Ah, Meromixia: such a pretty name for a girl.
Like I said, I was floating around the Black Sea with Neil Acherson. All good fun so long as you stay on the surface. But it turns out that below 120m it is the largest anoxic volume of water on this our aerobic planet. The whole basin contains in excess of 500,000 cu.km. and 90% of this is below the oxygen horizon. Don't be fooled into thinking that lack of oxygen means that the depths are dead. There is life down there, Jim, but not as we know it. The Dead Sea isn't dead either! It may not host fish or crustacea but has a wild collection of weird bacteria and archaea making a living with weird and wonderful chemical transformations. And just to put things into perspective. The Dead Sea is a paltry 115 cu.km is volume . . . and falling, as more water is abstracted from the R. Jordan and rainfall fails year after year. And even Lake Baikal which I previously bigged up, holds only 24,000 cu.km.
What seems to have happened is that when the glaciers melted at the end of the last Ice Age, the depression between Crimea and Turkey filled up with fresh water. At some stage the landbridge at the Dardanelles gave way and dense salty water gushed in from the South. Because it was [3%] dense with dissolved ions, it sank to the bottom of the bucket and became over-lain with water which was much less [1.7%] salty. The top layer is called the mixolimnion and the lower monimolimnion and they do not mix. The top 100m or so is literally in touch with the atmosphere and so absorbs oxygen, especially in Winter when the temperature is lower. The oxygen encourages the growth of Prochlorococcus and some larger photosynthetic algae. Every-aquatic-thing else depends ultimately on this base of the food-chain and the Black Sea supported fantastically rich fisheries. The surrounding polities have put paid to this by a combination of industrial pollution and eutrophication driven by sewage and, more significantly phosphate and nitrate run-off. With increasing frequency, algal blooms deplete the precious oxygen and everything else goes belly-up.
But that oxygen depletion is only temporary, for now, because there is a lot of atmosphere from which re-charge is possible. Down there in the monimolimnion there .. is .. no .. oxygen. But there can be a rich microbial life based on sulphur and methane metabolism: purple and green sulfur bacteria, methanotrophs complementing methanogens. There are coal, oil and gas measures under the floor of the Black Sea and these hydrocarbons leak into the monimolimnion as a baseline food source. Fish and fisherfolk at the surface pootle about oblivious of the dissolved carbon dioxide, methane and hydrogen sulphide . . . so long as the lake waters do not mix up. I described how a catastrophic mixing in Lake Nyos released a suffocating deep carpet of carbon dioxide in 1986 which killed 1700 people and millions of animals. Armageddonists speculate that a major earthquake or a meteor strike in the Black Sea might bring all those poisonous gases burping to the surface and kill everyone on the littoral like a dry tsunami. The Black Sea contains about 3 million times as much water+stuff as Lake Nyos but the killing gas-clouds won't scale up to kill everyone [1700 x 3million] on the planet . . . they think.