How do subsurface lake waters heat up in the spring?

It has something to do with a process that is called as vertical mixing. When this process takes place, overturning or "turning over" happens to the lake which sometimes causes fishes living in the lake to be killed in the process. It is important to know that density increases together with depth since vertical mixing revolves around this. So when the ice is thawed and the surface water heats up, the density would increase which would, in turn, sink it to the bottom. Now the water below moves up and is heated and then sinks back. This continues until stability is achieved.