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As the lake turns: The seasonal cycle of lake stratification

Published Sept. 24, 2015
In the summer months, deeper lakes like Lake Eufaula will stratify because of temperature but as the fall months bring in cooler weather the layers will mix as the water temperatures equalize.

In the summer months, deeper lakes like Lake Eufaula will stratify because of temperature but as the fall months bring in cooler weather the layers will mix as the water temperatures equalize.

TULSA - During the summer months a naturally occurring phenomenon unfolds below the surface of many Tulsa District, U.S. Army Corps of Engineers lakes, as organisms compete for precious oxygen molecules. As autumn approaches, and temperatures cool, many lakes will undergo changes.

Photosynthesis and Fish

As sunlight penetrates the lake’s surface, or epilimnion, photosynthesis occurs and oxygen, created by aquatic plant life like algae, is released into the water in the form of dissolved oxygen.  Surface waters also warm much more quickly than those deeper in the water column.

When fish swim through the water, their gills process this dissolved oxygen allowing them to breathe.
With so much light during the summer months, it seems natural that the water would have plenty of oxygen for fish, right? Not necessarily.

In warm water, oxygen becomes less soluble making oxygen less available for fish and other aquatic life.
Striped bass, for example, desire at least 5 milligrams of dissolved oxygen per liter of water.

When the water temperatures rise, the surface becomes less dense and oxygen molecules are less soluble, sometimes resulting in less hospitable conditions for fish, particularly when fish and other aquatic life are crowded in a limited volume of water.  This can become particularly intense during nighttime hours when photosynthesis is not producing dissolved oxygen and oxygen is consumed by decomposition.

Thermal Stratification

In deeper lakes like Lake Texoma and Lake Eufaula, an event called thermal stratification occurs during the summer months. Thermal stratification is simply the development of persistent layers, characterized by dense, cold water called a hypolimnion near the bottom of the lake and the warmer, less dense epilimnion near the surface.

“In the summer, a persistent layer of warmer water containing oxygen develops to a depth of around 20 to 25 feet from the surface,” said Steve Nolen, Chief of the Tulsa District’s Natural Resources and Recreation Branch. “When the water on top warms it causes a density difference between the two layers and these layers often don’t mix.”

The hypolimnion gets little or no light, minimizing or eliminating the possibility of photosynthesis. This layer also is not subject to aeration through wind and wave action.  This cold, deep layer is often oxygen-deficient and unwelcoming to fish.
“At the bottom of the lake you get what’s called an anoxic environment,” said Paul Balkenbush, environmental specialist at the Lake Texoma Project Office. “The fish won’t go down there because there is so little oxygen.”

Life Goes On

Though fish are absent, life is present in the hypolimnion in the form of bacteria feeding off decaying matter from the lake. These bacteria deplete the water of oxygen and release gases that are trapped in the hypolimnion.

“The lower layer doesn’t get oxygen and bacteria and organisms in the sediment and water column use the oxygen that is available as well as create conditions that promote release of various chemical compounds from the sediments,” said Nolen.
A relatively thin layer of water characterized by a steep temperature drop and depleted oxygen concentrations lies between the epilimnion and the hypolimnion. This layer, called the metalimnion or thermocline, is where you’ll find many fish.

If you’ve ever gone scuba diving in the summer, you’ve probably experienced a thermocline as you descend. You can’t see it but you can feel it as the temperature quickly drops.

When thermal stratification occurs, larger fish will tend to hang out in the metalimnion, where more dissolved oxygen is available and water is cooler.

Bigger Isn’t Always Better

Unfortunately for the larger fish, the oxygen in the metalimnion is limited as more fish use the available resource.

“You get what’s called a temperature/oxygen squeeze,” said Nolen. “If the dissolved oxygen falls too far hypoxia occurs and the fish get stressed, stop feeding, and sometimes die. That’s when you see fish kills.”

In such conditions, the big fish are at a disadvantage. Larger fish species generally require more oxygen to survive. 

“If you get a big fat striper, it has a lot less tolerance for temperature and low oxygen, than smaller fish,” said Balkenbush. “Temperature and oxygen changes are tough on those big fish. The little fish aren’t as susceptible to oxygen depletion.”

What’s that Smell?

In Oklahoma, lake surface temperatures begin to drop in October. When the epilimnion falls to a temperature approaching that of the hypolimnion, wind mixing causes the layers to disappear as the lake “turns over” or mixes.

“When lake turnover occurs, you’ll sometimes know it by the smell,” said Nolen. “Gases, like hydrogen sulfide, are mixed in surface water and you’ll notice a rotten egg-like smell. At the same time, mineral compounds containing iron and manganese become less soluble and are adsorbed back into the sediments.  Water supply users may experience brief periods of taste and odor in their finished water.”

Although lake turnover may occasionally be offensive to our olfactory senses, it’s actually good for the health of the lake as oxygen from the surface is mixed downward and nutrients from the lake bed are forced upward.

“It’s a natural process and it restores dissolved oxygen to all levels in the lake water column,” said Nolen. “It makes the entire water column once again useful for fish and other organisms that require oxygen.”