A volume of water is heaviest at 4 degrees Celsius (39.2 degrees F). That is just above freezing. The same volume of water becomes lighter as it gets warmer. So in a lake, warm lake water is at the top and the colder water is at the bottom (except in winter—see below).

As the sun continues to heat the water at the top, the difference in temperature between the top and bottom water becomes greater. Eventually there are 2 distinct layers, the epilimnion at the top and the hypolimnion at the bottom. Between these 2 layers is a third, less distinct, transition layer called the metalimnion.

Because of the temperature difference (and thus density difference) between the epilimnion and hypolimnion, they don’t typically mix together during the summer. It takes a major climactic event to accomplish this, though the lake will mix in the autumn as the surface water cools.
Often in the summer, the hypolimnion will become depleted of oxygen. The bacteria responsible for decomposition consume the oxygen and access to the atmosphere’s oxygen is cut off by the stratification.

If water becomes more dense as it gets colder, then it should freeze from the bottom up, right? Well, water is most dense at 4 degrees Celsius (or 39.2 degrees F), which is warmer than freezing. So as water continues to cool from 4 degrees C (39.2 degrees F), it becomes less dense and rises back to the top, leaving the slightly warmer water below.

At the surface, the cooler 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 degrees C for a brief time. The lake will mix thoroughly (“turn over”) 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.

Tony Thorpe