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Discharges of water and nutrients by the Vistula... – M. Pastuszak, Z. Witek
The source apportionment of nitrogen and phosphorus losses into inland surface waters, made for all the Baltic countries (HELCOM, 2004), shows that the respective contribution of nitrogen from diffuse sources, point sources, and natural background constituted 62%, 18%, and 20% in Poland in 2000; the respective contribution of phosphorus from diffuse sources, point sources, and natural background constituted 54%, 29%, and 17%. Source apportionment of nitrogen and phosphorus as well as N and P pathways in the Vistula and Oder basin are extensively discussed in Chapter 8. It is evident from that Chapter that diffuse sources (mainly N) and point sources (mainly P) play a very important role in nutrient emission into the Vistula and Oder basins. For the past decades, high contribution of Poland to overall loads of nitrogen and phosphorus reaching the Baltic Sea, has been a subject to severe, not fully justified, criticism on an international level. The high annual loads of N and P have never been denied by Poles, but that truth requires explanation which should be based on scientific findings on complexity of factors responsible for N and P emission from man utilized land to riverine systems and then to the sea (Haycock et al., 1993; Oenema and Roest, 1998; Oenema, 1999; Nixon, 1999; Hooda et al., 2000, 2001; Honisch et al., 2002; Oenema et al., 2003; Salo and Turtola, 2006). Among the main factors one must take into consideration: - (i) the area of the catchment, - (ii) the land use/cover, and within this, the area of agricultural land, - (iii) the geomorphologic features, - (iv) the geographical setting which is connected with climate, thus type and intensity of precipitation and consequently the volume of water drained from the catchment, - (v) the population in a particular catchment because that is connected with point sources discharges; besides, higher human population density is connected with accelerated nitrogen cycling through fertilizer use, food movement, atmospheric pollution, and land disturbance, and therefore is responsible for higher diffuse N outflow, as stated by Howarth et al. (1996). If 62% TN and 54% TP, discharged to surface waters of the Baltic catchment, originates from diffuse sources, it is advisable to show the agricultural land area in all the Baltic countries (Fig. 9.14).