The Sulejów Reservoir is a lowland dam reservoir that was constructed in 1973 on the Pilica River at the village of Smardzewice. It is 15.5 km long and has an elongated, trough-like shape. It is characterized by a low depth (mean - 4.5 m), and its mean annual retention time ranges from a dozen to four dozen days. The reservoir is strongly eutrophic due to large loads of phosphorus and nitrogen entering its catchment area. Since the creation of the reservoir, strong water blooms have been caused by Aphanizomenon flos-aquae, Anabaena flos-aquae, Microcystis aeruginosa, and Microcystis wessenbergii. These blooms were associated with the intensified development of green algae, mainly of the genera Coelastrum, Dictyosphaerium, Pandorina, Pediatrum, and Scenedesmus. Beside the blue-green and green algae, the diatoms were the richest in species, of which Asterionella formosa, Aulacoseira granulata, Cyclotella meneghiniana, Diatoma tenuis, Fragilaria capucina, F. Pinnata, F. Ulna, Melosira varians, and Stephanodiscus hantzschii constantly dominated. These three groups were accompanied by Dinobryon sertularia, Ceratium hirundinella, and Peridinium inconspicuum.