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Changes of the nutrient loads of the Odra River during the last century – their causes and consequences
Nutrient emissions by point and diffuse sources and their loads were estimated for the Odra catchment over the time period of the last 40 years by means of the model MONERIS. For nitrogen a change of the total emissions from 31 kt a-1 N at the begin of 1960s to a maximum of 76 kt a-1 N in the mid 1980s and a recent value of about 63 kt a-1 N were estimated for the total Odra Basin. The share of the point source discharges on the total N emissions varied between 23% (1960) and 31% (1990). The emissions from groundwater and tile drained areas represent the dominant pathway (42-58% of total N emissions) during all investigated time periods. Since the mid of 1980s the portion of the emissions from tile drained areas is higher as from groundwater For phosphorus the emissions changed from 6 kt a-1 P in 1960 to 15.5 kt a-1 P in 1990 and a recent level of 8.7 kt a-1 P. Point source discharges caused between 30 to 60 % of total P emissions and represent the dominant pathway for all investigated time periods. Erosion and discharges from paved urban areas and sewer systems was the dominant diffuse pathway of the total P emissions into the river system. The comparison of calculated and observed nutrient loads from the 1960s to 2000 for the main monitoring stations along the Odra River shows that the average deviation is 24 % for phosphorus and 16% for nitrogen. From the analysis it can be concluded that the present load of dissolved inorganic nitrogen (DIN) and total nitrogen (TN) of the Odra into the Baltic Sea is about 2 times higher than in the 1960s. The maximum DIN load (1980) was 2.5 times higher than in the 1960s. The change of the total phosphorus (TP) load is characterized by an increase from the 1960s to 1980 from 2.4 to 6.2 kt a-1 P (factor 2.6). The present value of TP load is 4 kt a-1, which is only 60% above the level of the 1960s.
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