The region in a body of water that divides the warmer, oxygen-rich surface layer from the colder, oxygen-poor deep water.
The warm water above the thermocline is relatively shallow: for most of the world's oceans the top layer is only about 150 to 300 feet deep.
The -cline of thermocline comes from a Greek word meaning "to slope" and refers to the gradual series of temperature changes that occur in this kind of zone. In a freshwater lake there is very little mixing between the layers of warm and cold water during the summer. During autumn, however, a major turnover occurs. The oxygen-rich surface water cools and sinks to the bottom, and the nutrient-rich water near the bottom is displaced to the top. The cycle is reversed the following spring.