## 5 Conclusions  
The conducted analysis clearly emphasizes the critical importance of continuous and comprehensive water quality monitoring in the Ribnica Reservoir, given its essential role in supplying drinking water and its position within the protected Zlatibor Nature Park. The comparative evaluation using the Serbian Water Quality Index (SWQI) and the Canadian Water Quality Index (CWQI) revealed overall improvements in water quality in 2022. However, it also uncovered specific risks associated with heavy metals, reduced dissolved oxygen, and localized ecological threats, highlighting the necessity for sustained vigilance. Heavy metals thresholds were defined by Canadian Councils of Ministers of Environment. Aluminum concentrations exceeded the threshold (0.005 mg/L) for overall and water quality for aquatic life in all samples and ranged from 0.083 mg/L to 0.438 mg/L. Taking into account that these values were many times higher than the threshold, it can be concluded that it had a detrimental effect on aquatic organisms. Iron concentrations were above the thresholds (0.3 mg/L) for overall, drinking, and water quality for aquatic life in most samples and ranged from 0.322 mg/L to 0.634 mg/L. Although these concentrations were not drastically increased, a certain level of concern is necessary, because the Ribnica Reservoir is used for drinking water supply. Chromium concentrations exceeded the threshold (0.001 mg/L) for overall and water quality for aquatic life in most samples and for irrigation water quality (0.0049 mg/L) in s few samples, and ranged from 0.0017 mg/L to 0.0196 mg/L. Increased mercury concentrations (0.007 mg/L) in all samples for overall and water quality for livestock comparing with the threshold (0.003 mg/L) were recorded in all samples. Increased chromium and mercury concentrations is also concerning, because these metals are considered as the most toxic in an aquatic environment. Their joint impact led to an increase in reactive oxygen species production and a reduction in antioxidant defenses in fish. Further, cumulative effects of aluminum, copper, and mercury cause enhanced DNA damage and bioaccumulation potential in aquatic biota. Given these findings, several targeted measures are recommended to enhance future water-resource management efforts. Firstly, establishing a continuous, multi-depth monitoring regime is vital to capture seasonal fluctuations, anthropogenic impacts, and long-term ecological changes. Secondly, expanding the current monitoring framework to incorporate comprehensive microbiological assessments and systematic measurements of heavy metals at various depths is critical to safeguarding public health and ecological integrity. Furthermore, it is highly recommended to initiate and maintain regular water quality monitoring programs for the C rni Rzav River and other tributaries within the reservoir's catchment area, as such data is currently lacking and would greatly enhance the understanding of pollutant sources and ecological interactions. Additionally, proactive management strategies such as regular sediment assessments, strict control of tourist and urban development activities, and mitigation of pollution sources through enhanced wastewater treatment practices are essential. Strengthening these approaches will contribute significantly to preserving the reservoir's ecological balance, maintaining high water quality standards, and ensuring its sustainable use. Overall, this study underscores the value of integrating local and international indices, providing a nuanced understanding of water quality dynamics. Continued application and refinement of these methodologies, supported by rigorous data collection protocols, are fundamental to maintaining the Ribnica Reservoir as a reliable source of drinking water and a significant ecological asset within a protected landscape. Furthermore, a more detailed analysis of heavy metal sources and their ecological interactions is warranted.