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Based on the archival data set of the Institute of Natural and Technical Systems from 1955 to 2015 the intra-annual variability of temperature, salinity, dissolved oxygen and PH in the surface layer of the waters of the northwestern shelf of the Black Sea, including the Danube estuary area, is analyzed. The temperature of surface waters on the NW shelf of the Black Sea and the Danube waters differ significantly on a seasonal scale. The Danube waters considerably cool down the surface layer of the estuarine zone during the winter months. Seasonal changes in salinity are associated not only with large seasonal fluctuations in the river flow, precipitation and evaporation, but also with seasonal variability of the wind field. In the autumn period in the spatial distribution there is an increased oxygen content off the coast of Romania, which is related to the prevailing northwestern wind in this region, which leads to the drift of the transformed Danube waters along the western coast.    Due to the prevalence of southeastern winds off the Romanian coast, the Danube waters are blocked in the coastal region, which is clearly seen from the spatial distribution of oxygen. The seasonal dynamics of the pH value corresponds to the development of production processes. From winter to summer, an increase in the pH value is noted in both water areas, and then its decrease by autumn.    The absolute predominance of the annual harmonic in the seasonal variability of most of the analyzed hydrological and hydrochemical characteristics in most of the examined shelf water area was confirmed. The exception is the salinity in the estuary area of the Danube. The semiannual harmonic describes more than 60% of the total variance due to the seasonal variability of salinity, which is related to the peculiarities of the wind regime in the region.

Analysis of seasonal variability of hydrological and hydrochemical characteristics of surface waters in the northwestern Black Sea