Long-term trends of major plant nutrients in the River Danube at Vienna (Austria), the nutrient source for the New Danube
Weilguni, H.; Humpesch, U.H.; Kavka, G.G.
The nutrient content of the New Danube flood-relief channel at Vienna is primarily influenced by that of the River Danube, through two pathways: continuously, by groundwater percolating through the aquifer of the narrow interface - Danube Island - between the main Danube and the New Danube, and occasionally when river-water enters the channel when flood-gates are opened. The impact of the main river on the nutrient content of the New Danube was examined by statistical analysis of a 39-year series of data (1957- 1995) for seven variables measured in the River Danube at the northern end of Vienna. Long-term trends of nutrient concentrations were analysed in relation to human activities (represented by time as an extra variable), water discharge and temperature. Trends in time were most pronounced for soluble reactive phosphorus and total phosphorus, but also occurred in ammonium and nitrate; nitrite showed no trend. Seasonal patterns were marked for nitrate, slightly less for total phosphorus, soluble reactive phosphorus and ammonium, and least for nitrite. Concentrations were minimal in summer and maximal in winter, inversely related to discharge (maximal in summer). Following reductions in nutrient inputs to the Danube, in both Germany and Austria, the mean concentrations in the river have fallen by at least half since the 1980s. Nitrate also shows a trend towards lower concentrations. From the results presented in this study, it is clear that management measures taken over a 15-year period have had a significantly positive effect on the nutrient content of the Danube in the last ten years, thereby reducing nutrient inputs that directly influence the New Danube when the flood-gates are opened. In contrast, the (major) indirect influence on nutrient concentrations in the New Danube, due to infiltration of groundwater through the gravel aquifer of Danube Island, is dependent on the biological processes occurring therein.