This is just a guess and not a the guaranteed correct answer, but its in my head for some reason, must have read or heard it somewhere. Anyways, I think its the thermocline, which is the point at which there is a big temp. change in the water. A cool property of water is that a 4 degrees (celsius) it is most dense, and sinks to the bottom, therefore in the summer the upper layer of water would be warmer, then you would have your thermocline layer, then your bottom layer of 4 degree water. And it the winter, top layer of ice, cold layer of water, thermocline layer, then bottom of 4 degree water. Good thing water does this or lakes would freeze bottom up...I think