INORGANIC NITROGEN DYNAMICS IN THE RIVER SEINE DOWNSTREAM FROM PARIS (FRANCE)

The River Seine, below Paris, receives the effluents from a large sewage treatment plant, increasing the ammonium concentration up to 6 mgN.1- in late summer. Careful measurement of ammonium, nitrate and organic nitrogen during the downriver travel of the water masses over 100 km below the outfall, along with direct determination of nitrification and benthic fluxes, allowed to establish a budget of nitrogen transport and transformations in this reach of the river. Nitrification is shown to start after a distinct period of several days required for the growth of a significant nitrifying bacterial population. Denitrification is active in the upper layer of bottom sediments but absent from the water column. Comparison of our data with those published for the period 1973-1976 shows that the nitrate load carried by the river has increased not only because of higher runoff of agricultural nitrate in the upstream part of the watershed, but also as a result of the severe reduction in the rate of denitrification processes, owing to the restoration of better oxygen conditions.