NITROGEN AND PHOSPHORUS IN THE VISTULA RIVER, POLAND - CHANGES FROM SOURCE TO MOUTH

Nitrogen and phosphorus concentration data representing samples collec ted once a month for nine months at 13 locations along the Vistula Riv er are considered in a preliminary discussion of the sources of the nu trients transported to the Baltic Sea. Concentrations in relation to f low data indicated substantial differences between subbasins. Based on those differences, on the area-specific nutrient loss for a six-month period and on the wastewater discharge in each subbasin, four regions could be recognized in the river basin: i) the southern region with a large impact of point sources, ii) the south central region, where di ffuse sources seemed to be of major importance, iii) the north central region with a combined effect of point and diffuse sources, and reten tion in two reservoirs, iv) the northern region where point sources se emed to be the dominating source, at least for phosphorus. Our results illustrate the importance of differences in phosphorus retention betw een the basins. Long-term retention along the course of the river, par ticularly in the two reservoirs, must be estimated to allow proper sou rce apportionment in the Vistula basin. Concentration decreases in the Wloclawek Reservoir varied between 44 and 68% for P, and 11 to 37% fo r N, in the months with significant retention. In some months, however , concentrations increased, indicating a release of nutrients.