This may not be the correct place to ask a question about limnology (which I just found out is the science of lakes) but it seems like the place most likely to produce a good answer.
In my diving lessons I learned that there is a layer of water in lakes where temperature suddenly drops. Judging from the pictures at Wikipedia: Thermocline the teacher was talking about the metalimnion which also sort of makes sense considering the depthof the lake he was talking about.
It is however still no clear to me why these layers form.
The article seems to suggest that the top layers warm-up due to sunlight while the bottom layer stays cold and there is little mixing because the colder water is denser.
But shouldn't that lead to a somewhat linear temperature gradient instead of layers?
Edit: Here is a nice picture showing the different layers from loon lake: