## 3 Results and Discussion
### Lead, Cobalt and Nickel Concentrations in Bottom Sediments  
When assessing the pollution levels present in bottom sediments of Lake Markakol, high levels of lead, cobalt and nickel were detected in all of the analyzed samples at 20 points. The lead content in the bottom sediments was detected as ranging from 4.6 to 11.3 mg kg\({}^{-1}\), with an average of 7.4 mg kg\({}^{-1}\). The accumulation of lead in deep waters is expressed by a weak correlation (r = 0.4) between the lead concentration present in the bottom sediments and lake depth. Figure 4 shows high lead concentrations existing in bottom sediments of deep-water areas at depths of 10 m and deeper, up to 21 m, in a range of 8.3-11.4 mg kg\({}^{-1}\), also in the zones influenced by the Tikhushka River (point 16) 9.6 mg kg\({}^{-1}\) and Topolevka River (point 4) 9.8 mg kg\({}^{-1}\). Chemically bound lead in the bottom sediments of a lake is generally a good prerequisite for relatively good Pb storage. However, with higher lead concentrations, there is greater potential for chemical transformation to bioavailable forms of lead, which can be dangerous for aquatic organisms and vegetation. For example, internal water exchange occurring in the lake between the bottom and the surface water body, driven by wind, mud flow, water temperature differences, pH and redox changes or earthquakes can initiate the transformation of chemically bound lead to bioavailable lead [12]. The cobalt found in bottom sediments is present in high concentrations across the entire water area, up to 10.3-18.0 mg kg\({}^{-1}\); the nickel values vary from 3.4 to 15.0 mg kg\({}^{-1}\) (Figure 4). On average, high cobalt concentrations were detected in deep water zones up to 15.7 mg kg\({}^{-1}\) (point 8); and in the zones influenced by rivers, this was up to 13.2 mg kg\({}^{-1}\) (points 11, 17) for cobalt and up to 10.0 mg kg\({}^{-1}\) for nickel (Figure 4). Low nickel concentrations in bottom sediments of the deep lake zone with a correlation coefficient (r = -0.3) may indicate that under high cobalt concentration conditions, nickel concentrations in the bottom sediments decrease. This can take place due to various processes, such as competition between metals for particle adsorption, fluctuations in water composition and the particle size distribution in the bottom sediments or the surfaces of bottom-forming rocks, vegetation and aquatic organisms [42; 43; 44; 45; 46; 47]. In past studies [7; 8; 9; 10], Lake Markakol has been described as a deep-water lake with an irregular, mainly stony bottom, transitioning to a muddy bottom in the shallow areas. According to information from the Ministry of Ecology and Natural Resources of the Republic of Kazakhstan, the soils of the lake catchment area are represented by several types, such as mountain-tundra, mountain-forest, and mountain-meadow soil types, as well as dark gray slightly podzolic, forest-black earth, mountain-forest-meadow, meadow-steppe, chernozem mountain-steppe leached soil types, as well as meadow-chernozem, name-meadow, meadow-swamp and swamp [8]. Referring to this soil information, it is understandable that during the field studies, the upper layer of the bottom sediments at the bottom of the lake was characterized by sapropels, dark types, and closer to the shoreline and shallow zone--brown type--which is significantly different from the lithogenic base of the surrounding landscape. However, this can be understood as a result of the erosion of humic soil layers from the surrounding lake catchment and their liquid underwater transformation. In all of the studied samples, a thick suspension was formed at the water-bottom section, changing into liquid sapropel. Sapropelic sediments are usually organic sediments rich in vegetation remains and other organic materials that alter the chemical and physical properties of bottom sediments and influence the adsorption mechanisms of heavy metals [48; 49; 50]. In turn, organic matter forms complexes with metals, affecting their bioavailability and migration.