The phytoseston in the Plociczna River originated mainly from lakes situated in its water course. It was more abundant and diverse in spring than in summer, which was related to differences in water discharge. The phytoseston abundance between the stations at the inlets and outlets of the successive lakes increased 9.7, 1.9 and decreased 1.4 times, respectively. A gradual reduction in the abundance of organisms was also noted at the outlets of successive lakes, which indicated a decrease in the trophic status of the water. The abundance of phytoseston between the outlets and inlets to the subsequent lakes (river stretch) decreased considerably. Disturbances in the river continuum system caused by the lakes situated in the river course resulted in the acceleration of potamoplankton succession.