The Black Sea has lost more than a third of its habitable volume
And yet, the level of oxygen didn't increase again. On the contrary, it remained the same for several years, when the winters were particularly cold, before decreasing again. This time, global warming was the culprit, by influencing ventilation. If the winters are warmer, a lower volume of dense water is generated, which reduces the oxygen content when these waters sink down to the halocline. "The phenomenon could well get worse. Before, this formation of cold water took place every year. And yet, the figures collected over the past ten years bear witness to an increasingly intermittent formation of cold water. We are currently in the process of analysing our results, but it would seem that this once annual ventilation now only takes place every two or three years. We still can't determine the consequences of this phenomenon, but in any case, we are witnessing a changing system."
Besides a less extensive and occasional mixing, this warming masks another effect leading to deoxygenation. One of the chemical properties of cold water means that it becomes saturated less quickly than warm water. The colder the water, the more it can contain dissolved gas, which obviously includes oxygen. As it heats up, the surface water is increasingly unable to accumulate oxygen. Subsequently, not only does oxygen no longer colonise the Black Sea at depth, but moreover, its concentration decreases in the entire water column. The deoxygenation caused by the increase in the water's temperature is a global problem that concerns all the oceans. Today, the problem is taken very seriously by the scientific community.
Implications to be quantified
The study aims above all to quantify the physical processes linked to the water column by collecting and analysing the data. The dynamics seem to be properly understood now, in terms of both space and time. The big unknown remains the influence that these variations will have on the ecosystem. The models that enable the study of the different scenarios in the Black Sea must now be integrated with this new halocline, thermocline and oxycline data, so that their real impact can be more precisely predicted. However, several avenues can already be explored. "The Black Sea is clearly facing significant compression of its habitable area. The whole ecosystem is formed in this layer, from phytoplankton to predators, which evolve in the deeper waters. The entire trophic chain is organised in the water column according to the presence of light or nutrients. Previously organised over a depth of 140 metres, the interactions between these trophic groups must now find a new balance over a depth of 90 metres. There will be an ecological and economic affect. Fishing, which is one of the major activities in the region, will probably have to adapt to this reorganisation." According to the FAO, the catch amounted to 376,000 tons in 2013. Barely two times less than for the whole of the Mediterranean.
A toxic outsider
One final process deserves to be monitored. As previously mentioned, biomass consumes oxygen as it decays. When there is no more oxygen, this biomass continues to decay, leading to the consumption of sulfates by the bacteria and the production of hydrogen sulfide (H2S), a highly toxic gas. The permanent stratification of the Black Sea acts as a lid over the deep waters, in which this hydrogen sulfide has accumulated and reaches now unprecedented concentrations. Nothing currently proves that the shoaling of the oxygen penetration depthdirectly correspond to a shoaling of the hydrogen sulfide onset depth. "The depth at which the H2S appears doesn't exactly correspond to the depth at which the oxygen disappears. There is a whole series of intermediary processes in a median zone that is suboxic and devoid of hydrogen sulfide. We focused on oxygen and our study revealed a rise in the upper boundary of this zone, but not the lower one. We can assume that the stratification of the Black Sea will remain stable overall. But it's possible that if the H2S were to rise up, unstable climate or geological conditions would cause the hydrogen sulfide to pierce through the oxygenated layer. This could have major repercussions on aquatic life. In order to determine the situation and to solve the dynamics of the H2S, we must now model these processes, and quantify and inventory its concentration."