Chapter 9 - Continental Atlantic Rivers: the Seine Basin.

The Seine River and its basin (70 000 km2, 500 m3/s mean annual discharge) are studied since 1989 by an interdisciplinary research programme (https://www.piren-seine.fr/). The river receives the effluents of about 17 M inhab (70% from Paris conurbation) and from industries. Intensive agriculture using agrochemicals is another cause of water quality alteration. Morphological alterations (such as channelization and reservoirs construction) were considerable. From the 1970s to the 1990s, the Seine water quality was very poor downstream of Paris: summer oxygen deficit, nutrient excess and eutrophication, severe contamination by metals, persistant organics and emergent pollutants, including plastic debris, all detrimental to aquatic life, as revealed by very low fish species richness. From the 1990's on, the measured quality status of the Seine has been significantly improved owing to urban and industrial wastewater treatment, yet its ecosystem remains fragile, with high agriculture impact. Basin scale mass balance budgets were established for most of the pollutants, and their trajectories reconstructed over past decades. Scenarios for more sustainable future development have been established. Basin-wide numerical models analyse the hydro-biogeochemical functioning of the Seine river network, from headwaters to the estuary, taking into account past, present and future human activities in the watershed