## 3 Results  
### Serbian Water Quality Index (SWQI)  
Water quality assessment for Ribnica Reservoir across 2021 and 2022 indicates notable variations in several key parameters, offering insights into potential ecological changes. pH values remained relatively stable, with surface waters generally exhibiting higher readings. In 2021, the pH fluctuated between 7.5 and 8.3, while in 2022, values ranged from 7.5 to 8.0. This consistency suggests minor seasonal and biological influences but no drastic deviations that might indicate significant acidification or alkalization trends. Electrical conductivity measurements show a slight decrease over time. In 2021, values varied between 158 and 201 \(\upmu\)S/cm, with deeper layers maintaining lower conductivity levels. A mild reduction was observed in 2022, where conductivity ranged from 68 to 188 \(\upmu\)S/cm. These changes could result from variations in dissolved ion concentrations, potentially influenced by hydrological inputs or precipitation patterns. Oxygen saturation levels demonstrated improvements between the two years, reflecting possible enhancements in aeration or reduced organic loading. In 2021, saturation spanned from 62.5% in deeper waters to a maximum of 95% in surface layers. By 2022, values rose to between 78% and 100%, indicating enhanced mixing conditions or a decrease in biochemical oxygen demand. The drop in biological oxygen demand (BOD) further supports this observation--values in 2021 ranged between 4.2 and 5.3 mg/L, whereas in 2022, they declined significantly, with recorded levels as low as 1.23 mg/L. This reduction suggests improved decomposition processes and reduced organic pollution. Suspended solids remained consistently measured at 4 mg/L throughout both years, indicating stable sedimentation rates and particulate matter distribution. However, nitro \begin{table}
\begin{tabular}{c c c} \hline \hline
**Index** & **Category** & **Score Range** \\ \hline SWQI & Excellent & 90–100 \\ \hline SWQI & Good & 84–89 \\ \hline SWQI & Medium & 72–83 \\ \hline SWQI & Bad & 39–71 \\ \hline SWQI & Very Bad & 0–38 \\ \hline CWQI & Excellent & 95–100 \\ \hline CWQI & Good & 80–94 \\ \hline CWQI & Fair & 65–79 \\ \hline CWQI & Marginal & 45–64 \\ \hline CWQI & Poor & 0–44 \\ \hline Source of data: [37; 38]. & & \\ \end{tabular}
\end{table} 
Table 4: Threshold ranges for water quality classification in SWQI and CWQI. gen oxides present a slight increasing trend, particularly in 2022. While 2021 values ranged from 0.185 to 0.715 mg/L, concentrations in 2022 reached up to 0.800 mg/L, with higher readings observed in surface layers. This shift could suggest a rise in nutrient availability, warranting further observation to assess potential eutrophication risks. Orthophosphate concentrations varied slightly across years, with values in 2021 fluctuating between 0.023 and 0.034 mg/L. In 2022, levels increased, reaching up to 0.054 mg/L in some instances. Elevated phosphorus concentrations may point to increased biological activity or external nutrient inputs that could influence productivity within the lake ecosystem [40]. Ammonium concentrations displayed moderate fluctuations. In 2021, values remained within 0.035-0.085 mg/L, while 2022 recorded a slightly wider range, peaking at 0.120 mg/L. This minor increase may be linked to microbial processes, sediment interactions, or shifts in nutrient cycling [41]. One notable limitation in this assessment is the absence of data on total coliforms, an essential component of Serbian Water Quality Index (SWQI) calculations. The lack of coliform measurements prevents a comprehensive evaluation of microbiological contamination, which is critical for assessing overall water quality from a public health perspective [42; 43]. Additionally, missing data for biochemical indicators such as detailed seasonal variations or organic pollutant concentrations could restrict a full interpretation of ecological trends.