Purpose. The work is purposed at studying the influence of water exchange processes between the Black and Azov seas upon the characteristics of major ion composition (MIC) and other hydrochemical indicators of the Kerch Strait waters, as well as the impact of changes in the relative content of major ions of water salt composition upon the accuracy in determining salinity values. The MIC transformation during mixing of the sea surface waters and the Taman Bay ones in the Kerch Strait is investigated. The errors in calculating salinity by the standard methods are assessed for the Kerch Strait, the northeastern Black Sea and the Taman Bay waters.

Methods and Results. The concentrations of major ions determining MIC in the Kerch Strait, Black Sea and Taman Bay surface waters in 2019–2023 were defined by the potentiometric titration method. The water salinity values were obtained in four different ways.

Conclusions. It was established that the salinity value ~18.66 calculated by a sum of the major ions corresponds to the surface waters in the northeastern part of the Black Sea, that conforms to the practical salinity value ~18.10 calculated using the CTD probe data. On the average, MIC of these waters is characterized by the following relative content of major ions: Cl⁻ = 54.1%, SO₄²⁻ = 8.2%, HCO₃⁻ = 1%, Na⁺ = 30.8%, K⁺ = 1.3%, Ca²⁺ = 1.3% and Mg²⁺ = 3.4%. It is shown that the Kerch Strait waters, even in case of their similar salinity, can have different ratios of the major ions characterized by high spatial and temporal variability which, in its turn, is subjected to a significant impact of the waters inflowing from the shallow Taman Bay. The largest differences were between the sum of major ions and the practical salinity. For the Kerch Strait waters, the differences averaged ~2.5%. The ionic variations contributed to underestimating the practical salinity values calculated for all the waters under study. In calculating salinity using the chlorine coefficient, the deviations from the sum of ions constituted ~2%, whereas those obtained using the TEOS-10 equations – ~1%.