In cold countries, when the temperature falls below the freezing point, water, lakes and rivers get frozen. Under such conditions, how do aquatic plants and animals survive?
Generally all liquids expand on heating, but water is an exception. At 0°C, if water is heated gradually, its volume decreases and this contraction continues till the temperature rises to 4°C. Above 4°C water starts expanding and keeps on expanding with further rise in temperature. This shows that at 4°C, the volume occupied by a given mass of water is minimum. In other words, the density of water at 4°C is maximum. This irregular expansion of water is known as anomalous expansion.
This anomalous expansion of water plays an important role in nature. Due to this only the upper layers of water in the ponds and lakes in cold countries get freezed. Lower layers remain as water, and as a result aquatic animals survive.
In cold countries, during winter when the atmospheric temperature is very low, the upper layers of water in the lakes and ponds start cooling. When the temperature of the surface layer falls to 4°C, the water acquires maximum density and sinks down. The lower layers of water then rise up. This water also gets cooled to 4°C and again sinks down. The process continues till the temperature of the whole water falls to 4°C. As the temperature falls below 4°C, the density of water decreases and as a result water at the surface becomes lighter and does not sink down. The surface water finally freezes while the lower layers remain at 4°C. As ice is a bad conductor of heat, freezing in the lower layers is a very slow process. Thus underneath the frozen layer, fish and other aquatic animals and plants survive. Had the expansion of water been uniform, the lakes or ponds would have been completely frozen, along with the aquatic plants and animals. In this situation no plant and animal would have survived.