Why are the deep waters of the Black Sea nearly biologically dead?

The deep waters of the Black Sea are considered nearly biologically dead not because of pollution, but due to a lack of oxygen in the lower water levels.2

This condition arises from the sea's highly stratified nature, which severely limits vertical water intermixing. Because the water is so stable and compact, it takes hundreds of years for water to cycle from the depths to the surface. Consequently, there is only weak ventilation of the deep layers, preventing the replenishment of oxygen below depths of approximately 230 to 500 feet (70 to 150 metres). In these oxygen-depleted reaches, the water is contaminated by hydrogen sulfide, creating an environment that can only support specialized bacteria.12