Long-term trends in nutrient concentrations in Polish coastal rivers

This paper analyzes the long-term changes in concentrations of nutrients in seven rivers located in the Polish part of the southern Baltic coastal zone in response to economic changes that took place between 1988 and 2013. These changes included: construction or modernization of 390 biological waste water treatment plants, reduction of untreated municipal waste waters discharged into water and soil, a decrease in fertilizer application, and a decline in animal production by about 60% LU. Statistically significant downward trends in nutrients concentrations were observed in six out of seven studied rivers. The mean annual concentration of Ntot changed from about 3.6 to 3.8 mg N dm−3 in the late 1980's and early 1990's to about 2.2 mg N dm−3 in the last 5 years of the study period. The average concentrations of total phosphorus showed the highest values between 1989 and 1991, with a maximum of about 0.5 mg P dm−3 in 1990, and were then gradually decreasing to about 0.12 mg P dm−3 in the late 2000's and early 2010's. Additionally, during the last 25 years the Ntot/Ptot ratio increased significantly, which is indicative of the improvement of ecological status of studied rivers. Correlation analyses indicate that significant reduction in point emission of nutrients and in consumption of organic fertilizer played an important role in the change of nutrient concentration in study rivers.