Suboxic and Anoxic Conditions of Deep Waters in the Gdansk Basin of the Baltic Sea

Long-term variability of the concentrations of oxygen and hydrogen sulfide in deep waters of the Gdansk Basin of the Baltic Sea for the last 25 years was considered. The dependence of suboxic and anoxic conditions in deep waters on the intensity and periodicity of the inflow events of the North Sea waters was shown. During the years of 1993, 2003, and 2014-2016, three intervals of strong increase of the concentration of dissolved oxygen after Major Baltic Inflow events (MBI) were distinguished. It was shown that the near-bottom layer of the Gdansk basin was aerated during two or three months after MBI in January-March whereas the two-year absence of MBI resulted in the appearance of hydrogen sulfide. However, the most pronounced hydrosulfide contamination within the period of surveys was registered in the middle and late 1990s, with as high hydrogen sulfide concentrations as 2.8 mL/L. A quality of reanalyzing the distribution of dissolved oxygen was evaluated as well. One may note a good repeatability of the field data by reanalysis of SHMI in terms of quality; however, the observed maximum concentrations in the distribution of dissolved oxygen were underestimated, sometimes even several times.