|Thermal stratification, which contributes much to lake structure, is a direct result
of heating by the sun. Thermal stratification is the phenomenon in which lakes develop
two discrete layers of water of different temperatures: warm on top (epilimnion) and cold
below (hypolimnion). These layers are each relatively uniform in temperature but are
separated by a region of rapid temperature change (the metalimnion or thermocline).|
The distribution of water as measured by temperature is a reflection of the
differences in its density. Colder, denser water is on the bottom; zone of rapid change
above, and warmer, less dense water at the surface of the lake. Although storms may stir
the warm waters of the epilimnion into furious motion, little energy is transmitted through
the thermocline to the cool quiescent hypolimnion. Consequently, the epilimnion is often
called the mixed layer.
The remarkable mixing of the epilimnion is explained by the greater density change per degree of temperature change in warm water than in cold. Thirty times as much energy is required to completely mix equal volumes of 24 degrees Celcius and 25 degrees Celcius water as it takes to mix the same volumes of water at 4 degrees Celcius and 5 degrees Celcius. The resistence to mixing linked to these density differences maintains the stratified structure in lakes, which is important to the distribution of dissolved chemicals, gases, and biota. Thermal stratification is most characteristic of deep lakes. Shallow lakes never stratify for more than short periods of time.