LONG-TERM VARIABILITY OF TOTAL NITROGEN AND TOTAL PHOSPHORUS CONCENTRATION AND LOAD IN THE SOUTH PART OF THE BALTIC SEA BASIN

A high decrease was observed in concentrations and loads of nitrogen and phosphorus discharged from the Warta River (catchment area 54 519 km(2)) to the Oder (catchment area 119 074 km(2)). One half of the monitoring stations are strongly influenced by point sources, which allows for verifying the functioning of sewage treatment plants. Nitrogen compound concentrations are positively correlated with the river runoff, requiring the application of water years when studying river eutrophication. A high retention of nitrogen in the Warta River basin is documented. Calculated area-specific loads show that the Warta River introduced considerably less N and P than rivers discharging into the North Sea. Monthly loads of total nitrogen show a significant positive correlation with the river runoff. In 1992-2011, nitrogen and phosphorus load time trends were negative and statistically significant. The participation of non-point sources in the nutrient emission, used in the MONERIS model, is too high and of that of point sources is too low.