Water is its densest (heaviest) at 4 degrees Celsius. Why is this property of water very important for ensuring lifeforms living in lakes, ponds, rivers and brooks in Canada, during the winter months, continue to be able to live?

i rlly dont understand what this question is asking

It means the water at 4 degrees above freezing sinks to the bottom where the fish can live while the cold water up on the surface turns to ice.