Hydrogen Sulfide layers in water

[codecruncher]

This page on Seasonal Lake Stratification I found pretty interesting and mentions hydrogen sulfide.



Excerpt:
During summer stagnation the lake bottom can become anoxic (i.e., without oxygen) and anaerobic bacteria begin to decompose organic material without the aid of dissolved oxygen.  If dead algae accumulate at a faster rate than bacteria decompose the organic matter, sediment deposited in the lake will be rich in organics.  This is likely because without thorough mixing to provide the surface water with nutrients from the bottom, the algae eventually begin to limit the available nutrients in the epilimnion. Lack of available nutrients can cause large die-offs of algae, adding to the organic matter on the lake bottom.  Frequently, anaerobic bacteria produce hydrogen sulfide gas (H2S), so the organic-rich sediment may have the odor of “rotten eggs”.

How does the H2S gas end up becoming a floating layer in the water column instead of simply escaping into the air on the surface?