With rivers providing an abundant supply of fresh water, the upper layers of the Black Sea are less dense than its saltier lower layers. A permanent boundary between the two prevents any vertical mixing. The oxygen, derived from the atmosphere and photosynthesis, remains restricted to these surface waters. However, this precious gas is essential to the development of the majority of living species. Recent research, carried out by the MAST (Modelling for Aquatic Systems) group at the University of Liège, has shown that this oxic boundary shoaled from 140 to 90 metres between 1955 and 2015. A compression of almost 40 % of the habitable space in the Black Sea, directly linked to its eutrophication and global warming. This phenomenon could be accompanied by major ecological and economic consequences. Furthermore, a high concentration of hydrogen sulfide, an extremely toxic gas, lies dormant in the deepest layers of the Black Sea. For the moment, there is no evidence of a correlation between the compression of the oxic zone and this gas rising. But if the stratification of the water column weakens, even locally, an imbalance could endanger the aquatic life in the surface layer.

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