Transformations of nutrients (N, P, Si) in the turbidity maximum zone of the Seine estuary and export to the sea

Nutrients (N, P, Si) were studied in the turbidity maximum zone (TMZ) of the Seine estuary at various moments of tidal cycles from April to October in 2001, 2002 and 2003, covering a salinity gradient from 0 to 27. Contrasted hydrological conditions were observed (extremely wet in 2001, unusually dry in 2003). Nutrient fluxes were analysed in terms of an indicator of coastal eutrophication potential (ICEP) using nutrient stoichiometry. Besides these three years, nutrient fluxes and corresponding ICEP values were analysed for a 17-year period, from 1990 to 2006. nd Si forms did not show ample transformations in the salinity gradient, revealing that the estuary plays a limited buffer role at the interface between river and coastal zone, however not inconsiderable during dry hydrology when longer water and particle residence times are compatible with the biological growth rates. The simplified LIFT model of the TMZ, already used for analysing organic matter transformation, was implemented here to simulate the nutrients’ behaviour in the salinity gradient and to quantify nutrient fluxes after their transit in the TMZ. Abatements ranging from 15% to 40% of incoming N and P were calculated, without significant changes for silica. Although P-ICEP values did not show a severe risk for coastal eutrophication for the past few years, contrary to the situation from 1970 to 2000, the high N-ICEP, due to a steady use of fertilisers in agriculture, is a real threat for eutrophication and harmful algal blooms.