Nutrient input and trophic conditions in an artificial flood protection reservoir along the river Danube in Vienna are evaluated from a 10-year data base. Nutrient concentrations were largely governed by concentrations in the River Danube (total phosphorus, P-tot = 236±41 µg/l; total nitrogen, N-tot = 2.53±0.78 mg/l), because the major input was groundwater percolating from the river through the separating island to “Neue Donau”. Concentrations were reduced by about 40% during subsurface transport (time lag approx. one day), because the sediments were mainly composed of gravel (correlation coefficient r = 0.77 for P-tot of the river vs. Neue Donau). The amount of water strongly depended on the complex hydrological regime. The decreasing concentration gradients that developed downstream in the Neue Donau are analysed in detail for the year 1991, for total phosphorus, chlorophyll-a, and phytoplankton biomass. Long-term changes of these parameters are discussed for the years 1981 to 1991. Average phosphorus concentrations for the summer period (June - September) increased from >100 µg/l in 1981 to >400 µg/l in 1984. Average chlorophyll concentrations ranged from 40 to 80 µg/l during this period, peaking in the year 1985. Both variables declined drastically in the years 1986 and 1987 and more slowly decreased thereafter. Lowest values of 40 and 10 µg/l respectively were reached in 1990 which were not significantly different from both 1989 and 1991.