Since the amount of organic matter from the Black ...

Since the amount of organic matter from the Black Sea Rivers with a depth of 2,210 m is greater than that of bacteria in the seawater, they normally provide oxygen from sulfur ions, a component of seawater, rather than dissolved oxygen normally found in seawater. As a result of this process, highly toxic hydrogen sulfide (H2S) gas emerges and prevents life under 200 meters. The Black Sea is the largest hydrogen sulfide reserve in the world. There is no life below depths of 150-200 meters. Water does not contain oxygen and is loaded with H2S. Hydrogen Sulfide kills the entire ecosystem where it is found, destroys coastal fishing, and if it comes to the surface, it paints the gold of the ships black with the chemical compound it creates. a Wikipedia