Andes dissected flank with its own characteristics. In addition, the geological formations that affect the water quality of the Caplina Basin were identified based on the geological cartography. The sources with the greatest influence on water quality were determined. These included the Aruma and Paralocos geothermal springs which produce water with high contents of arsenic, lead, and sodium, in concentrations exceeding the Environmental Quality Standard (ECA, Spanish acronym), and high iron concentration in the water channel from the Barroso Chico spring to the Ancoma Creek. The relationships among the geology, climate, and hydrology were identified, which reflect changes in the concentrations of the elements in the water during seasons with low water levels versus those with high levels.

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.

На основании архивных данных за период 1923-2020 гг. анализируется внутригодовая изменчивость растворенного кислорода и температуры в слое 0-150 м глубоководной части Черного моря. Подтверждено, что в верхнем перемешанном слое концентрация растворенного кислорода достигает максимума в марте-апреле, а минимума в августе. Амплитуда сезонного хода с глубиной уменьшается и на горизонте 150 м она падает практически до уровня шумов. Сезонный ход кислорода, согласованный с внутригодовыми изменениями температуры, принципиально не изменился за почти столетний период наблюдений. Вместе с тем, его амплитудно-фазовые характеристики значимо изменяются под действием климатических вариаций. Промежуточный слой на масштабах нескольких десятилетий характеризуется понижением кислорода и ростом температуры, которые продолжаются и в последние годы. Ключевые слова: растворенный кислород в верхнем 150-метровом слое, температура, уменьшение концентрации кислорода в промежуточном слое вод Черного моря.

The article briefly describes the main facts and milestones in the development of methods for monitoring the hydrological and hydrochemical state of the Black Sea waters from the 19th century to the present day. It is shown that the modern understanding of the hydrological and hydrochemical processes of this unique sea basin resulted from the intensive work of many oceanographers over the past centuries. Par-ticular attention is paid to the oxygen regime of the Black Sea.