The Seine estuary is a megatidal estuary located in northern France. It is the third largest estuarine ecosystem in the country after the Gironde and Loire along the French Atlantic coast; its waters flow into the English Channel. The geographical zone of influence of the Seine estuary runs from just upstream of the Poses dam, at the limit of the tidal penetration into the estuary — 160 km upstream of Le Havre —to the eastern part of the Bay of Seine. It can be divided into three sections: the fresh water upstream section (125 km), the mixing zone characterized by varying salinity levels (35 km), and the marine downstream section under the influence of the Seine River. The megatidal regime generates a turbidity maximum in the mixing zone (middle estuary) between the marine and fluvial sections of the estuary. The Seine estuary is a typical estuarine ecosystem: highly stressed by natural fluctuations and anthropogenic pressures, and hosting a rich ecological system. The Seine valley and its estuary are of major economic importance for France, with the presence of two maritime ports. It’s watershed (79,000 km2) is the home to 17 million people, and accounts for 50 % of the river traffic in France, 40 % of the country's economic activity, and 30 % of its agricultural activities. In addition to the Greater Paris area (> 11 million inhabitants), which contributes heavily to the Seine estuary's upstream inputs, two other major river settlements — Rouen (500,000 inhabitants) and Le Havre (300,000 inhabitants) — are maritime ports of international importance. Due to economic development, the Seine estuary has been subjected to major hydrodynamic, ecological, biogeomorphological and biogeochemical changes. The importance of studying the effects of anthropogenic impacts on estuarine ecosystems has increased in the last decade, especially under the Water Framework Directive, which aims to achieve “good environmental status” of all European water bodies.
