The Black Sea's oxygen-free depths

The Black Sea has a strongly stratified water column. Beneath the oxygenated surface layer is a transition zone; oxygen-free waters begin at roughly 100–150 m across much of the basin, though the boundary varies by location and conditions. Hydrogen sulfide accumulates in deeper layers. Familiar large animals do not inhabit these waters, but diverse microorganisms do, participating in sulfur, nitrogen, and carbon cycles.