Question

How is aquatic life benefited by the fact that water has max density at 4 degree celcious

Answer 1

Water contracts till it reaches a temperature of 4 degree Celsius, but contracts after that unlike any other compound.

Now, during winters when the atmosphere's temperature drops, the temperature of water drops and say the temperature of the whole water body drops to 4 degree Celsius. Below that temperature when water starts to freeze it expands and hence it's density decreases which makes the colder liquid to remain on top and the temperature of water below it does not drop. The ice starts forming from top and acts as an insulator.