LONG-TERM NUTRIENT LOADING AND BIOLOGICAL RESPONSE IN A FLOOD-WATER RESERVOIR (NEUE DONAU) IN VIENNA, AUSTRIA

Nutrient input and trophic conditions in an artificial flood protectio n reservoir along the river Danube in Vienna are evaluated from a 10-y ear data base. Nutrient concentrations were largely governed by concen trations in the River Danube (total phosphorus, P-tot = 236+/-41 mug/l ; total nitrogen, N-tot = 2.53+/-0.78 mg/l), because the major input was groundwater percolating from the river through the separating isla nd 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 depen ded on the complex hydrological regime. The decreasing concentration g radients that developed downstream in the Neue Donau are analysed in d etail for the year 1991, for total phosphorus, chlorophyll-a, and phyt oplankton 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 mug/l in 1981 t o > 400 mug/l in 1984. Average chlorophyll concentrations ranged from 40 to 80 mug/l during this period, peaking in the year 1985. Both vari ables declined drastically in the years 1986 and 1987 and more slowly decreased thereafter. Lowest values of 40 and 10 mug/l respectively we re reached in 1990 which were not significantly different from both 19 89 and 1991.