Why does surface ice help water beneath a lake remain liquid in winter?

Surface ice helps keep the water beneath a lake liquid during winter by acting as an insulating cover.1

When water cools to the freezing point, it forms a layer of ice on the surface. Because ice is less dense than liquid water, it floats rather than sinking to the bottom. Once this ice cover is established, it effectively blocks the exchange of energy between the cold air above and the warmer water below.213

By serving as a barrier, the ice prevents the cold air from directly cooling the deeper water, which helps regulate the temperature beneath the surface and protects aquatic life during cold periods. While heat losses continue, they are manifested primarily in the production of more ice at the surface rather than in dropping the temperature of the water column below.12