How does anomalous expansion of water help the aquatic organisms in cold climates?

In lakes, as the temperature drops, the surface water cools, sinks and mixes with the deeper water, this process continues until the temperature reaches 4°C. Beyond that point, the surface water expands, becomes lighter and it does not sink. So, further cooling takes place only at the surface while temperature of lower layers of water does not change. The water at the surface freezes to form a layer of ice that insulates the deeper water and allows aquatic organisms to survive in the water beneath the ice.