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Conditions
Thermal stratification is most common in deep bodies of water in colder climates. During the winter and early spring, the water has a relatively even temperature distribution until heat from the sun and the air above the water begin to increase its temperature. As the water is heated from above, the density of the water near the surface decreases, limiting its ability to mix with the water near the bottom.
Layers
Thermal stratification divides water bodies into three horizontal layers known as the epilimnion, metalimnion and hypolimnion. The epilimnion is the highest and warmest layer; the metalimnion is the transitional layer between the warm upper regions of the water and the cool layer near the bottom, the hypolimnion. The metalimnion layer also includes the thermocline -- a region of water between the upper and lower layers where the change in temperature is greater than 1 degree Celsius per meter.
Exceptions
Thermal stratification is uncommon in warm, shallow bodies of water found in southern regions. The tendency of water to form horizontal layers is also disrupted by extremely low temperatures. When the temperature of a lake or pond drops below the freezing point, the coldest layer moves to the top. Cold bodies of water that are above freezing are called isothermal, because the temperature of the water becomes relatively even at all depths.
Impact
The process of thermal stratification directly impacts aquatic life. The temperature of the water effects how active aquatic organisms are and how quickly they can grow. Most aquatic organisms prefer warmer temperatures and will become more active during warmer periods of the day and year. The temperature of the water also impacts its chemical properties. Cold water can hold more oxygen and carbon dioxide than warm water, while the opposite is true of dissolved minerals. The quantities of minerals and oxygen available have a significant impact on the type and amount of aquatic life a body of water can support.