## 3 Results and Discussion
### Hydrochemical Conditions of the Studied Rivers
Of particular concern were the exceptionally high maximum TSS concentrations observed in some rivers: Molstowa (318 mg\(\cdot\)L\({}^{-1}\)), Lubieszowa (230 mg\(\cdot\)L\({}^{-1}\)), Piaskowa (224 mg\(\cdot\)L\({}^{-1}\)), and Sapolna (208 mg\(\cdot\)L\({}^{-1}\)) (Table 4). The elevated levels of TSSs can be largely attributed to the high proportion of agricultural land within the catchments of the studied rivers. According to the Regional Inspectorate for Environmental Protection in Szczeccin, agricultural land in the West Pomeranian Voivodeship accounts for 48.7% of the total area, including 38.5% arable land, 6.6% meadows, 3.4% pastures, and 0.2% orchards [24]. Within the municipalities through which the tributaries of the Rega River flow, the share of agricultural land ranges from 38% to 72%. The highest percentages are found in the municipalities of Brojce (72%; Molstowa, Lubieszowa), Gryfice (67%; Rega), Radowo Male (66%; Ukleja, Piaskowa), Nowogard (65%; Sapolna, Wolczenica, Gardominka), Ploty (61%; Ukleja, Rekowa, Lubieszowa), Loce (56%; Brzeznica-Wegorza), Resko (50%; Ukleja, Rega), and Przybiernow (38%; Loznica) [25]. Numerous studies have shown that soil erosion from agricultural areas is a major source of nutrient salts and mineral matter, primarily in the form of fine soil particles, entering surface waters [26; 27]. In the case of the Sapolna River, the exceptionally high TSS concentrations are further exacerbated by wastewater discharges from a treatment plant [15]. These elevated values pose a serious threat to in-stream spawning habitats, especially artificial salmonid reds. High TSS levels can lead to oxygen depletion within spawning nests, hindering embryo development and significantly reducing hatching success [28; 29; 30]. Moreover, sediment-laden flows may accelerate the degradation of artificial spawning structures [18]. For most of the studied rivers, dissolved oxygen (DO) concentrations ranged between 7.6 and 9.6 mg O\({}_{2}\cdot\)L\({}^{-1}\), corresponding to Class I and II water quality [17] and indicating conditions suitable for both salmonid and cyprinid fish (Directive 2006/44/EC of the European Parliament and of the Council of 6 September 2006). However, in the Gardominka and Rekowa rivers, DO levels fell below the Class II threshold of 7.5 mg O\({}_{2}\cdot\)L\({}^{-1}\), making these waters unsuitable for sustaining fish populations according to EU standards (Tables 3 and 4). Critically low DO values were recorded in the Gardominka (1.9 mg O\({}_{2}\cdot\)L\({}^{-1}\)), Lubieszowa (2.9 mg O\({}_{2}\cdot\)L\({}^{-1}\)), and Rekowa (3.1 mg O\({}_{2}\cdot\)L\({}^{-1}\)) rivers (Table 5). These oxygen deficits occurred primarily during spring and summer. Low DO levels are indicative of various forms of pollution, primarily of anthropogenic origin. Historical monitoring by WIOS in 2008 already identified poor water quality in the Gardominka, Rekowa, and Lubieszowa rivers [16]. One contributing factor is likely the reduced flow velocity due to siltation, especially in straightened river sections, such as those in the Lubieszowa catchment, dominated by meadows and cultivated fields. In the Gardominka River, low DO levels may be linked to beaver activity. The construction of dams by beavers creates impoundments, resulting in areas of reduced flow or stagnant water. These conditions promote the accelerated decomposition of organic matter and the oxidation of minerals--processes that significantly deplete dissolved oxygen in the water [31; 32]. When DO concentrations drop below 3-4 mg O\({}_{2}\cdot\)L\({}^{-1}\), the functioning of aquatic ecosystems may be disrupted, potentially leading to fish mortality. Maintaining DO above this threshold is essential to support healthy aquatic communities and preserve the self-purifying capacity of rivers. The intensity of organic matter decomposition is also reflected in the biochemical oxygen demand (BOD\({}_{5}\)). In eight of the nine rivers, BOD\({}_{5}\) levels remained below 3.0 mg O\({}_{2}\cdot\)L\({}^{-1}\), indicating good water quality.