explain why seasonal stratification of temperatureand oxygen takes place in deep

Hey crazycat,

The thermal stratification of lakes refers to a change in the temperature at different depths in the lake, and is due to the change in water's density with temperature. Cold water is denser than warm water and the epilimnion generally consists of water that is not as dense as the water in the hypolimnion. However, the temperature of maximum density for freshwater is 4 °C. In temperate regions where lake water warms up and cools through the seasons, a cyclical pattern of overturn occurs that is repeated from year to year as the cold dense water at the top of the lake sinks. For example, in dimictic lakes the lake water turns over during the spring and the fall. This process occurs more slowly in deeper water and as a result, a thermal bar may form. If the stratification of water lasts for extended periods, the lake is meromictic. Conversely, for most of the time, the relatively shallower meres are unstratified; that is, the mere is considered all epilimnion.

Lake stratification is the separation of lakes into three layers:

Epilimnion - top of the lake.
Metalimnion (or thermocline) - middle layer that may change depth throughout the day.
Hypolimnion - the bottom layer.

Hey, check out this awesome resource...

http://faculty.gvsu.edu/videticp/stratification.htm