Hydrogen sulfide is formed by anaerobic bacteria under conditions in which dissolved O2 levels are very low or absent. I don't know about the water circulation patterns in the Black Sea, but there may be bottom water areas with slow circulation current turnover rate in which dissolved O2 has been depleted up-current by aerobic micro-organisms (e.g. in organic decomposition processes), and thus where sulfates or sulfur-containing organic molecules can be reduced to hydrogen sulfide by anaerobic bacteria.

In the deep part of the Black Sea, two circulation cells are generated. Water moves counterclockwise. In the centers of the cycles, water moves from the bottom up. The mechanism of oxygen exchange in deep waters is present.

Borys--Oh, but perhaps there are some stagnant bottomwater (inter-nodal?) areas where turnover times are very slow? Otherwise perhaps there are seabed areas of exposed sulfide-containing minerals--would expect most seabed areas to be covered by sediments from materials that have dropped out of the water column over the millenia; however I speculate maybe could be some seabed areas where native sulfide-containing rock is exposed due to special bottomwater current conditions and/or active fault zones. Do you know if there are areas of seabed geothermal springs?

I personally took samples of the bottom sediments of the Black Sea at a depth of 2 km. There is blue silt. I didn't see sulfides. In addition, there are places at the bottom where oxygen enters the water from the earth's crust. See the graph from my discussion "Combustion of fuel takes oxygen from the atmosphere. There is more oxygen at the poles than in the tropics. Oxygen is released from the lithosphere?". It shows where oxygen is released on the seafloor and graphs oxygen content at different depths.

As it has all been said: Sulfide is formed from sulfate in an oxygen-poor environment. The decisive factor is the lack of oxygen. Sulphide is also formed in fresh water if there is no circulation.

Do you confirm my thought? The main thing is the presence of sulfur in the water. The second is the lack of oxygen.

My remark. There is no sulfur in fresh water. There is no sulfur in the continental crust. There is sulfur in the ocean. There are more than 2 grams of SO4 ion in a liter of sea water, and more than 2 kg in a cubic meter of water. If you block the Bosphorus, then hydrogen-sulfide will disappear in the waters of the Black Sea. Did I understand you correctly?

There is sulfate in fresh water, not as much as in salt water, it is mg/l, not grams. Still, it's enough to smell like rotten eggs. And to keep various sulfur bacteria alive. https://www.encyclopedia.com/places/britain-ireland-france-and-low-countries/british-and-irish-political-geography/freshwater

Arne's comment is accurate, since all organisms contain some sulfur (e.g. in the amino acids cysteine and methionine), and nearly all fresh waters contain some living organisms, if only micro-organisms.

However, Borys does have a point; it remains a mystery how there exist such large areas of anoxic and sulfide-rich waters in the Black Sea (see e.g. https://depths.energy/en/environment-of-the-black-sea/). It seems that this widespread anoxic & sulfide rich condition must be related to the patterns of water inflow/outflow and water circulation/turnover, gyres etc. in the Black Sea (including of course the Bosphorous Strait currents); a couple of fairly recent such investigations are linked here: Article Interannual Change in Mode Waters: Case of the Black Sea

Article Variation of the Cold Intermediate Water in the Black Sea Ex...

Looks like pretty variable and complicated mixing conditions within the Balck Sea; many investigations likely needed to unravel mystery of the presence of the widespread anoxic & sulfide rich water conditions.

I agree.

But there is a nuance.

In the aquatic environment, there is a deficiency of substances that are necessary for the growth of plants and animals. If nitrogen and phosphorus are needed for plant growth, then sulfur is needed for animal growth (for the formation of body protein mass). I think that for this reason there is no excess of sulfur in fresh water.

It is not very often that a rotting freshwater pond smells of "rotten eggs".

Meromixis is a phenomenon caused by density differences, which means that the water does not circulate. It could be salt water, which has penetrated and settled on the bottom. Or it can be substances that come from the ground. For example, calcium or iron. In modern times, road salt can trigger meromixis. Under such conditions, H2S is formed.

Look at the vertical section of the Caspian Sea.

http://algology.ru/1427

These are oxygen concentrations. There is practically no oxygen at the bottom. These are the conditions of "Meromixis". There is no inflow of ocean water into this sea (no sulfur input). There is no hydrogen sulfide either.