Most of the time I haven't had enough time to explain what takes place in fresh water ecosystems, but you need to know about thermocline. It's very important to your fishing. I hope I can help you to understand it. Lakes go through seasonal water temperature changes. There are times when the layers mix and then separate.
Most lakes have three layers.
The epilimnion, is the top layer. When water temperatures rise in early summer, so does the density between the surface layer and the deeper layers. The hot surface water forms a thermal energy which opposes wind energy. As this occurs, water doesn't mix very far down. This is how barriers are formed.