean-Corsica basins, and - to subject to more stringent treatment discharges of urban waste water from agglomerations with a population equivalent (p.e.) of more than 10 000 into sensitive areas or areas which should have been identified as sensitive, the French Republic had failed to fulfill its obligations pursuant to Article 5(1) and (2) of, and Annex II to, Council Directive 91/271/EEC of 21 May 1991 concerning urban waste water treatment. Since the Commission and the French Government did not agree on the scope of the definition set out in Article 2(11) of Directive 91/271, it was necessary for the Court to clarify the meaning of eutrophication for the purposes of that directive. The Court emphasized that the objective pursued by Directive 91/271 went beyond the mere protection of aquatic ecosystems and attempted to conserve man, fauna, flora, soil, water, air and landscapes from any significant harmful effects of the accelerated growth of algae and higher forms of plant life resulting from discharges of urban waste water. The definition of eutrophication in Article 2(11) of Directive 91/271 had to be interpreted in the light of that objective. The Court defined eutrophication as being characterized by the confluence of four criteria: - the enrichment of water by nutrients, especially compounds of nitrogen and/or phosphorus; - the accelerated growth of algae and higher forms of plant life; - an undesirable disturbance of the balance of organisms present in the water; - deterioration of the quality of the water concerned. Any proliferation of a particular species of algae or other plant therefore constituted, as such, a disturbance of the balance of the aquatic ecosystem and, accordingly, of the balance of the organisms present in the water, even when other species remained stable. An undesirable disturbance also had to be considered to be established where there were significant harmful effects not only on flora and fauna but also on man, the soil, water, air or landscape. Species changes involving loss of ecosystem biodiversity, nuisances due to the proliferation of opportunistic macroalgae and severe outbreaks of toxic or harmful phytoplankton therefore constituted an undesirable disturbance of the balance of organisms present in the water. The fourth criterion referred not only to deterioration of the quality of the water which produced harmful effects for ecosystems but also deterioration of the color, appearance, taste or odor of the water or any other change which prevented or limited water uses such as tourism, fishing, fish farming, clamming and shellfish farming, abstraction of drinking water or cooling of industrial installations. In conclusion, the Court held that the French Republic had failed to fulfill its obligations pursuant to Article 5(1) and (2) of, and Annex II to, Directive 91/271.