Nature Bulletin No. 320 Forest reserve District of Cook County
November 8, 1952 William N. Erickson, President
Roberts Mann Supt. of Conservation David H. Thompson Senior Naturalist
In many ways lakes are like living things -- especially a tree. A lake breathes and has a circulation; it is warmed and fed; it harbors many other living things; and in cold weather it goes into a winter sleep. If it were not for the special character of a body of standing water which we call a lake, the things that live in it would be radically different or, perhaps, not exist at all.
Water is a very strange substance in many ways. or example, it is remarkable because it expands, becomes lighter and floats when it freezes into ice. If, like most other substances, water shrank when it changed from a liquid to a solid, it would sink, then, ponds and lakes would freeze from the bottom up and become solid blocks of ice. This would make life impossible for most kinds of aquatic plants and animals and indirectly affect all living things. Further, water is a poor conductor of heat otherwise lakes would freeze much deeper and, again, most living things in it would perish.
As warm water cools it shrinks and becomes more dense and heavy until about 39 degrees Fahrenheit. Then, as the temperature drops toward the freezing point, it starts to expand and become lighter. That trait of having its maximum density about 7 degrees above the freezing point has important consequences for the things that get their oxygen from water. It causes a complete circulation or overturn at least twice each year in spring and again in autumn like deep breaths carrying oxygen-rich water to all parts of the lake from top to bottom.
When a pond or lake is covered with ice, all the water beneath it is at or near 32° F, and without movement, as if it were holding its breath and hibernating until spring. Its main chance of getting fresh oxygen is from winter rains or melting snows which bring in oxygen-rich water. There is one other possibility. By a process called photosynthesis, green aquatic plants, with the aid of sunlight, produce pure oxygen even under a layer of clear ice. But if the ice is blanketed with snow, and if there are no rains or melting snows, the oxygen dissolved in the water is slowly exhausted and the fish, then many other forms of life, begin to smother and die.
In spring, after the ice melts and the surface water warms toward 39 degrees, it sinks and pushes the oxygen-starved bottom water up to the surface. At this time, winds may also cause circulation in the entire lake. Later, as the surface water becomes warmer and lighter, only the upper layer is circulated by moderate winds pushing water toward one shore or the other. The colder bottom layer is left undisturbed unless there is the complete turnover, from top to bottom, that sometimes occurs as the result of a violent storm. In the cleaner deeper lakes without too much rotting vegetation there is enough oxygen in the bottom layer to last thru the summer. In others it is used up, so that the fish and other animals are forced up to the surface to avoid suffocation. In July, a cage of minnows may live well at depths of 5, 10 or 15 feet but die within a few minutes when lowered to 20 feet and all greater depths. The circulation or overturn in a lake is important only to the animals that breathe by means of gills. For some reason, a few forms of life such as certain worms, insect larvae and single-celled animals can live a long time without oxygen.
With the coming of chilly nights in autumn, the surface water cools and sinks again, forcing up the bottom water. Then, with the temperature near 39 degrees, winds cause the entire body of water to circulate and our lake gets its second long breath before it freezes over and goes into its long winter sleep.
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