## 2 Materials and Methods  
### Study Area and Sampling  
The study area is located in western Serbia, encompassing Zlaitbor--a gently undulating plateau with an average altitude of approximately 1000 m. Hydrography is dominated by the Crni Rzav river, originating at an elevation of 1440 m, flowing through Zlaitbor, and merging with Beli Rzav near Vardiste to form the Rzav River, eventually draining into the Drina River (Black Sea basin). The Crni Rzav basin covers an area of 288 km\({}^{2}\), including significant tributaries such as Obudojevica, Skakavac, and Ribnica. Downstream from Ribnica, the river's average flow rate is 1.85 m\({}^{3}\)/s, with pronounced seasonal variations [18]. Zlaitbor's hydrography is influenced by lithology, tectonics, geomorphology, and climatic conditions [19]. Ribnica Reservoir, created in 1971 by damming the Crni Rzav, supplies drinking water to Cajetina and Zlaitbor and is recognized for its clean water and biodiversity. The reservoir covers approximately 0.458 km\({}^{2}\), contributing significantly to the local hydrological system.  
As an integral part of Zlaitbor Nature Park, Ribnica Reservoir is situated within this protected landscape, contributing to both its hydrological significance and ecological diversity.  
Zlaitbor Nature Park spans 41,923.26 ha, characterized by plateaus, tectonic formations, deep canyons, and river valleys. Within this diverse terrain, 32 geosites of significance are documented, including geomorphological, geological, speleological, and architectonic-petrological features. The biodiversity of Zlaitbor is shaped by serpentine geology, supporting rare and endangered flora. Among 1044 plant taxa, 34 are strictly protected, 112 are protected, and 76 are endemic. Native forests, predominantly black pine and mixed black-white pine, constitute essential habitats. Fauna include freshwater fish such as huchen and brown trout, 18 amphibian and reptile species, 154 bird species (127 strictly protected), and 38 mammal species. Zlaitbor is also designated a Prime Butterfly Area (PBA) [20].  
Management of the park involves three protection levels, with Zone II encompassing an area of 19,255.59 ha, representing 45.93% of the total park surface. The Ribnica Reservoir locality is situated within this zone, covering 283.42 ha, which includes both the lake itself and its surrounding terrestrial ecosystems. Conservation measures in Zone II prioritize habitat preservation, biodiversity management, and the sustainable use of natural resources. Due to its role in supplying drinking water and supporting ecological diversity, Ribnica Reservoir holds critical hydrological and ecological importance within the protected area [20].  
* Sampling was conducted at three profiles within Ribnica Reservoir (Figure 1): Profile A1: 43.68547\({}^{\circ}\) N, 19.66423\({}^{\circ}\) E. Profile B1: 43.68358\({}^{\circ}\) N, 19.67232\({}^{\circ}\) E. Profile C1: 43.68136\({}^{\circ}\) N, 19.68158\({}^{\circ}\) E. Sampling depths included: Profile A1: 50 cm, 200 cm, 350 cm, 500 cm, 800 cm, 1000 cm, 1200 cm. Profile B1: 50 cm, 200 cm, 350 cm, 400 cm, 450 cm. Profile C1: 50 cm. Samples were collected by the Serbian Environmental Protection Agency in May and in September 2021 and May 2022 [21]. However, no data on water quality are available for the years preceding or following this period, highlighting a significant gap in monitoring efforts, which is particularly concerning regarding the reservoirs' location within a protected area.