Coastal eutrophication of the southern bight of the North Sea: assessment and modelling

Massive blooms of the colony-forming Haptophyte Phaeocystis sp. are a recurrent phenomenon in the eutrophicated continental coastal waters of the North Sea. This coastal zone is receiving the discharge of seven major West-European rivers the watershed of which is characterized by high population densities and intense industrial and agricultural activities. The basic mechanisms of Phaeocystis blooms development in response to riverine nutrient enrichment are being investigated since 1988 in the scope of national and EC research projects on Phaeocystis bloom dynamics. The ultimate objective is to set up a computer tool that could provide guidance for making selection among the control actions available for counteracting coastal eutrophication in this area. For this purpose, an integrated research methodology has been implemented, combining field observations of Phaeocystis blooms development and associated physico-chemical and biological variables at reference coastal stations, physiological studies of Phaeocystis nutrient metabolism and the development of the mechanistic biogeochemical MIRO model. Results of this long-term study are presented here. Observational and mathematical evidence is given that the present-day extent of Phaeocystis colony development observed in the whole area, mainly results from changes in riverine nitrate and silicate delivery, driven by changes in both anthropogenic activities and rainfall conditions on the watershed. Furthermore, realistic nutrient reduction scenarios as scheduled for North-Westem Europe for the next 25 years, making use of the current MIRO model, gives additional illustration of the complex interaction between the continent and coastal sea systems and of the appropriateness of the mechanistic model as a decision support for selecting the most appropriate measure to counteract coastal eutrophication