Nutrient dynamics and control of eutrophication in the Marne River system: modelling the role of exchangeable phosphorus
Résumé
The Marne River (12,762 km2) is one of the main tributaries of the Seine river, upstream from the City of Paris. Its population counts 2 million inhabitants of which 70% are concentrated in the downstream part of the basin. It has a high industrial activity and an intensive agriculture. The Marne River water supplies drinking water production to a large number of Parisians. A diversion reservoir (350×106 m3) is used mainly to sustain summer low flows. On the basis of field observations and experimental studies, a hydrological and ecological model of this river system was built using the Riverstrahler approach (Billen, G., Garnier, J., Hanset, Ph., 1994. Modelling phytoplankton development in whole drainage networks: the riverstrahler model applied to the seine river system. Hydrobiologia 289, 119–137; Garnier, J., Billen, G., Coste, M., 1995. Seasonnal succession of diatoms and Chlorophyceae in the drainage network of the river Seine: observations and modelling. Limnol. Oceanogr. 40, 750–765). The modelling strategy was to consider separately nine sub-basins and the main branch, to which was coupled a model of the reservoir. The point and diffuse sources of nutrients were analysed over more than 10 years (1991–2001) taking into account the role of exchangeable phosphorus (Némery (2003). Origine et devenir du phosphore dans le continuum aquatique de la Seine, des petis basins à l'estuaire. Rôle du phosphore échangeable sur l'eutrophisation. Thèse Univ. Paris 6, p. 258; Némery, J., Garnier, J., Morel, C., 2004. Phosphorus budget in the Marne watershed (France): urban vs. diffuse sources, dissolved vs. particulate forms. Biogeochemistry (in press)). The model was validated through its ability to reproduce available water quality observations. Different realistic scenarios of future reduction of phosphorus load were tested, in various hydrological conditions (dry and wet years). Phytoplankton development can be slightly reduced by a further 85% abatement of phosphorus in all the wastewater treatment plants of the basin, but a reduction both of diffuse and point phosphorus sources would be necessary to further decrease eutrophication.