Purpose. The purpose of the work is to study the variability of major ionic composition and salinity in the coastal zone the northeastern Black Sea over the period 2022–2025, to analyze its causes, as well as to assess the impact of major ionic composition on the accuracy of salinity determination by the classical (chlorinity) and modern (TEOS-10) methods.

Methods and Results. The water samples were collected in the coastal water area from the city of Anapa to the Lazarevskoye settlement (Sochi) during the expeditions in September 2022, in June, September and December, 2023, in August 2024, and in March and August, 2025. The concentrations of major ions (Cl⁻, SO₄²⁻, HCO₃⁻, Ca²⁺, Mg²⁺) were determined by the potentiometric titration method, K⁺ gravimetrically, and Na⁺ by calculation method. The water salinity values were calculated in three ways: by the sum of major ions, by chlorinity, and by density using the TEOS-10 thermodynamic equation. Density was measured by a high-precision laboratory densitometer. Salinity of the studied water samples varied from 8.77 to 19.11 g/kg (average is ∼ 18.40 g/kg). The relative content of Cl⁻ varied within the range 52.6–54.6%, SO₄²⁻ – 7.8–12.9%, HCO₃⁻ 0.8–1.4%, Na⁺ – 29.9–31.5%, Ca²⁺ – 1.1–1.8% and Mg²⁺ – 3–3.7%. The highest deviations of major ionic composition from the oceanic one correspond mainly to the low salinity waters, and this fact indicates the decisive role of freshwater continental runoff in the modification of major ionic composition. The deviation of chlorinity-based salinity calculation from the one based on sum of ions were up to 5% (∼ 0.9 g/kg) in the coastal sea waters with salinity 19 g/kg and 11% (∼ 0.9 g/kg) directly in the river mouth waters with salinity 9 g/kg and the deviation of density-based salinity calculation using the TEOS-10 equation constituted 4% (0.7 g/kg) in the coastal sea waters with salinity 19 g/kg. A correlation between the increase in SS – S_Cl difference and the increase in SO₄²⁻, HCO₃⁻ and Ca²⁺ contents in water composition was established; when assessing the SS – S_4ρ difference, no such correlation was observed.

Conclusions. A comparison of three methods for determining salinity – by the sum of ions (SS), by the chlorine coefficient (S_Cl) and using the S_4ρ density values – has shown that the most accurate values are obtained by the ion sum method. This method makes it possible to define the causes of salinity change due to seasonal variations in the concentrations of certain ions (SO₄²⁻, HCO₃⁻ and Ca²⁺) in the coastal waters. A comparative analysis of the results revealed a trend towards increasing salinity in the coastal waters of the Black Sea northeastern shelf over the observation period 2022–2025.