The Black Sea has lost more than a third of its habitable volume
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.
Of all the planet's seas, the Black Sea has a very particular profile. Surrounded by land, it could even be mistaken for a great lake if it weren't for the fact that it is directly connected to the Mediterranean Sea through the Bosphorus Strait, a small one-kilometre-wide waterway. A sea surrounded by land that determines its special characteristics. "The main supply of water to the Black Sea comes from rivers. Especially the Danube", explains Arthur Capet, the first author of the publication on the decline of oxygen in the Black Sea (1) and researcher at MAST, led by Marilaure Grégoire, FNRS research director. "This fresh water, which is less dense than sea water, colonises the upper layers of the water column without mixing with the lower layers." Because the lower layers are far more saline. The origin is to be found to the south-west of the Black Sea, in the Bosphorus. "Here, there is an exchange with the Mediterranean Sea in two layers. The fresh water at the surface flows out, and lower down, the salt water flows in and sinks directly towards the denser levels."
The permanent stratification linked to salinity, the halocline, deprives the deep waters of oxygen. The marine food chain therefore develops above this boundary below which the waters are devoid of oxygen. "All the same, the Mediterranean inflow supplies a small amount of oxygen to the intermediary layers. Not only does it contain oxygen, but as it descends, it entrains surface water with it. However, this oxygen is very rapidly consumed as the organic matter decays." What happens is that the organic matter (plankton, algae, etc.), produced on the surface by photosynthesis, breaks down or is consumed and expelled by other species in the trophic chain. In both cases, this eventually sinks. Since it requires oxygen to break down, the few reserves that exist in the lower layers are exhausted.
"The oxygenated and therefore habitable area of the Black Sea is a very restricted space. This is the case horizontally, because the basin is almost completely closed, and also vertically, owing to this permanent stratification. Compared with other seas, this restricted volume is exposed to major external influences. It is therefore more sensitive and capable of evolving rapidly", Arthur Capet explains. It is this type of evolution that the researcher was able to observe. By compiling the data gathered over the past 60 years, he noted that the oxygen-rich top layer of the Black Sea had shrunk from 140 metres to 90 metres deep. Impressive figures that correspond to a more than 40 % decrease in the habitable volume.
Permanent stratification compared with seasonal stratification
The salt content favours the permanent vertical stratification in the Black Sea. In addition to this permanent stratification is a seasonal stratification due to the temperature of the water. "In winter", Arthur Capet continues, "lower temperatures accompanied by higher winds make the surface water colder and richer in oxygen. However, cold water is denser than warm water. Therefore, this cold water sinks and takes the oxygen it contains with it. This creates a ventilation effect" It is this periodic phenomenon that supplies the deeper layers with oxygen. In the case of the Mediterranean, the surface waters cooled in winter sink to the bottom, supplying the entire basin with oxygen. However, in the Black Sea, these waters are blocked in the permanent halocline, even though they are colder than the deep waters. In terms of density, salt eventually wins over temperature. The cold waters end their journey here, and retain their oxygen. In summer, the surface waters warm up and no longer sink, thus creating a new stratification of the water column, the thermocline.
(1) Arthur Capet, Emil V. Stanev, Jean-Marie Beckers, James W. Murray, and Marilaure Grégoire, Decline of the Black Sea oxygen inventory, Biogeosciences, 13, 1287–1297, 2016