How do fish survive in a frozen pond?

Aquatic life is a wonderful world in the depths of rivers, oceans and lakes and ponds. During winter when the temperature falls to a significant degree, we notice that the water in the ponds, lakes and the like, freezes. How then does aquatic life survive?

The answer to this question is a basic property which water exhibits. Liquids in general expand when their temperature is increased and contract when the temperature is lowered. Expansion causes a decrease in the density while contraction increases the density.

Water behaves in a slightly different manner than other liquids. When the temperature of water is decreased, it contracts until its temperature drops to 4°C. Its density goes on increasing and is maximum at 4°C. If the temperature of water is further decreased to 0 C to form ice, water starts expanding instead of contracting and its density decreases. This unique property is known an ‘anomalous expansion of water’.

This property of water to expand below 4°C is what causes water in the ponds to freeze from the top. A drop in the temperature in the surroundings causes the water to freeze. Ice which is less dense and hence lighter than water causes the water to be displaced below it. Fishes and other aquatic forms of life are thus able to survive in ponds even when the top most layers are frozen.


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