One of the properties of water that permits life survival in all bodies of water that freeze over, is its unique expansion and contraction graph vs temperature. Water contracts and gets denser down to 39 degrees Fahreneheit. Then as it gets colder than 39 it begins to expand again until it reaches 32 degrees and freezes, which is why ice floats. If this were not so, ice would sink to the bottom and ponds would freeze from the bottom up instead of from the top down.
Convection would rapidly produce solid blocks of ice out of ponds that merely have a crust of ice on them in the winter. This protective property of water can be nullified by deeply aerating a partially frozen pond and breaking up the protective stratification so that the deepest parts no longer are in the 39-42 degree safety zone of many fish. There are a number of posts regarding winter aeration that illustrate this principle.
