## 3 Results and Discussion
### Hydrochemical Conditions of the Studied Rivers
Only the Brzeznicka-Wegorza slightly exceeded this value, at 3.1 mg O\({}_{2}\cdot\)L\({}^{-1}\) (Tables 4 and 5). These results are consistent with Class I and II water quality and confirm the general suitability of these waters for both salmonid and cyprinid species [17; 21]. Long-term mean concentrations of total phosphorus (TP) were below 0.2 mg P\(\cdot\)L\({}^{-1}\) in all rivers, which is within the limit for salmonid waters under Directive 2006/44/EC (Tables 4 and 6) [21]. However, the more stringent threshold for Class I water quality (0.100 mg P\(\cdot\)L\({}^{-1}\)) [17] was exceeded in the Lubieszowa, Loznica, and Sapolna rivers. Particularly high TP concentrations were observed in the Sapolna River, where maximum levels reached 0.676 mg P\(\cdot\)L\({}^{-1}\) (Table 6). Although the long-term mean TP levels remained below 0.2 mg P\(\cdot\)L\({}^{-1}\), the findings point to considerable anthropogenic influence across the studied catchments. [PERSON] [33] suggests that TP concentrations in undisturbed rivers should remain below 0.025 mg P\(\cdot\)L\({}^{-1}\). The elevated values observed here likely stem from the predominantly agricultural land use, especially in meadow areas where mown grasses are often left in place during summer, as required for farmers to receive EU subsidies. Additionally, the growing presence of beaver habitats and their associated modifications to river hydrology--particularly slowed water flow and increased sedimentation--may further contribute to phosphorus accumulation. According to the Central Statistical Office of Poland, the beaver population has increased significantly over the past 28 years--from 5000 to approximately 127,000 individuals [34]. Beaver activity can affect water quality in both negative and positive ways. It may contribute to reduced DO levels and increased TP concentrations [31; 32], while also promoting reductions in N and TSS levels [35; 36]. However, a comprehensive inventory and targeted research are needed to fully understand the ecological impact of beaver activity on nutrient dynamics in these river systems. The long-term mean concentrations of nitrite nitrogen (NO\({}_{2}\)\({}^{-}\)-N) in the studied rivers ranged from 0.012 mg\(\cdot\)L\({}^{-1}\) (Rekowa) to 0.026 mg\(\cdot\)L\({}^{-1}\) (Sapolna), with statistically significant differences between rivers (\(p\leq 0.05\)) (Table 6). According to Directive 2006/44/EC, these values are within the acceptable range for waters supporting cyprinid fish (Table 4). However, the maximum NO\({}_{2}\)\({}^{-}\)-N concentrations observed in the Molstowa, Ukleja, Gardominka, Rekowa, Brzeznicka-Wegorza, and Sapolna rivers exceeded the threshold of 0.03 mg\(\cdot\)L\({}^{-1}\), which is considered the upper limit for cyprinid habitats (Tables 4 and 6). Such elevated levels suggest anthropogenic nutrient loading, consistent with findings by [PERSON] et al. [37]. In the Molstowa River, high NO\({}_{2}\)\({}^{-}\)-N concentrations were particularly evident during low-flow periods. This may be linked to a rainbow trout hatchery located upstream of the sampling point in Molstowo (Figure 1, Site 2-2), as well as the presence of agricultural land further downstream. In the Ukleja River, sampling points 7.2 (Troszczyno) and 7.3 (Midogoszcz) are located downstream of weirs (Figure 1; Tables 2 and 6), where impounded water promotes sediment accumulation and nutrient retention, especially in summer. Although the river is bordered in places by forested buffer zones, agricultural land beyond these buffers likely contributes additional nutrient inputs. In the Gardominka River, beaver dams and lodges located both upstream and downstream of the sampling site have created flooded areas that extend into meadows and crolands, increasing the retention and release of nutrients into the water. The Rekova River, a small watercourse with channel features resembling a drainage ditch, collects runoff from surrounding agricultural fields, explaining its elevated NO\({}_{2}\)\({}^{-}\)-N levels.