Abstract
Large and harmful cyanobacterial blooms appeared in two newly-built artificial reservoirs shortly after being filled with water. Taxonomic composition of cyanobacterial communities was highly variable in both water bodies and fast species replacement was observed. In the first year of the operation of the smaller Konstantynów Reservoir, the mass development of Anabaena flos-aquae and Planktolyngbya limnetica (48.7 and 53.6% of the cyanobacterial abundance) occurred in summer, while in autumn the dominance of Planktothrix agardhii (99.9%, 14.95 × 106 ind. Dm−3) was noted. The surface scum developed in summer consisted of An. flos-aquae that contained high amounts of anatoxin-a (1412.4 μg AN-a dm−3 of scum) and smaller amounts of microcystins (10 μg eq. MC-LR dm−3 of scum). In the larger Kraśnik Reservoir, Aphanizomenon flos-aquae occurred in high abundance in spring and summer, however, it was replaced by different species of Microcystis (1.3 × 107 ind. dm−3) which created thick surface scum. Simultaneously, a hazardous increase in the total concentration of microcystins (from 13.6 to 788.5 μg eq. MC-LR dm−3 of water with scum) and anatoxin-a (from 0.03 to 43.6 μg dm−3) was observed.