What is the role of anomalous behaviour of water in preserving aquatic life in regions of cold climate?

The anomalous behaviour of water is that from 0oC to 4oC it contracts and beyond 4oC it expands. So, the density of water is at the peak at 4oC. However, when the surrounding temperature goes down, the water in oceans and rivers also cools down and the temperature of whole water reaches 4oC. The water therefore reaches its maximum density at this temperature. Below this temperature (4oC), the water layer on the surface expands caused by anomalous behaviour of the water because of it’s decrease in density. Hence, this colder layer remains on top and converts into ice. This ice acts as an insulator and does not allow the temperature of the water layer below it to fall below 4oC. This ensures that a liveable temperature is maintained for aquatic life under the oceans and rivers.