Many lakes and ponds undergo a regular yearly process known as thermal stratification. A lake that is thermally stratified simply means that there is a noticeable temperature gradient as the water becomes deeper. You may have noticed this in summer while swimming. If your lake is stratified, you may feel the deeper water around your lower legs or feet is noticeably colder than the surface water. Stratification is not a major concern, but it can contribute to lake management problems if specific circumstances arise. An understanding of lake stratification can be a valuable tool for effective lake management.
Normal thermal stratification of a lakes or ponds, if it is to occur, generally begins in May or early June and ends in September or early October. Lakes can stratify because water attains maximum density at 39°F. It becomes less dense (lighter in weight) both above and below 39°F. Soon after the ice melts in the early spring, the water temperature throughout the lake rises to 32°F. Winds blowing across a lake's surface can cause the water to pile up on the downwind side. The water moves downward, across the lake bottom, to the upwind side. The entire lake begins to circulate from top to bottom, maintaining a uniform temperature. As long as winds are strong enough, the lake temperature will remain uniform, even as the lake begins to warm during spring. This is a period known as "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 destroyed if high winds occur, only to redevelop when speeds decrease again. If wind remains low, the stratification will strengthen and become harder to destroy.
During summer, the temperature differential between the warm, upper layer (called the epilimnion) and the colder, bottom layer (the hypolimnion) increases. Normal summer wind and weather conditions cannot cause the two layers to mix, and the lake 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.
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 lake.
As fall progresses into winter, water cools to 39°F and below, the colder upper layer becomes less dense. This causes the lake 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 lake surface and preserving the stratification until ice-out. Little mixing occurs during winter.
Not all lakes 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 lakes (> 12 feet) almost always stratify. Winds are rarely strong enough to prevent thermal stratification from forming.
Lake location plays an important factor in stratification development. Lakes that are located in open areas are subject to all the wind's energy and rarely stratify. Only very deep lakes (> 16 ft) stratify in open areas. Conversely, even shallow lakes that are well protected from wind will stratify.
Weather patterns also influence stratification development. Calm, hot summers can cause virtually all lakes to stratify in summer whereas cooler, windy summers prevent stratification in many lakes except the deep or well-protected lake. Differences in weather between summers is why a lake 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 rain water causes the lake to prematurely overturn. This can lead to catastrophic summer fish kills (see the FAQ titled "Winter and Summer Fish Kills in Lakes").
Lake owners can also prevent stratification from occurring. Small lakes that are aerated extensively often do not stratify because aeration keeps the lake 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 aeration may be required. Should lake stratification be prevented with aeration? That depends on the management goals of the lake owner. Lake owners should learn the pros and cons of aeration, consider their goals, and then make an informed decision.
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Acknowledgement to William E. Lynch Jr.
Ohio State University Extension Associate, Aquatic Ecosystem Management
Copyright © 2001 BLU DOT INC. All rights reserved.
Revised:
03/09/05.