Three Typical Hydrological-Hydrochemical Situations Near the Danube River Mouth Based on the Marine Hydrophysical Institute Research Expeditions in 1997-2013

Purpose. The paper is aimed at studying the present state of water ecology of the Black Sea northwestern shelf (NWS). Since the early 60-ies of the last century, ecology in this region deteriorated seriously due to intensive anthropogenic load that resulted in eutrophication of waters and regular fish mortality in the warm period of a year. Methods and Results. In 1997-2013 Marine Hydrophysical Institute realized 7 hydrological and hydrochemical expeditions near the Danube mouth. The dissolved oxygen and nutrients (nitrate, nitrite, phosphate and silicic acid) contents were analyzed in the water samples collected at the bottom (0.5-1 m above the bottom) and surface (0-1 m) horizons. Conclusions. Revealed are three hydrological-hydrochemical situations typical of the region: distribution of the transformed Danube flow far to the east, pressing of the Danube flow to the west coast of the Black Sea and alongshore upwelling. Silica and nitrate are rapidly and almost completely extracted from the seawater (the latter one – even in early winter) during photosynthesis in the surface waters on the NWS. The processes of particulate organic matter mineralization in the bottom waters of the NWS are assumed to be an additional source of silicic acid and nitrates. As for silicic acid, the process takes place during a warm season, and for nitrates – even in early winter. The alongshore upwelling in summer can lead to oxygen deficiency in the coastal waters and that, in its turn, can result in fish mortality on the shelf. The waters of the northwestern shelf open part which, in autumn 2010, pressed the Danube transformed freshwater flow to the Black Sea west coast, contained very small concentrations of biogenic elements (low content of silicic acid should be particularly noted). The study confirms the fact that the hazardous process of change in the phytoplankton species composition still continues, thus, altering the whole food chain in the Black Sea.