## 3 Results
### Water Chemistry
In 2017, the mean total nitrogen concentration in the surface water layer of Karczemne Lake was 3.191 mg N 1\({}^{-1}\) (\(\pm\)1.320), and near the bottom, it was 2.975 mg N 1\({}^{-1}\) (\(\pm\)1.008). In 2019, in the surface water layer of the lake, the mean concentration of TN was 3.188 mg N 1\({}^{-1}\) (\(\pm\)1.042), and the near-bottom content was 3.280 mg N 1\({}^{-1}\) (\(\pm\)0.998) (Figure 6, Table 4). In 2017, in Klasztorne Male Lake, the average TN concentration in the surface water layer was 1.740 mg N 1\({}^{-1}\) (\(\pm\)0.182), while at the bottom, it was 19.450 mg N 1\({}^{-1}\) (\(\pm\)2.306) (Table 4). In 2019, the average total nitrogen concentration in the water of the lake was 2.177 mg N 1\({}^{-1}\) (\(\pm\)0.314), while at the bottom, it was 35.973 mg N 1\({}^{-1}\) (\(\pm\)7.047) (Table 4). In the surface water layer of Klasztorne Duze Lake, in 2017, the mean content of TN was 1.740 mg N 1\({}^{-1}\) (\(\pm\)0.273), and that of the near-bottom water layer was 3.664 mg N 1\({}^{-1}\) (\(\pm\)1.565) (Figure 6, Table 4). In 2019, after modernization of the city's sewage system, the mean concentration of TN in the surface water layer of Klasztorne Duze Lake was 2.035 mg N 1\({}^{-1}\) (\(\pm\)0.449), while at the bottom, it was 4.065 mg N 1\({}^{-1}\) (\(\pm\)2.319). The cutting off sewage inflow to lakes did not cause significant variability in the chlorophyll a content and SD visibility values in the lakes (Table 3). In 2017, the average chlorophyll a level in the water of Mielenko Lake was 28.9 ug l\({}^{-1}\) (\(\pm\)13.88), and in 2019, after protective treatments were implemented, it was 35.4 ug l\({}^{-1}\) (\(\pm\)19.38) (Figure 7, Table 4). In the water of Karczemne Lake, the mean content of chlorophyll a in 2017 was 90.4 ug l\({}^{-1}\) (\(\pm\)32.60), while in 2019, it was 77.4 ug l\({}^{-1}\) (\(\pm\)50.70) (Figure 7, Table 4). In 2017, in Klasztorne Male Lake, the average chlorophyll a level was 45.3 ug l\({}^{-1}\) (\(\pm\)12.65), and in 2019, it was 43.8 ug l\({}^{-1}\) (\(\pm\)24.27) (Table 4). In 2017, the average chlorophyll a content in the water of Klasztorne Duze Lake was 42.1 ug l\({}^{-1}\) (\(\pm\) 16.81), while in 2019, it was 25.6 ug l\({}^{-1}\) (\(\pm\) 14.53) (Figure 7, Table 4). Figure 6: Mean annual values of total nitrogen (TN) in the water of Kartuzy lake complex in selected research years (box \(\pm\) SEM, whisker plots \(\pm\) Standard Deviation). The mean value of Secchi disc visibility (SD) in 2017 in Mielenko Lake was 0.525 m (\(\pm\)0.138), and in 2019, it was also 0.525 m, but with standard deviations of \(\pm\)0.282 (Figure 8, Table 3). In 2017, the average Secchi disc visibility in Karczemne Lake was 0.312 m (\(\pm\)0.112), and in 2019, it was 0.293 (\(\pm\)0.067) (Figure 8, Table 4). In Klasztorne Male Lake, in 2017, the mean Secchi disc visibility was 0.575 m (\(\pm\)0.267), while in 2019, after modernization of the city's sewage system, it was 0.531 m (\(\pm\)0.185) (Figure 8, Table 4). In 2017, the average Secchi disc visibility in Klasztorne Duze Lake was 0.812 m (\(\pm\)0.467), and in 2019, it was 0.881 m (\(\pm\)0.494) (Figure 8, Table 4). Figure 8: Mean annual values of transparency of Kartuzy lake complex in selected research years. (box \(\pm\) SEM, whisker plots \(\pm\) Standard Deviation)  
Figure 7: Mean annual values of chlorophyll a in the water of Kartuzy lake complex in selected research years (box \(\pm\) SEM, whisker plots \(\pm\) Standard Deviation).