## 2 Materials and Methods
### Data Processing and Analysis  
A comprehensive assessment of water quality was achieved by integrating measurements from 27 parameters, with temperature and pH representing the common variables between the Serbian and Canadian indices. Table 1 provides a summary of the methods and standards employed by the Serbian Environmental Protection Agency (SEPA) in the water quality assessment of the Ribnica Reservoir. The Serbian Water Quality Index (SWQI) originates from the internationally recognized Water Quality Index (WQI) methodology but has been adapted by the Serbian Environmental Protection Agency (SEPA) to address regional environmental conditions and regulatory requirements. It aggregates ten key physico-chemical and microbiological parameters--including oxygen saturation, biochemical oxygen demand (BOD), ammonium ion (NH\({}_{4}\)+), pH, total nitrogen (TN), orthophosphates (PO\({}_{4}\)3\({}^{-}\)), suspended solids (SS), temperature (T), electrical conductivity (EC), and total coliforms (TC)--into a composite indicator for surface water quality assessment. Each parameter is weighted according to \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Method/Standard** \\ \hline pH & SRPS H.Z1.111:1987 \\ \hline Electrical Conductivity & US EPA 120.1:1982 \\ \hline Oxygen Saturation & SEV:1977 \\ \hline Biochemical Oxygen Demand & SRPS ISO 5815:1994 \\ \hline Ammonium Ion & HACH Method 8155 \\ \hline Total Nitrogen & UP 1.27/PC 12, Chemiluminescence detector CLD \\ \hline Suspended Solids & SRPS H.Z1.160:1987 \\ \hline Orthophosphate & HACH Method 8048 \\ \hline Total Coliforms & SRPS EN ISO 9308-1:2010 \\ \hline Turbidity & UP 1.88/PC 12 \\ \hline Dissolved Oxygen & SRPS ISO 5813:1994 \\ \hline Calcium & ISO 6058:1984 \\ \hline Sulphate & HACH Method 8051 \\ \hline Chloride, Nitrate, Nitrite & SRPS ISO 9297:1997 \\ \hline Arsenic, Cadmium, Chromium, Copper, Iron, Nickel, Zinc, Manganese & EP A 6020A:2014 \\ \hline Mercury & EPA Method 245.7 \\ \hline Lead & EPA 6020 A: 2007 \\ \hline Source of data: [21]. \\ \end{tabular}
\end{table} 
Table 1: Summary of methods and standards used for water quality analysis in the Ribnica Reservoir. its impact on environmental stability, ensuring a standardized and reliable framework for monitoring water resources in Serbia [37]. Complementing this localized index, the Canadian Water Quality Index (CWQI) incorporates a broader array of 19 parameters, encompassing both physico-chemical properties and heavy metal concentrations. In this study, CWQI assessment included turbidity, dissolved oxygen (DO), pH, calcium (Ca), sulphate (SO42\({}^{-}\)), chloride (Cl\({}^{-}\)), nitrate + nitrite (NO3\({}^{-}\) + NO2\({}^{-}\)), aluminum (Al), arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) [38]. This comprehensive set of parameters allows for a multidimensional evaluation of water quality conditions, addressing both conventional indicators and potential ecological risks. Metal thresholds are defined by experts from the Canadian Council of Ministers of the Environment (Table 2). For the Serbian Water Quality Index (SWQI), individual parameter measurements were converted into sub-indices using pre-defined national thresholds. Each sub-index was then weighted according to its environmental significance. The SWQI was calculated as the weighted sum of the following sub-indices:  
\[SWQI=\sum q_{i}\times w_{i}, \tag{1}\]  
where \(q_{i}\) is water quality of the \(i\)th parameter, and \(w_{i}\) is weight unit of the \(i\)th parameter. Weight unit is defined for each parameter by the Scottish Development Department in 1976, who developed the original version of the WQI (Table 3). This approach enables a clear categorization of water quality in accordance with Serbian national standards [37]. In parallel, the Canadian Water Quality Index (CWQI) was determined by evaluating deviations from set objectives through three components: Scope (\(F_{1}\)), Frequency (\(F_{2}\)), and Amplitude (\(F_{3}\)).