Échanges de sels nutritifs entre la zone côtière et un système estuarien intertidal : la baie de Somme (Manche, France)

Abstract

Nutrient exchanges between coastal water and an intertidal estuary: the Bay of Somme (English Channel, France). The Bay of Somme is the second ranked estuarine system, after the Seine estuary, on the French coasts of the Eastern Channel. The nutrient retention and regeneration capacities of this estuary have been calculated from continental and coastal hydrological data and from estimation of the residence time. Measurements at the mouths of the main rivers over a one-year period (from October 1994 to November 1995) were used to construct a budget of continental inputs which showed that the smaller rivers could, at times, be important contributors. The annual budget shows continental inputs of nitrogen, phosphorus, silicon and suspended particulate matter of 9 000 t · y −1, 380 t · y −1, 5 400 t · y −1 and 67 800 t · y −1. Estimation of coastal inputs to the system shows two fluxes of different origins: a flux of nitrogen, mainly nitrate, from the Somme river and a flux of phosphate from the southern coastal area. The interaction of these two inputs may explain the high biological activity of this system. A high degree of retention of nitrogen was calculated and is attributed to uptake by phytoplankton which is itself exported as organic particulate matter. A new estimate of primary production is put forward which distinguishes between new production, regenerated production and microphytobenthic production. The estimated total primary production (600 gC·m −2·y −l) is comparable with that of the Wadden Sea (The Netherlands). The model thus developed for studying exchanges is a useful tool for studying system function and provides a more accurate estimate of the role of continental inputs from the Bay of Somme in the context of coastal water nutrient enrichment in the Eastern Channel and southern Bight of the North Sea. © 2000 Ifremer/CNRS/IRD/Éditions scientifiques et medicales Elsevier SAS English Channel / Bay of Somme / nutrient fluxes / primary production