Point and diffuse nutrient emissions and loads in the transboundary Danube River Basin. - II. Long-term changes

Nutrient emissions by point and diffuse sources and their loads were estimated for the Danube catchment over the time period of the last 50 years by means of the model MONERIS. For nitrogen a change of the total emissions from 393 kt a -1 N in the 1950s to a maximum of 923 kt a -1 N in the mid 1980s and a recent value of about 756 kt a -1 N were estimated for the total Danube Basin. The share of the point source discharges on the total N emissions varied between 15% (1950s) and 30% (1990). The emissions from groundwater and natural interflow represent the dominant pathway (35-45 % of total N emissions) during all investigated time periods. For phosphorus the emissions changed from 41 kt a -1 P in 1955 to 110 kt a -1 P in 1990 and a recent level of 68 kt a -1 P. Point source discharges caused between 25 to 69% of total P emissions and represent the dominant pathway since the 1960s. During the 1950s erosion was the dominant pathway of the total P emissions into the river system. This pathway was the major single source of diffuse P emissions over all study periods. The change of retention due to the establishment of the Iron Gate dam was taken into account, when calculating nutrient loads. The comparison of calculated and observed nutrient loads from the 1950s to 2000 for the main monitoring stations along the Danube River shows that the average deviation is 22 % for phosphorus and 12% for nitrogen. From the analysis it can be concluded that the present load of dissolved inorganic nitrogen (DIN) and total nitrogen (TN) of the Danube into the Black Sea is about 1.9 times higher than in the 1950s. The maximum DIN load (1980) was 2.2 times higher than in the 1950s. The change of the total phosphorus (TP) load is characterized by an increase from the 1950s to 1970 from 18 to 29 kt a -1 P (factor 1.6). After the establishment of the Iron Gate dam the TP load decreased until the mid 1970s but increased again to the level of about 32 kt a -1 in 1980. Since this time the TP load was decreasing to the present value of 22.5 kt a -1 , which is only 27% above the level of the 1950s. While the variation of the nitrogen load can be explained mainly by changes in the quantity of emissions, additional effects such as the construction of the Iron Gate dam have to be taken into account for explaining the changes of the phosphorus load.