## 2 Materials and Methods
### Laboratory Analyses  
The methodology for the metal determinations of the water samples and bottom sediments using a flame atomic absorption spectrometer includes several steps [33; 34]: (1) Sample preparation: the water samples were delivered to the laboratory for filtering using blue tape filter paper with a pore size of 2-3 microns; (2) the water samples were filtered directly into 250 mL glass bottles with tight-fitting lids; (3) the filtered samples were stored and transported in a portable refrigerator, model ALPICOOL CF 45 (China), at a temperature of \(10\,^{\circ}\)C; (4) the time that passed between sampling and filtration was 5-6 h, and between filtration and analysis, 10 to 15 days. The time period between sampling and analysis primarily depends on the length of the field research days; (5) sample preservation was not necessary because the lake water is ultra-fresh and does not contain high concentrations of dissolved minerals, according to the ST RK ISO 8288-2005 method; (6) the essence of the method used to analyze mobile forms of metals is to process samples of bottom sediments with an ammonium acetate buffer solution followed by the subsequent determination of metals in the resulting solution via atomic absorption analysis; (7) in order to avoid different light absorption and eliminate interfering influences during atomic absorption analysis, which can distort the results of metal determination, solutions were prepared using an ammonium acetate buffer solution with pH 4.8; and (8) according to the RD 52.18.289-90 procedure, the bottom sediment sample was dried to an air-dried state. An air-dried sample of the bottom sediments weighing 5 g was transferred to clean polyethylene bottles, and 50 mL of ammonium acetate buffer solution was added, which was kept for 24 h at room temperature, stirring 5-7 times in a circular motion. Then, we filtered the samples with the solution into clean 100 mL polyethylene bottles with a tight-fitting lid using a folded paper filter \"white tape\" with a pore size of 5-8 microns. The volume of the filtrate in the volumetric flask was supplemented with a buffer solution to 100 mL.  
For the construction of calibration graphs (r = 0.99) for quality control and the calibration of the spectrometric determination of metals, different concentrations of State Standard Samples (SSO), according to [33], registered in the Register of the State Measurement System of the Republic of Kazakhstan in different concentrations:  
* An aqueous solution of lead ions (SSO 7012-93) (2K-1) in concentrations \(\mathrm{C_{Pb}}\) = 0.0125; 0.025; 0.05; 0.1 mg/L;
* An aqueous solution of cobalt ions (SSO 7880-2001) (NK-EC) in concentrations \(\mathrm{C_{Co}}\) = 0.1; 0.15; 0.2; 0.5 mg/L;
* An aqueous solution of nickel ions (SSO 7873-2000) (NK-EC) in concentrations \(\mathrm{C_{Ni}}\) = 0.1; 0.15; 0.2; 0.5 mg/L.  
The determination of heavy metals was carried out via the flame atomic absorption spectrometric method [33; 34] using an atomic absorption spectrophotometer AA-7000 (Shimadzu Company, (Kyoto Japan) operating with an acetylene-air mixture. The method is based on the property of metal atoms to absorb light of certain wavelengths in a ground state (Pb--283.3 nm; Co--240.7 nm; and Ni--232.0 nm), which they emit in an excited state. The obtained atomic absorption analysis data were used to estimate the metal content in the water and sediment samples.