## 4 Discussion
In the case of Mielenko Lake, which was the lake with the least pollutant loading from the outlets of the sewage collectors, after the modernization of the city's sewage system, a 37% decrease in the external phosphorus load and a 32% decrease in the external nitrogen load were observed. In Karczemne Lake, a 32% decrease in phosphorus load and a 39% decrease in external nitrogen load were noted. In the case of Klasztorne Male Lake, up to a 66% decrease in phosphorus external load and a 40% decrease in nitrogen load were found. In Klasztorne Duze Lake, the external loads of nutrients (P and N) decreased by 54% and 32%, respectively. The multiannual discharge of raw domestic- and storm-derived sewage into lakes resulted in an extremely high concentration of phosphorus compounds, similar to those found in diluted wastewater. The total phosphorus (TP) concentration in the bottom layer of the waters of Klasztorne Male Lake exceeded 20 mg P L\({}^{-1}\). Such high values were not recorded in any other lake degraded by sewage [47; 48; 49; 50]. In a moderately eutrophic lake, the phosphorus cycle is regulated mainly by phytoplankton, and during its maximal developmental stage, the phosphate level may decrease to analytical zero. In the water of the Kartuzy lake complex, despite its extremely high primary production (a strong oxygen oversaturation, a high pH above 9.5, a water transparency of 0.2-0.3 m, and a high organic matter content), the total depletion of phosphorus in the water was not noted. The concentrations here were so high that they were in excess of the amount that phytoplankton could consume. After severing the sewage inflow, these conditions did not change significantly. According to [PERSON] et al. [51] and [PERSON] et al. [22], it is certainly related to phosphorus release from bottom sediments. This process took place even in the shallow lakes of Mielenko and Karczenne because the bottom sediments of these lakes are rich in phosphorus. Although the range of release from bottom sediment under aerobic conditions is generally lower than that in anaerobic conditions, nutrients are introduced directly into the trophogenic zone, where they are used by phytoplankton [52; 53]. The extremely high concentration of phosphorus in the near-bottom layer of Klasztorne Male Lake was related to the permanent anoxic conditions in the over-bottom water and thus restricted P binding by bottom sediment. Under low redox potential conditions, phosphates released by desorption and in the organic matter destruction processes in the bottom sediment diffuse toward the over-bottom water layer [54]. The discharge of raw municipal sewage into the study lakes resulted in extremely high concentrations of nitrogen. The average total nitrogen (TN) concentration reached 43 mg N 1\({}^{-1}\). After the sewage inflow was cut off, these conditions did not change. [PERSON] et al. [55] noted that nitrogen depletion in water can be caused by the increased sedimentation of organic matter and its deposition in sediment or by the ammonification and denitrification of organic compounds to free nitrogen. It should be emphasized that the formation of free nitrogen is only possible after prior nitrification, which was possible in the Mielenko and Karczenne Lakes. Microorganisms play a major role in nitrogen transformation, and their activity is dependent on many environmental factors. Ammonification may occur under both aerobic and anaerobic conditions over a wide range of pH values and temperatures, and the intensity of these processes depends on the number of respective physiological groups of bacteria [56]. According to [PERSON] and [PERSON] [57], increased temperatures favor the release of ammonia nitrogen from sediment to water. In the Klasztorne Male and Klasztorne Duze Lakes, where the bottom water layer was anaerobic, ammonia nitrogen was present in very high concentrations and was the dominant nitrogen compound form.