Acta Scientiarum Polonorum

Abstract: Investigations covering water in the Warta river between Oborniki and Skwierzyna in the years 1992/1993–2001/2002 revealed an unexpected decrease in the concentration and load of total nitrogen and main nitrogen forms in the lower stretch of the river, i.e. Kłosowice–Skwierzyna. The drop was seen in each year. During the study decade the nitrogen load carried by the river between Oborniki and Kłosowice (upper stretch) increased by 2181.4 Mg ^. year^-1 on average, whereas on the lower stretch it decreased by 4804 Mg ^. year^-1. The largest share of the load was contributed by organic nitrogen. The rate of decrease in the concentration of nitrogen compounds contained in the river water on the Kłosowice–Skwierzyna stretch was found to depend on the mean annual concentration of total nitrogen recorded in Kłosowice.