Abstract
The Koronowo Reservoir (16.6 km2) is located in the lower reaches of the Brda River, in the Kujawy-Pomerania Province, 20 km from Bydgoszcz near the town of Koronowo, hence the name of the reservoir. The reservoir was built in the 1960s and started from damming the waters of the Brda River by a ca. 20 m high earth dam in the village of Pieczyska. This reservoir is used mainly for water retention and production of electric energy in the hydroelectric power station; in addition, it plays a significant recreational function. In terms of the latter function, the spread of cyanobacteria is a serious problem. The aim of the study was to assess longterm changes in the structure of cyanobacteria in waters of the Koronowo Reservoir. The results from 1996, 2008, 2011 and 2013 were compared. As evidenced by the analysis of the phytoplankton composition in the Koronowo Reservoir in terms of functional groups, especially the occurrence of dominant and subdominant species from the groups P, H1, S1, LM, J, G and W2, the waters of the reservoir can be classified as eutrophic, potentially evolving towards the hypertrophic conditions. The HPLC analysis of cyanobacterial toxins revealed the presence of microcystins at two sites of the water body. The species structure, the distribution and the dominance of specific species of cyanobacteria is recurrent over the 17 years; the following species form algal blooms: Aphanizomenon flos-aquae (L.) Ralfs et Bornet et Flahault and Microcystis aeruginosa (Kütz.) Kützing.