## 2 Materials and Methods  
### Study Area  
This study covered nine rivers within the Rega River catchment (Figure 1), with brief descriptions provided below. Figure 1: Location of water sampling sites in the studied rivers of the Rega catchment. **Molstowa** is the longest first-order tributary of the Rega River (Table 1). Its upper reaches have a mountainous character and flow through forested areas, while its lower course runs through agricultural land and meadows. A water quality assessment conducted by the Regional Environmental Protection Inspectorate [16] in 2008 at the confluence with the Rega indicated that physicochemical parameters fell below the threshold for good ecological status. Biological elements were classified as class I, and the overall ecological condition was assessed as moderate [16]. Three sampling points were established along the Molstowa in the villages of Rzesznikowo, Molstow, and Bielikowo (Figure 1, Table 2). **Ukleja** is a left-bank tributary of the Rega with the largest catchment area among the studied rivers (Table 1). It originates from Lake Dlusko and flows through Lakes Woswin, Mielno, and Okrzeja, acting as a connecting river between them. It then transitions into a gravel-bed lowland stream and finally becomes a sandy-clay lowland river before joining  
\begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**River** & **GPS Coordinates** & \multicolumn{2}{l}{**Mean Width**} & \multicolumn{2}{l}{**Mean Depth**} & \multicolumn{1}{l}{**Guidelines**} \\  & & & **(m)** & **(m)** & **(RMI 2021)** \\ \hline \multicolumn{6}{c}{River’s first-order tributaries} \\ \hline Molstowa & 53°44′43′′ N 15°36′03′′ E & 54.4 & 377.0 & R1\_poj \\ Ulleja & 53°29′24′′ N 15°29′15′′ E & 45.4 & 457.0 & R1\_poj \\ Gardominka & 53°51′32′′ N 15°13′47′′ E & 26.6 & 112.6 & R1\_poj \\ Rekowa & 53°41′39′′ N 15°27′′11′′ E & 22.0 & 110.4 & P1\_poj \\ Piaskowa & 53°41′39′′ N 15°27′′11′′ E & 18.0 & 86.70 & P1\_poj \\ Lubieszowa & 53°56′12′′ N 15°14′22′′ E & 14.3 & 65.94 & P1\_poj \\ Loznica & 53°38′23′′ N 15°37′32′′ E & 13.4 & 88.14 & PN \\ \hline \multicolumn{6}{c}{River’s second-order tributaries} \\ \hline Brzeznicka- & 53°29′25′′ N 15°43′30′′ E & 37.8 & 276.0 & R1\_poj \\ S  
the Rega. Just before its mouth, it receives its largest tributary, the Sapolna River, from the left. Various fish species are found along different sections of the river, with the most abundant being gudgeon (_Gobio gobio_ L.), brown trout (_Salmo trutta fario_ L.), bullhead (_Cottus gobio_ L.), and spined loach (_Cobitis [PERSON]_ L.) [14]. A 2008 assessment by WIOS at the mouth of the Ukleja rated its overall water quality as good [16]. Sampling sites were located near the villages of Mieszewo, Troszczyno, and Milogoszcz (Figure 1, Table 2). **Gardominka** is a left-bank tributary of the Rega, with a length of 26.6 km (Table 1). It receives effluents from a mechanical wastewater treatment plant in Grebocin and a mechanical-biological facility in Trzygflow. In the upper section, the dominant fish species is the three-spined stickleback (_Gasterosteus aculeatus_); in the middle section, roach (_Rutilus rutilus_ L.), pike (_Esox lucius_ L.), and tench (_Tinca tinca_ L.); and in the lower section, stone loach (_Barbutula barbutula_ L.) and gudgeon [14]. A single sampling site was located near the village of Truskolas (Figure 1, Table 2). **Rekowa** is a right-bank tributary of the Rega, also 26.6 km long (Table 1). It originates between the village of Gardzin and the hamlet of Gozdno. Since 2010, its lower section has been protected within the Rekowa River Nature Reserve, which lies within the Natura 2000 site \"Dorzecze Regi\" (PLH320049). In 2008, a water quality assessment at the river's mouth showed physicochemical parameters below the good status threshold, while biological elements were classified as class I. The overall ecological status was moderate, and the general two-tier evaluation rated the river in poor condition [16].