Ecological functioning of the Marne reservoir (upper Seine basin, France)

The Marne reservoir (Der and Champaubert Lakes, 48 km(2), 8 m mean depth) i s one of the three major reservoirs constructed on the upper reaches of the Seine river to regulate its flow. The major features of the ecological fun ctioning of the reservoir were analysed on the basis of results obtained in a 3-year routine sampling programme where the main variables of water qual ity were determined upstream, downstream and within the reservoir. Plankton ic and benthic processes were also investigated in 1 year. A model of the r eservoir was developed by coupling the RIVE model (which describes the majo r microbiological processes involved in the ecological functioning of aquat ic systems), with a description of the morphology and hydrology of the rese rvoir. The model was used to simulate the seasonal and year-to-year variati ons of nutrients (N, P, Si) and phytoplankton biomass, and to compare them with the observed data. The model was then used to explore the effect of di fferent control factors on denitrification, namely residence times and phos phorus loading. The results showed little influence on the absolute amount of nitrate denitrified within a realistic range of residence times and phos phorus loading. The Marne reservoir appears however to be a significant sin k for nitrate (40% of the influx is eliminated), but this reduction represe nts only 10% of the nitrate flux at the outlet of the Marne sub-basin. Copy right (C) 2000 John Wiley & Sons, Ltd.