The reservoir where the studies were carried out was originated by modification of an old oxbow lake of the Vistula River. The most important sources of inflowing waters are industrial waters from ?Pulawy Fertilizer Factory?. The highest variability of concentration, was NH4+-N and also Cl-, Ntot., and PO4-3. Some significant correlations between chemical properties of water and algae (especially green algae and diatoms) were found, which suggested that this phytoplankton was mainly responsible for the biogeochemical cycles in the shallow, strongly polluted reservoir. Another interesting phenomenon was the statistically positive correlation between the density of cyanobacteria and potassium, which suggested that it may play an important role in their abundance.