Why does ice form first at the surface of freshwater lakes?

Ice forms at the surface of freshwater lakes primarily due to the unique density properties of water.12

As a lake cools during the autumn and winter, the water becomes denser and sinks until it reaches its maximum density at approximately 4 °C (39 °F). As the surface water cools further—dropping below 4 °C toward the freezing point of 0 °C (32 °F)—the water molecules begin to expand and form a rigid, open hexagonal lattice.3245

This structural change makes ice less dense than the surrounding liquid water. Because of this lower density, the ice remains buoyant and floats on the surface rather than sinking to the bottom. This surface layer of ice acts as an insulating barrier, protecting the aquatic life below from the extreme cold of the air above.4523