Water is unique in the way it changes density at different temperatures. Unlike almost all other liquids, water is most dense at 39 degrees Fahrenheit 4 degrees Celsius , and is lighter at both warmer and colder temperatures.
In other words, when water reaches the critical temperature of 39 degrees Fahrenheit, further cooling causes the water molecules to become less dense and rise to the surface. This unusual characteristic allows water to form distinct layers within an otherwise uniform liquid. This phenomenon explains why ice forms at the surface and does not sink.
There are two ways that help visualize this phenomenon. First, think of diving down into a lake in the summer. The top layer of water is very warm. As you swim deeper you often feel a distinct and sudden drop in temperature. Now imagine it is winter and the lake is frozen over. Solid ice floats and stays on top of the lake because it is less dense than liquid water, despite the ice being colder than the water it floats on.
The weather keeps the water near the surface cool, making it less dense than the warmer water deep in the lake. Ice also acts as an insulating blanket, preventing deeper lakes from freezing completely solid. Due to seasonal changes in sunlight intensity, surface water temperature begins to transition in the spring and fall. In the spring, the water surface warms. In fact, water temperatures in this layer typically range from F C even during the hottest summer months.
The epilimnion and thermocline, however, seal the hypolimnion from any contact with the atmosphere. Since it is the deepest layer of water in the pond, little light penetrates to it and plants which produce oxygen can not live here.
Decomposition of plants and animals which sink to this depth and the decay of organic material which washes into the pond rapidly uses all the dissolved oxygen found in the hypolimnion. By mid-July this layer may be completely devoid of oxygen. Fish can not survive in the hypolimnion so there is little point in fishing the bottom of the pond in the summer. As temperatures drop toward 50 F in the fall, the epilimnion cools and becomes both more dense and heavier.
This cooler, more dense water sinks and fall winds stir the epilimnion which gradually erodes the thermocline. Eventually, the pond waters become uniform in density and temperature. These changes are due to the pond-wide circulation of decaying particles of organic material and gasses which accumulated in the hypolimnion all summer. Over time, the freely circulating water becomes thoroughly oxygenated from the surface to the deepest part of the pond.
As winter approaches, water in the pond reaches its point of maximum density at 39 F. A layer of colder, less dense water soon forms on top of the deeper 39 F water. Eventually, this colder water reaches 32 F and a layer of ice forms on the top of the pond. This gradual separation results in a pond which has colder water on top of warmer water. As the winter goes on, the water at the bottom of the pond becomes depleted of oxygen from the decay of organic material just as it does in the summer.
Fish move higher in the water column as the winter progresses. Each spring the ice melts and the sun and seasonal breezes warm the top layer of cold, less dense water. Eventually, this upper layer of water reaches 39 F, becomes heavier, and sinks. Soon the pond achieves a uniform temperature and density throughout the water column and it becomes well mixed.
Obviously, no two ponds are exactly alike. Some ponds stratify strongly during the summer and some remain well-mixed from top to bottom. Ponds in northern Missouri tend to have well developed periods of winter stratification.
Ponds in the southern part of the state may have only brief periods of winter stratification. The depth of your pond has a lot to do with its annual stratification cycle. Shallow ponds may never truly stratify, or do so for only brief periods of time. They are warmed or cooled to uniform temperatures quite quickly and are readily mixed by high winds.
A floating fountain can also increase circulation, which increases oxygenation and gas release, but is only recommended for lakes less than 15 feet deep. Mitigating the harmful effects of lake turnovers is important for the health of fish and plants in the lake. Healthy lakes maintain that balance of consistent temperatures and oxygen levels throughout the water. Sub surface aeration systems are the most helpful resource in maintaining healthy water consistency and quality as you prevent lake turnover.
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