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The Black Sea has lost more than a third of its habitable volume
9/1/16

Several diagnostics to check the presence of oxygen 

To diagnose this shrinking of the oxygen-rich top layer, Arthur Capet had to take into account two sources of variabilities that had to be distinguished to avoid biased conclusions. On the one hand, temporal variability, providing a view of the evolution in time of the presence of oxygen in the sea, and on the other hand, spatial variability. "Oxygen penetration isn't consistent in all areas. Especially close to coastlines, where the interaction between the current and the seabed induces increased vertical mixing, or close to the Bosphorus Strait. It was necessary to take into account every place where the measurements were taken to get a clear image of this evolution in time. And then there was another difficulty: the dominant currents in the Black Sea create forces that lift the vertical structure in the middle of the basin and lower it in the periphery. This mean that at the same depth, the water will be less dense close to the coast than in the middle of the basin." In other words, rather than forming a horizontal boundary, the halocline resembles a dome. To overcome this additional difficulty, the researcher quantified the oxygen concentration by expressing the depth in metres on the one hand, and in terms of density on the other. Which then made it possible to find a consistent average for the whole of the basin and establish an accurate overall vertical profile for the water column.  

Oxygen Black Sea

The drivers behind this astonishing decline 

Several historic databases contained information, collected during a number of campaigns, on oxygen distribution in the Black Sea. By compiling these figures and those collected by the ARGO  http://www.argodatamgt.org buoys, which drift freely and send satellite information on the evolution of the temperature, salinity and oxygen, it was possible to compare more than 4000 profiles, taken between 1955 and 2015. By proposing an average of all these diagnostics and by inventorying the quantity of oxygen in the Black Sea, the final observation was very accurate and unequivocal. The oxygen penetration declined throughout the second half of the 20th century, shrinking from 140 metres in 1955 to a mere 90 metres in 2015.  

There were two successive causes behind this gradual drop. A greater abundance of nutrients initially, then global warming. Up until the 1990s, the intensity of ventilationlinked to the dynamics of the cold waters didn't decrease. It even increased in certain years, during harsher winters. Therefore, there should have been a larger quantity of dissolved oxygen. However, its concentration continued to fall in the entire water column. It was necessary to look for the cause elsewhere than in the physical reaction linked to the climate. "In reality", Arthur Capet contextualises, "this shortage can be explained by the extensive eutrophication of the basin during this period. It corresponds to a major economic boom in the USSR, when huge farms and extensive cattle breeding were developed. What's more, this boom wasn't accompanied by environmental considerations." Fertilisers and organic waste linked to breeding found its way into the rivers and ended up in the Black Sea. They had a very high nitrate and phosphate content which encouraged the primary production. "Just as the fertilisers encourage plants to grow, they also influence the production of algae. These algae consume oxygen when it decays or is consumed. A greater biomass therefore leads to a greater consumption of oxygen." In 1990, this influx of nutrients fell significantly. Once again, it seems that it was associated with a geopolitical and economic context, since it coincided with the fall of the Soviet empire and the economic difficulties encountered in the region. It is also the moment when the first wide-scale environmental measures were applied.

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