The Dynamics of Lakes
Lakes, ponds and reservoirs go through fascinating changes throughout the seasons that are not observed in rivers and streams. Most of these changes are due to the unique properties of water. For example, water becomes more dense as it cools until it freezes, when it floats. Wind and waves are powerful forces too, allowing for the mixing of lake layers.
During winter, a protective layer of ice forms on the surface. This actually insulates the waters below, allowing fish to survive the winter.
As the ice melts, cooler surface waters mix with the warmer insulated layer below. Nutrients and oxygen are able to spread through out the entire lake body, which allows for plant growth.
As sun heats the lake in the summer, layers once again set up. This time a warm layer of water sits above cooler water below. The lack of mixing can mean lower oxygen levels in the cool, deep layer.
Colder weather cools down the surface of lakes, until the layers mix and oxygen and nutrients are once again able to circulate throughout the entire lake body.