Biological importance of the anomalous expansion of water
The peculiar expansion of water has an important bearing on the preservation of aquatic life during very cold weather (Fig. 15.15). As the temperature of a pond or lake falls, the water contracts, becomes denser and sinks. A circulation is thus set up until all the water reaches its maximum density at 4°C. If further cooling occurs any water below 4 °C will stay at the top owing to its lighter density. In due course, ice
forms on the top of the water, and after this the lower layers of water at 4°C can lose heat only by conduction (see chapter 17). Only very shallow water is thus liable to freeze solid. In deeper water there will always be water beneath the ice in which fish and other creatures can live.