Pond Boss Forums INTRO Questions & Observations Thermocline

We are taught that heat rises. For the most part, that's true, except for special cases with water.
As sun, wind and waves carry energy, in the form of heat, into water, that same heat tries to escape by rising.
Warm water expands, sucks in more heat, and more heat tries to rise.
Late spring, early summer, that heat can only go "down" so far, then it rises.
That's what creates the thermocline.
Look at water as a layer cake. Distinct layers seperate warm layers from the cooler waters below.
The consequences are...warmer water becomes most productive water. Deeper, larger lakes have more surface area, more volume, therefore the warm layer is deeper. The lower layer can't mix with surface air, and becomes stratified away from the rest of the lake. As a result, no contact with air means this water loses its oxygen content. Fish waste, organic matter, beer cans, normal bio byproducts make their way into the lower layer, with little outlet for escape.
So, fish can come into the cold layer, but they can't stay. Some fish head for cover, other species suspend. But, if they go down and stay down, they die.
Water is an amazing substance. It's most dense at 39 degrees, F. Warmer water is less dense, as is colder water. That's why ice floats and steam rises. So, in summer, the upper layer is warmer, but during cold months, the lower layer is warmer, somewhere near 39 degrees.
Great question.