Qualitative and quantitative aspects of the phytoplankton of the river Šešupė were studied in 1991, at a point 272 km from the source. Totally 69 taxa of phytoplankton were identified. The members of Bacillariophyceae and Chlorophyceae were dominating, representing respectively 69 % and 18 % of total species number. Cyanophyceae were represented only by a few species (7 % of total species number), but were abundant in cell number. Total cell abundance varied from less than 90 000 cells l-1 to more than 1 200 000 cells l-1. Phytoplankton development dynamics could be described as seasonal changes of three dominating groups of algae – diatoms, blue-green and green algae. Water discharge, temperature and light conditions seem to be the main controlling factors of phytoplankton dynamics.
Keywords: phytoplankton, species composition, abundance, seasonal dynamics, the Sesupe river, Lithuania