Phytoplankton species composition, biomass, diversity, nutrients and chlorophyll a were studied at monthly intervals from December 1991 to December 1992 in a selected area of the river Strymon. SRP ranged from 53 to 182 pg-' I-' and DIN from 265 to 850 pg-' I-'. Nutrient values do not indicate strong anthropogenic effects. Chlorophyll a ranged from 1 .O to 35.3 pg-' I-' and followed the temporal distribution of total phytoplankton biomass. Phytoplankton biomass exhibited maxima in winterspring and summer (6.8 g m-3 in' December 1991, 4.8 g m-3 in April 1992 and 9.3 g m-' in August 1992) composed mainly of diatoms, chlorphytes, cyanophytes and dinophytes. Nanoplankton was the most important component of phytoplankton biomass (69.5%) revealing increased values in winter and early spring. Phytoplankton diversity ranged from 0.8 to 3.2. The hydrological conditions in the river Stryrnon seem to be appropriate for the algae to reproduce themselves in the running water and so, to develop as a true potamoplankton. However, significant populations of phytoplankton must have been carried out from the Kerkini reservoir, situated at the north of the sampling station. The phytoplankton species composition and their periodicity in the river resemble those of typical, large, lowland and nutrientrich rivers of Europe.
S. acuminatus (LAGERH.) CHOD.