###### Abstract
Several studies have investigated water quality in Serbian artificial lakes and reservoirs: Garasi Reservoir [10; 11; 12; 13; 14; 15]; Bukulja Reservoir [10; 13]; Radoinjsko Lake [11; 13; 16]; Gruza Lake, Bovan Lake, Prvonek Lake, Celije Lake, Vrutci Lake [11; 13; 14]; Sava Lake, Srebroo Lake [14]; Grlste Lake [11; 14]; Vlasina Lake [11; 14; 17]; Zlatar Reservoir [11; 12]; Pridvorica Reservoir, Bojnik Reservoir, Bresnica Reservoir, Nova Grosnica Reservoir; Barje Reservoir, Uvac Reservoir [11; 13]; Potpec Rezervoir, Over Sanja Reservoir, Medurrsej Reservoir, Bajina Basta Reservoir, Zvornik Reservoir, Bor Reservoir, Krajkovac Reservoir, Divcibare Reservoir, Zavoj Reservoir, Backa Topola Reservoir, Bela Crkva Reservoir [11]. However, Ribnica Reservoir has received little scientific attention. The chemical properties of Ribnica Reservoir were studied by [PERSON] et al. [16]. [PERSON] et al. [14] studied 10 reservoirs and found elevated levels of iron (-84 ug/L) and manganese (-80 ug/L), as well as critical values of calcium (50.7 mg/L in Lake Grlste) and residual sodium carbonate (3.08 mg/L in Srebroo Lake), indicating potential risks for irrigation and ecosystem health. [PERSON] et al. [11] assessed 28 lakes and reported ammonium levels up to 0.28 mg/L, total phosphorus at 0.15 mg/L, and total nitrogen exceeding 1.2 mg/L in some reservoirs, surpassing drinking water guidelines and raising concerns over eutrophication. [PERSON] et al. [15] examined the Garasi Reservoir using SWQI and CWQI. SWQI values ranged from good (85) to excellent (97) in surface layers and declined from bad (59) to medium (67) at greater depths, indicating a strong vertical stratification in water quality. CWQI values varied more widely, ranging from marginal (50) to excellent (100), with the lowest scores observed at depths exceeding 1000 cm, highlighting the need for depth-specific monitoring when assessing overall reservoir condition. [PERSON] et al. [17] analyzed long-term trends in Vlasina Lake, where SWQI ranged from 78 to 100, but CWQI dropped to 46 in 2022, pointing to tourism-related seasonal degradation in this protected area. These findings underscore the variability and complexity of water quality across Serbian reservoirs and highlight the need for site-specific, long-term monitoring frameworks. Despite Ribnica Reservoir's ecological importance and protected status, it remains largely unexamined in this regard, particularly in terms of depth-related dynamics and cumulative anthropogenic impacts. This study addresses the identified gap by providing a comprehensive evaluation of spatial and temporal variations in Ribnica Reservoir, with the aim of supporting sustainable management and conservation in protected areas. Specifically, this research applies two standardized water quality indices--the Serbian Water Quality Index (SWQI) and the Canadian Water Quality Index (CWQI)--to evaluate the lake's condition. The combined use of these indices provides a comprehensive framework by incorporating a broad range of physical and chemical parameters. This approach enables an in-depth analysis of water quality dynamics across different depths and timeframes. Considering the dual pressures of increased tourism and conservation responsibilities, the use of standardized water quality indices is significant for effective long-term monitoring. This study contributes to the sustainable management of Ribnica Reservoir and supports wider initiatives aimed at protecting water resources and maintaining ecological integrity in protected environments. The findings are intended to guide adaptive strategies that mitigate environmental risks and ensure the sustainable use of resources.