Why is the greatest density of water at 4 degree celsius important?

Answer 1

Fishes living in lakes which are prone to freezing in winter, take advantage of this property of water ( having greatest density at 4 degree celsius).

When temperature starts reducing in winter, lake water cools upto 4degree celsius by convection as density increase with reduction in temp.

On further cooling, due to abnormal expansion of water , density reduces & cold water remains at top .Then top layer frezees.Water being bad conductor of heat ,lake water below ice remains at 4 degrees & thus fishes can survive in winter also.

Answer 2

Because water at 4° Celsius is denser than water at 0° Celsius, it sinks to the bottom of lakes and ponds instead of floating on the top and chilling further to the point of freezing.

This means the water at the bottom of the lake doesn't usually become cold enough to become solid, so fishes and other life can survive down there across the winter.

Most things are lighter (less dense) the warmer they are, so this characteristic of ICE being lighter than liquid water that's actually warmer, means life in the lakes and ponds does NOT have to start evolving from scratch every spring.