Why do lakes have three temperatures?

Temperature can vary depending on the time of year and the size and depth of a body of water. A unique phenomenon for lakes is the stratification of the water into layers due to changes in temperature at different depths. These layers occur because as water temperature changes so does its density.



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The epilimnion is the upper, warm layer, and is typically well mixed. Below the epilimnion is the metalimnion or thermocline region, a layer of water in which the temperature declines rapidly with depth. The hypolimnion is the bottom layer of colder water, isolated from the epilimnion by the metalimnion.

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We can thank lake breezes for these cooler temperatures. Lake breezes develop when the land becomes warmer than the water. The warm air over land rises, and gets replaced by the relatively cool air which resides immediately above the lake surface.

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When winter approaches and the air temperature becomes lower than the water temperature, water on the surface will cool off and then sink because it is denser, which forces the now slightly warmer water at the bottom of the lake to the top.

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During the early morning hours, the land and the water start out at roughly the same temperature. On a calm morning, a given pressure surface will be at the same height above both the land and water. A few hours later, the sun's energy begins to warm the land more rapidly than the water.

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Lake water temperatures have been rising with warmer air temperatures (particularly in the winter), more rain than snow, declining snowpack, and earlier snowmelt. Other factors affecting lake water temperature are ice cover, cloud cover, humidity, and wind.

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When water cools down to 0 degrees Celsius (0°C), ice begins to form and floats on top of warmer water; water at the bottom of a lake or river is typically 4°C (if it is not frozen). During the summer, the opposite occurs as warmer water floats on top of colder water. Many factors can affect water temperature.

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When the upper water layers warm in the summer months, they become separated from deep water by a transition zone known as a thermocline. In a thermocline, the temperature decreases rapidly with small increases in depth. This phenomenon linking temperature change with depth is called temperature stratification.

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The hypolimnion is the bottom layer and is colder and denser than either the epilimnion or metalimnion. When a lake or reservoir is thermally stratified, the hypolimnion becomes largely isolated from atmospheric conditions and is often referred to as being stagnant.

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If water were most dense as a solid, lakes would freeze from the bottom up, eventually freezing solid. In that case, little or nothing would survive in the lake. Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below.

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Enough oxygen However, colder water can hold more dissolved gas than warmer water can, so water below freezing holds the most oxygen. Then, because fish metabolism has slowed down, they are using less oxygen. Some fish even go into a state called torpor that decreases their need for oxygen even more.

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The bottom of a lake do not freeze in severe winter. The reason is that ice is a poor conductor of heat hence once the surface is frozen no further heat is liberated or absorbed by water beneath ice. Therefore water below ice never freezes.

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