Modeling the response of water quality in the Seine river estuary to human activity in its watershed over the last 50 years

The unique database of water quality measurements made in the Seine estuary over 45 years by the Service de la Navigation de la Seine at Rouen is used here to reconstruct the evolution of oxygen status in the estuary and the nutrient fluxes to the Seine Bight during the last half century. The Riverstrahler model is used to establish the link between these long-term trends in the functioning of the Seine system and the evolution of agricultural, domestic, and industrial activity in the watershed over this period taking into account natural and man-induced hydrological variations. Oxygenation of the fluvial sector below Paris has increased considerably owing to improved wastewater treatment, but a large part of the estuary remains completely anoxic during the spring and summer months. Nitrogen input to surface waters from urban sources has remained essentially constant while diffuse inputs from agricultural soils have increased 5-fold as a result of more intensive agricultural practices as well as the loss of retention capacity in riparian zones. Phosphorus inputs from domestic and industrial sources increased 3-fold from 1950 to 1980, but have decreased gradually in recent years. The generally Wgh level of phosphorus contamination has favored strong algal development in all large tributaries of the Seine River upstream of Paris since the 1960s. Silica inputs, originating mainly from the weathering of rocks, fluctuate widely depending on hydrology. In-stream retention of silica, linked to diatom blooms, has increased but remains limited. These changes have induced several shifts in the nutrient limitation conditions of the Seine Bight.