Why does weak mixing of deep and surface water affect the Black Sea?

The weak vertical mixing of water in the Black Sea is primarily due to the sea's compact and highly stratified nature, which limits the exchange between surface and deep layers. Because this process is so restricted, it is estimated that hundreds of years are required for water to complete a cycle from the depths to the surface.1

This lack of effective ventilation has profound consequences for the sea's environment:

• Oxygen Depletion: Oxygen is only dissolved in the upper water levels, where it supports rich sea life. Below depths of approximately 230 to 500 feet (70 to 150 metres), there is no oxygen.12
• Hydrogen Sulfide Zone: The deep, unventilated layers are contaminated by hydrogen sulfide, creating a saturated, "dead" zone.1
• Biological Impact: This environment is largely lifeless, inhabited only by adapted bacteria. The presence of this toxic layer significantly limits the distribution of benthic plants and animals compared to other bodies of water like the Mediterranean.13