The Mirgenbach reservoir (vol. : 7.3 Mm3, area : 90 ha, mean depth : 7.7 m) is a safety and thermal water buffer for the nuclear power station in Cattenom (Eastern France). Water used for cooling is pumped out of river Moselle, filtered, overheated, concentrated by evaporation in the atmospheric cooling towers that discharge the cooled water into this artificial lake where it continues to cool. The levels of temperature, conductivity, hardness and eutrophising capacities are higher than those of river Moselle waters. The hydrobiologjcal study was conducted from April 1986 to December 1988. Before August 1986, the lake had developed to a dimictic stratified type. Immediately after the first input in August 1986, it developed towards a polymictic unstratified type. The oxygen levels, the conductivity, the pH and the temperature were homogeneous from surface to bottom everywhere in the reservoir at any time.The trophic level study has enabled us to investigate the interrelationships between the physical and chemical factors, and the biocenoses in which the relationships between the various trophic levels could be observed. Considering the level of eutrophication and the amoun of biomass produced, exploitation of the algal and fish productions was proposed. Applied studies are planned : the aquaculture of the green alga Hydrodictyon and intensive fish farming in floating nets with an addition of dry Hydrodictyon in artificial feeding.