## 3 Results and Discussion
### Hydrochemical Conditions of the Studied Rivers
In the Brzeznicka-Wegorza, the sampling site near the village of Brzezniak (Figure 1, Site 9-1) is influenced by water flowing from the eutrophic Lake Zabice, which experiences seasonal algal blooms that impact water quality further downstream. In the case of the Sapolna River, the maximum NO\({}_{2}\)\({}^{-}\)-N concentration reached 0.101 mg\(\cdot\)L\({}^{-1}\), likely due to the discharge of treated wastewater [15]. These elevated NO\({}_{2}\)\({}^{-}\)-N concentrations are concerning, as they can impair fish health, reducing reproductive success and increasing physiological stress [38; 39]. In contrast, ammonium nitrogen (NH\({}_{4}\)\({}^{+}\)-N) levels remained low across all rivers, ranging from 0.043 mg\(\cdot\)L\({}^{-1}\) (Loznica) to 0.111 mg\(\cdot\)L\({}^{-1}\) (Sapolna) (Tables 4 and 6). These values meet Class I standards [7] and indicate that the waters are suitable for salmonid fish [21]. Nutrient-driven eutrophication remains a critical issue in European waters. More than 60% of water bodies fail to meet the target of \"good ecological status\" (GES) established under the EU Water Framework Directive [22]. Agricultural runoff, along with household and municipal wastewater, represents one of the main pressures on surface waters [40; 41; 42; 43]. Long-term mean alkalinity in the studied rivers ranged from 161.0 to 222.0 mg CaCO\({}_{3}\cdot\)L\({}^{-1}\), while water hardness varied from 225.3 to 260.0 mg CO\({}_{3}\)\({}^{2-}\cdot\)L\({}^{-1}\), with statistically significant differences between rivers (Table 6). These values indicate medium water hardness according to standard classification. Chloride ion (Cl\({}^{-}\)) concentrations ranged from 24.7 to 59.5 mg\(\cdot\)L\({}^{-1}\) and also varied significantly among rivers (Table 6). In natural freshwater systems, typical Cl\({}^{-}\) concentrations range between 15-35 mg\(\cdot\)L\({}^{-1}\)[44]. Increases beyond this range are commonly attributed to anthropogenic sources, such as agricultural runoff, wastewater discharge, and road salt use [45; 46; 47]. The highest long-term mean chloride concentration (59.5 mg\(\cdot\)L\({}^{-1}\)) and maximum value (127.8 mg\(\cdot\)L\({}^{-1}\)) were recorded in the Sapolna River, suggesting contamination likely linked to treated wastewater inputs [15]. Changes in chloride and other ion concentrations in surface waters can significantly alter aquatic habitat quality and pose risks to aquatic plants and animals [48]. Water quality in terms of habitat suitability for aquatic organisms can be assessed using selected fish species as bioindicators. This method also enables evaluation of the effects of river engineering on the functioning of aquatic fauna [49]. Such monitoring was carried out in June 2019 on the tributaries of the Rega River as part of the project _\"Strengthening natural populations of the most valuable ichthyotaxa (including migratory salmonids) through environmentally friendly innovative initiatives such as novel hatchery techniques, creation of spawning grounds, development of fish feed for stocking purposes, and assessment of their impact on fish health and reproductive potential, as well as environmental monitoring for socio-economic infrastructure development in the region\"_ (unpublished data). The assessment recorded the presence of brown trout, with 22 individuals found in the Loznica River, 18 in the Molstowa, and 4-5 individuals in the Ukleja, Piaskowa, and Brzeznicka-Wegorza rivers. In addition, individual bullheads were observed in the Molstowa, Ukleja, and Piaskowa rivers. Minnow (_Phoxinus phxinus_ L.) was recorded only in the Ukleja River (8 individuals), while stone loach was found in the Sapolna River (5 individuals) (unpublished data). The occurrence of these fish species, known for their sensitivity to pollution, is consistent with the high water quality in these rivers. Approximately 78% of the assessed water quality indicators indicated a very good chemical status (Supplementary Table S2).