1. Helcom recommendation 13/9. Reduction of nitrogen, mainly nitrate, leaching from agricultural land: http://www.helcom.fi/ fullrecs/rec13 9.html
2. Behrendt, H, Huber P, Kornmilch M, Opitz D, Schmoll O, Scholz G, Uebe R (2000): Nutrient emissions into river basins of Germany. UBA-Texte 23/00
3. Buszewski B, Fudala J, Kosobucki P, Kowalkowski T, Kowalska J, Zbytniewski R (2002): Stan zlewni rzeki Wisly i jej wplyw na strefe przybrzeżna Baltyku. Wisla czysta, ale. EkoProfit 3 (63) and 4 (64) 166–174 (in Polish)
4. The state of the environment in Poland. (1998): Library of Environmental Monitoring, Warsaw
5. Environmental Protection (1996): Central Statistical Office, Warsaw