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The results of more than twenty years of own research and state environmental protection authority reports show that from 1986–2009 the quality of water in the lower Vistula, representative of the eastern (right bank) part of Poland, continuously and significantly improved. This improvement was the result of changes in water and sewage management in the Vistula basin. Earlier, in the 1970s, the Vistula was quite heavily polluted, but it has never been the dirtiest river in Europe. It has been shown that Wloclawek Reservoir has improved the water quality in the Vistula, substantially reducing all the indicators examined (Tab. 2). The only exception is concentration of oxygen, lower in the reservoir than in the undammed Vistula, but not to such an extent that it would threaten the development of its biocoenosis, which is richer than in the undammed Vistula. Reduction in the nitrogen (by 12.4%) and phosphorus (by 21.5%) load carried by the Vistula in the reservoir is particularly important for protection of the Baltic Sea against excessive eutrophication. This way, Wloclawek Reservoir contributes to fulfilment of Poland’s obligations under the Helsinki Convention ratified in 1999.

Changes in the quality of water in the lower Vistula River in 1986–2009