## 4 Conclusions  
An assessment of the pollution level of Lake Markakol relating to heavy metals present in the \"water-bottom sediments\" system was carried out for the first time. The results showed the presence of significant concentrations of lead in the surface water layer (4.3 \(\upmu\)g/L), in the bottom water layer (14.4 \(\upmu\)g/L) and in the bottom sediments (7.35 mg kg\({}^{-1}\)). Cobalt was recorded in the lake with an average of up to 50.5 \(\upmu\)g/L, and nickel was recorded up to 30.8 \(\upmu\)g/L, both in the surface layer and in the bottom layer. The concentrations in the bottom sediments reached up to 13.59 mg kg\({}^{-1}\) for cobalt and up to 9.32 mg kg\({}^{-1}\) for nickel. The heavy metal pollution of Lake Markakol relates to several key points that should be emphasized:  
Sapropel is an organic substance formed as a result of the decomposition of organic matter in underwater bottom sediments. The presence of sapropel of various types can be an indicator of the state of the bottom sediments and the initial stages of the eutrophication of the lake. Exceeding the maximum permissible standards of heavy metals in water is an alarming signal, which is associated with anthropogenic activities, water agitation or natural-climatic changes. The lack of historical data makes it difficult to assess changes over time. Comparative future analyses could help in understanding trends and identifying causal relationships. High concentrations of heavy metals in water can affect fishes by impacting their activity and accumulating in their tissues. The accumulation of toxicants in fishes can also lead to potential human health effects due to the consumption of contaminated fish. Pointing out the importance of providing healthy fish products to the public emphasizes the socio-economic importance of addressing lake pollution. We recommend repeating such investigations in relation to climate change or/and extreme weather conditions. The observation of different point (river inflow) and non-point sources (c.f. coastal erosion) of potential metal impacts into the Markakol Lake should be realized with the help of environmental monitoring, both event-based and permanent monitoring. The present results and their assessment can serve as the basis for future analyses and the development of measures to reduce pollution, restore the lake ecosystem and ensure the safety of fishery products for humans. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14177487/s1](https://www.mdpi.com/article/10.3390/app14177487/s1)]([https://www.mdpi.com/article/10.3390/app14177487/s1](https://www.mdpi.com/article/10.3390/app14177487/s1)), Figure S1: Spatial distribution of Pb in the system \"water--bottom sediments\" taking into account lake depth and water layer; Figure S2: Spatial distribution of Co in the system \"water--bottom sediments\" taking into account lake depth and water layer; Figure S3: Spatial distribution of Ni in the system \"water--bottom sediments\" taking into account lake depth and water layer. **Author Contributions:** Conceptualization, L.I., A.M. and C.O. ; methodology, L.I., A.M. and C.O. : software, A.Z. and S.Z. ; validation, A.M. and C.O. ; formal analysis, L.I., A.Z., B.S. and S.Z. ; investigation, L.I., A.M., A.Z., B.S. and S.Z. ; resources, A.M. and C.O. ; data curation, A.M.; writing--original draft preparation, L.I. and A.M.; writing--review and editing, A.M. and C.O. ; visualization, A.M., A.Z., S.Z. and L.I. ; supervision, A.M. and C.O. ; project administration, A.M.; funding acquisition, A.M. and C.O. All authors have read and agreed to the published version of the manuscript. **Funding:** This research is funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP14870595 \"Monitoring of the condition and assessment of the micro- and macroplastic pollution level of the Lake Markakol aquatic environment\").