Why does ice float on water? how does this affect life in earth’s aquatic environments?

The density of the ice is less than water. That's why it doesn't sink in water. This is a basic chemistry rule. Less dense objects tend to float in the liquid with greater density. Due to this, the regions where water freezes due to low temperature, the layer of ice is formed on the top of the ocean or sea or river water. This breaks the contact of oxygen with the air above the water body and the organisms are not able to get sufficient oxygen.

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