During all the years, also high seasonal variability of the studied parameters was observed in the River upstream from WR, in the reservoir and at the sites downstream from WR. For instance, during the season of 2000 and 2001, the concentration of seston at the site of “Płock” ranged from 17.0 to 51.8mgdm−3 (max/min = 3), and the concentration of Ntot ranged from 1.5 to 3.6mgdm−3 (max/min = 2.4) (unpublished data).