When the surface of a body of water, say a pond or a lake, reaches near freezing, it becomes more dense... and thus, it SINKS. The colder water gets, the HEAVIER it gets. That accounts for the seasonal "turnover" of otherwise stagnant lakes, so that oxygen from the surface water is driven down to the bottom and freshens the entire lake.
BUT --
If water gets denser as it gets colder, surely the freezing of the water would begin at the BOTTOM of the lake, where denser water sinks, and then gradually freeze the whole lake from the bottom up... wouldn't it?
Well, no. Because freezing is actually a molecular STRUCTURING of water, forcing the molecules into a kind of latticework structure that actually takes up MORE space, and is therefore LESS dense.
Water is at its most dense between 40F and 32F. Once it passes 32*F, it becomes rapidly LESS dense, when it freezes and assumes the latticework crystalline state of ice.
Freezing therefore causes the frozen water to float to the SURFACE. The resulting sheet of surface ice actually INSULATES the rest of the water from getting colder, as it prevents the surface water from SINKING. The rest of the pond can remain between 40 and 32 degrees, as it is oxygen-enriched by the previous process of the sinking of the colder water.
See? The colder the water gets, the more the oxygen-rich surface water sinks and brings oxygen to the rest of the water... UNTIL freezing, at which point the coldest water, as ice, rises to the surface and protects the rest of the lake from FREEZING SOLID.
Thus saving the fish and other aquatic life through the winter.
What a wonderful stroke of random, unplanned luck when water was created by nature, by chance, without any kind of design.