Water freezes from the top down
One would assume that if water becomes more dense as it gets colder, it should freeze from the bottom up. As explained earlier, however, water is most dense at 4° Celsius (or 39.2° F), which is warmer than freezing. So as water continues to cool from 4° C (39.2° F), it becomes less dense and rises to the top, leaving the slightly warmer water below.
At the surface, the cool water is exposed to freezing air temperatures and may eventually freeze. Once ice forms, the water beneath cannot be mixed by the wind.
When the ice melts in the spring, the entire water column will be at approximately 4° C for a brief time. The lake will mix thoroughly (“turnover”) with just a bit of wind. A calm, warm day can heat the surface water and initiate the stratification process.
Stratification may also occur due to changes in salt content as well as temperature. Oceans, particularly in places where freshwater enters, may be stratified by salinity. The problems with Gulf Hypoxia may be attributed partially to the inability of the dense and salty bottom water to mix with the oxygen-rich, less salty water above.