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The Behavior of Lakes
Nature Bulletin No, 320-A November 9, 1968
Forest Preserve District of Cook County
Richard B. Ogilvie, President
Roland F. Eisenbeis, Supt. of Conservation
THE BEHAVIOR OF LAKES
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. For example, it is
remarkable because it expands, becomes lighter and floats when it
freezes into ice. If, like most 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 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
through 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 , 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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Update: June 2012
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