Black Sea Becoming Inhabitable; More Than One-Third Devoid of Oxygen Over 60 Years

Recent changes to the Black Sea have stripped oxygen from the bottom 40 percent of the oxygenated water, rendering it inhabitable.

The Modelling for Aquatic Systems (MAST) group at the University of Liege recently completed a study about the oxygen decline in the Black Sea. The group is worried about the negative ecological and economic outcomes related to their findings. Their paper, published in the open access journal Biogeosciences, links the decreased oxygen levels to global warming.

The Black Sea is made up of different layers. The bottom layers are much saltier, denser and lack oxygen. Conversely, the river-fed upper layers have more oxygen and less salt, which allow oxygen needing creatures to thrive. A boundary keeps the layers separate. In the past sixty years, the boundary moved from a depth of 140 to 90 meters, thereby causing the loss of more than 40 percent of the sea's habitable water.

"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," lead author and researcher at MAST Arthur Capet said in a release.

"In winter, 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," he added.

The researchers tied the depletion of oxygen to man-made activities, namely eutrophication and climate chang-induced warmer waters. Eutrophication is a type of water pollution that occurs when runoff from land introduces a critical mass of nutrients in the water that kills animals by depleting oxygen in the water.