Ice is formed at 0 degrees Celsius. Thus, ice will remain at the top of the water body. Sun shining through the ice will serve to warm the water below slightly, keeping the temperature just above freezing. Water at 4 degrees C is the most dense, and will sink to the bottom and be replaced by lighter 1 - 3.9 degrees C water. In addition, temperate lakes stratify during the summer because of water's temperature-dependent density.
Stratification is a seasonal phenomenon.
The heat of the summer substantially raises the temperature of the upper water layers. As water temperatures increase, the density decreases. During the spring, stratification will break down allowing mixing of oxygen and nutrients. Eventually, the surface water layer falls below 4 degrees C, becomes less dense, and remains at the surface. EPA has developed an equation to determine maximum weekly average temperatures for growth and survival of fish species.