How Can Restoration Improve the Environmental Conditions of a Meromictic Urban Lake?

This study was conducted on Klasztorne Małe Lake located in the city of Kartuzy. For about 100 years, the lake served as a receiver of domestic, industrial, and rain sewage. The lake was completely degraded. In addition, excessive sewage inflow caused the accumulation of dense pollution in the area of the lake’s deepest part, which caused the formation of a monimolimnion layer. The bradymictic KML became a meromictic lake. After cutting off the inflow of pollutants, the lake was restored by the P inactivation method using coagulants PIX 111 and PAX 18. Before the restoration process of this lake, concentrations of nutrients in the bottom water layers achieved very high values of TN (total N) 65 mg/L and TP (total P) 30 mg/L. An average concentration of chlorophyll was 65 µg/L and visibility did not exceed 1 m. The method used brought very good results. In the entire volume of water, over 90% reduction in TP and a several-fold decrease in the content of TN were noted. The withdrawal of mineral P from the water limited the production processes in the lake, which was reflected in the reduction in chlorophyll amounts (19 µg/L) and the increase in water visibility (2 m). In addition, oxygen conditions in the lake improved. The application of coagulants caused a decrease in the density of the monimolimnion water, which was indicated by a significant decrease in water conductivity. However, the restoration method did not improve the water dynamics.