Thermoclines are layers in a body of water where there is an abrupt change in temperature. In the ocean, they are usually mobile and temporary, (submarines hide under them because they tend to reflect sonar) but in small, enclosed bodies of water they often separate fresh, oxygenated surface waters from deep layers saturated with poisonous gases in solution from decomposing algae, etc. They are also anerobic. The almost-landlocked Baltic has had one for years, the surface waters can be fished, but the depths are toxic, without oxygen, and with hydrogen sulfide gas in solution. There is a similar one in the Black Sea.

A few years ago a volcanic lake in Africa underwent some kind of temperature reversal where the toxic water underneath changed places with the clean, productive surface waters. At lower pressures, the H2S precipitated out of solution and bubbled out killing hundreds of villagers who lived along the shore.

And this was a completely natural phenomenon.