Eutrophication as a Priority Issue of Water Quality in Small Retention Reservoirs

The study aimed at comparing changes in the trophic status of waters in two small retention reservoirs "Otapy-Kiersnwek" and "Jasionwka", located in the Podlasie region, which have a similar way of management and the similarity of catchments, using the TSI trophy indicators. The obtained results show that total phosphorus is the limiting factor in both reservoirs. Analyzing the average annual value of TSI in the tested waters, it was found that both reservoirs can be classified as hypertrophic. Trophic state of the tested waters is strictly dependent on the processes taking place in their vicinity. The reservoirs revealed a clear effect of a surface runoff on water quality. Factors that favor the poor quality of water in both reservoirs are unfavorable morphometric conditions of reservoirs due to their small depth, which allows continuous mixing and strong heating of water in spring and summer. Additional causes can include an open space around the reservoirs and the lack of high vegetation that would shield the edges against the wind, which facilitates often mixing of waters to the bottom and resuspension of bottom sediments, which in turn increases the concentration of phosphorus in water.