The European coastal zones are areas of great concern because of growing problems associated with increasing inputs of nutrients since the late 1960s. These have resulted in a higher incidence of harmful algal blooms and other eutrophication phenomena and caused deleterious impacts on fisheries and tourism (Lancelot et al., 1989) .

Since the late 1980s, agreements have been made at national and international levels to substantially reduce the levels of nutrient emissions to the aquatic environments. The last directives from the European Commission have defined a strategy where every effort to combat eutrophication in the maritime area is requested, in order to achieve, by the year 2010, a healthy marine environment where eutrophication does not occur.

After ca 20 years of measuring sanitation, dramatic changes in the discharge of phosphorus to the North-Sea have been observed, whereas nitrogen emissions did not change much over the same period (de Jonge et al., 2002) .

The implementation of these measures represents a high cost for the community (Conley et al., 2002) and this demands an efficient assessment of their effects on the environment with respect to their objectives. Indeed critical levels for nutrient supply may vary from one system to another and uniform policy for nutrient sanitation may be either too restrictive or too permissive with respect to their objectives (avoidance of undesirable disturbance).