Resumen
The research was conducted in Karczemne Lake (area, 40.4 ha; maximum (max.) depth, 3.2 m) and Klasztorne Male Lake (area, 13.7 ha; max. depth, 20.0 m) located in the Kashubian Lake District (Northern Poland). From the beginning of the 1950s, these reservoirs have received municipal and storm wastewater. The long-term process of lake contamination has shaped the specific buffer capacity conditions and influenced the circulation of carbonate and bicarbonate in the water of these ecosystems. Extremely high concentrations of nutrients (Karczemne Lake: max. total phosphorous (TP) level, 7.5 mg P L-1; max. total nitrogen (TN) level, 5.6 mg N L-1; Klasztorne Male Lake: max. TP level, 20.6 mg P L-1; max. TN level, 43.3 mg N L-1) have caused very intensive primary production processes (Karczemne Lake: max. chlorophyll-a level, 193.40 µg m-3; max. Secchi disc visibility, 0.85 m; Klasztorne Male Lake: max. chlorophyll-a level, 160.01 µg m-3; max. Secchi disc visibility, 1.15 m). In the polymictic Karczemne Lake, the pH value of all water columns exceeded 10.0 (max. pH, 10.41), and in the meromictic Klasztorne Male Lake, the pH of the surface water layers oscillated around 9.5. In the polymictic Karczemne Lake, despite intensive photosynthesis, the calcium content and alkalinity were similar throughout the whole water column due to constant circulation. In the meromictic Klasztorne Male Lake, during the growing season, a decrease in calcium concentration and alkalinity of the surface water layers and an increase in calcium concentration at the bottom were noted.