Study results show continuous summer-autumn dominance of toxic cyanobacteria in plankton not only in the strongly eutrophicated lowland Siemianówka reservoir, but also along 130 km of Narew river below the lake. The negative effects of eutrophication of the reservoir reach far outside its boundaries. One of the symptoms of eutrophication was a mass development of cyanobacteria, including the toxin-producing Planktothrix agardhii. In the reservoir, the biomass of the species strongly correlated with the concentration of microcystins. Redundancy analysis (RDA) identified seven environmental variables as significantly influencing phytoplankton composition in the Narew river: discharge, conductivity, temperature, dissolved oxygen, orthophosphates, silicate ions, and total phosphorous. Higher discharge in the river and higher rates of flushing induced faster dilution of limnoplankton in downstream river and had positive effects on the decrease of cyanobacterial biomass and microcystin concentration.
Keywords: Dammed river; Discharge; Phytoplankton; Toxic cyanobacteria.