Thermal Stratification in Ponds and Lakes

Understanding Pond and Lake Stratification and Pond Aeration

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Pond and Lake Stratification


The Use of Pond Aeration and Water De-stratification Devices will Prevent Thermal Stratification from occurring in A Pond and Lake. The Use of a Pond Aerator that circulates deep water will reduce thermal stratification and provide Much needed oxygen to the water.

Many ponds and lakes around the country undergo a regular yearly process known as thermal stratification.

A pond that is thermally stratified means that there is an obvious change in temperature, as the water gets deeper. Most of us noticed this in the summer while swimming. If your pond is experiencing thermal stratification, you probably noticed that the deeper water is colder than the surface water. Stratification is not a major concern, but it can contribute to pond problems if certain conditions arise.

An understanding of pond stratification can be valuable in helping you manage your pond effectively

The Thermal Stratification Process

Normal thermal stratification of a pond, usually begins around May or early June and typically lasts until September or early October. Ponds can stratify because water reaches it's highest density at about 39°F. Water becomes less dense (lighter in weight) both above and below 39°F. Soon after ice melts in the early spring, water temperatures throughout the pond rises to above 32°F. Wind blowing across a pond's surface cause the water to pile up on the downwind side. As the water moves downward, across the pond bottom, to the upwind side, the entire pond begins to circulate from top to bottom, creating a more uniform temperature. The wind must be strong enough, for extended periods of time for the pond temperature to remain uniform, as the pond begins to warm during spring. This is a period known as "spring turnover." Unfortunately during many seasons, the wind is not nearly strong enough, or can even be non existent therefore the natural process of "spring turnover" may never occur.

Early Spring-Overturn

As spring advances and the weather warms, there is typically a period of little or no wind and circulation is reduced substantially. The surface waters warm quickly, causing the initial thermal stratification to develop. In some years, stratification may be eliminated if high winds occur, only to begin again when wind speeds decline again. If wind remains low, the stratification will strengthen and become harder to eliminate.

late Spring-initial Statification

During summer, the temperature change between the warm, upper layer Of the pond or lake (called the epilimnion) and the colder, bottom layer (the hypolimnion) increases. Normal summer wind and weather conditions will not cause the two layers to mix, and the pond will remain stratified until fall. Between the epilimnion and hypolimnin is a relatively thin layer of water called the thermocline. This layer is characterized by a rapid decrease in temperature.

summer-strong stratification

As summer wanes and fall begins, water temperatures in the warm, upper layer begin to cool. As the upper layer cools to approximately the same temperature as the lower layer, thermal stratification disappears as wind can now mix the two layers together. This is known as "fall turnover" and is considered to have occurred when temperatures are the same from the surface down to the deepest area of the pond.


As fall progresses into winter, water cools to 39°F and below, the colder upper layer becomes less dense. This causes the pond to become stratified again. However, in winter the colder water is near the surface rather than at the bottom. Occasionally, strong winds will break up winter stratification for a few days. It will reform once calm weather returns. Often a layer of ice forms, sealing the pond surface and preserving the stratification until ice-out. Little mixing occurs during winter.


Factors Affecting Pond Stratification

Not all ponds undergo stratification. Shallow ponds (< 8 ft deep) often do not stratify because even moderate summer and winter breezes can keep the pond completely mixed. Conversely, deep ponds (> 12 feet) almost always stratify. Winds are rarely strong enough to prevent thermal stratification from forming.

Pond location also plays an important factor in whether a pond will stratify. Ponds that are located in open areas are subject to all the wind's energy and rarely stratify. Only the very deep ponds (> 16 ft) stratify in open areas. Conversely, even shallow ponds that are well protected from wind will stratify.

Weather patterns also influence whether a pond stratifies or not. Calm, hot summers can cause virtually all ponds to stratify in summer whereas cooler, windy summers prevent stratification in many ponds except the deep or well-protected pond. Differences in weather between summers are why a pond may stratify one year but not the next. Occasionally, summer stratification can be "broken up" during a major rain event in which a large volume of cold rainwater causes the pond to prematurely overturn. This can lead to catastrophic summer fish kills (see Ohio State University Extension Fact Sheet A-8-01, Winter and Summer Fish Kills in Ponds).

Pond owners can also prevent stratification from occurring. Small ponds that are aerated extensively often do not stratify because aeration keeps the pond in a continuous circulation and prevents differences in temperature from forming. The same level of aeration in larger ponds and lakes may not prevent stratification. In these instances, additional pond aeration may be required. Should pond stratification be prevented with aeration? That depends on the management goals of the pond owner. Pond owners should learn the pros and cons of aeration, consider their goals, and then make an informed decision.

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