Primary production and its fate in the storage basin of Méry-sur-Oise

The basin of Mery-sur-Oise (50 km NE of Paris) is an artificial basin, fed by the river Oise and used as a storage reservoir for a drinking water plant managed by the Compagnie Generale des Eaux. Its volume is about 400,000 m3, with an average depth of 9 m. Water from the Oise is in¬jected with high energy, preventing any strarification of the water column. The mean residence time of the water is about 4 days. The river Oise at Mery (15 km from its confluence with the river Seine) is about 75 m wide and 5 m deep. Its mean annual discharge is about 150 m3 • sec-l, but values of 50 m3 • sec-1 are typical for the summer period. A comparison between these two ecosystems offer a unique opporrunity to study the effects of sorne physical and morphological factors on the biological function of aquaric systems. Indeed, the basin of Mery-sur-Oise is somehow intermediate between a river (because of its short residence time and the absence of vertical stratification) and a lake (because of its geometry and the low ve¬locities of the warer masses). With this in mind, measurements of primary production, zooplankton grazing, bacterioplankton activity were performed during the year 1982, both in the basin and in the river Oise. ln addition, the inputs and outputs and the sedimentation in the basin were evaluated, allowing to establish a budget of organic carbon. Sorne of these data are presented in another paper in this vol¬ume (BILLEN er al. 1984). They are discussed here in the perspective of comparing lotic and lentic trends in rhe ecological function of aquatic systems.