| Summary: | Purpose. The work is aimed at assessing the effect of variations in the major ion-salt composition on the accuracy of determining water salinity in the inland seas and other seawater areas. The main goal of the study is to assess the representativeness of the results of the CTD salinity measurements (standard in oceanological practice) for the areas where the ion-salt seawater composition differs from that of the ocean. Methods and Results. Salinity values of the seawater samples collected in the expeditions in the Black Sea and the Kerch Strait, and also in the Kara and Caspian seas in 2014–2021, were obtained in four different ways: 1) measurements with a CTD-probe based on electrical conductivity (practical salinity); 2) based on the measured density values, calculation by the TEOS-10 equation of state with due regard for the regional correction for the areas under study (absolute salinity); 3) calculation by chlorine content using empirical dependencies for the corresponding water basins; 4) direct calculation based on a sum of components of the major ionic composition (similar to chemical determination in a laboratory). Conclusions. Differences in the ratios of the main ions in the water chemical compositions of the water areas and basins under study significantly affect the accuracy of salinity determination by standard oceanographic equipment. The variations in the major ionic composition, especially in the surface layer of the sea coastal part, are assumed to be largely influenced by the continental freshwater runoff. The ionic composition variability, having been not taken into account, leads to the errors in the measurements of physical parameters at traditional CTD-probing.
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