LSU AgCenter scientist explains small pond turnover

(10/26/23) BATON ROUGE, La. — Following the extremely hot temperatures of the past summer, some landowners have experienced what is commonly called pond turnover.

LSU AgCenter aquaculture specialist Greg Lutz said several phenomena are impacting Louisiana ponds because of the extremely hot, dry weather, and most of them involve something called phytoplankton.

“The characteristic greenish tint of most pond water is the result of single-celled algae suspended in the water,” he said. “These microscopic plants are the principal source of oxygen in most ponds, but they can also cause problems at high concentrations.”

Lutz said high nutrient levels from fish feed and fertilizer runoff can combine with warmer temperatures to result in extremely high levels of phytoplankton.

“If you’ve ever waded out into a pond in the summertime and felt a layer of cold water several feet down, near the bottom, the same factors that cause that water to be cold also cause it to be a potential problem,” he said.

This is a sign that the pond may be vulnerable to the effects of a turnover during late summer and into the fall.

Layers of distinctly different temperatures in pond water indicate a phenomenon referred to as stratification.

This is caused by single-cell algae in pond water that thrive during the warm summer weather, Lutz said.

“There can be up to 20 million algal cells in a teaspoon of warm, fertile pond water,” he said. “These microscopic plants not only give the water a greenish color, but they also capture most of the sunlight that hits the pond, keeping the deeper waters shaded and dark.”

Over time, as the water near the surface gets warmer and the water at the bottom gets colder, the pond begins to stratify into layers of water that no longer mix with each other.

Water at the bottom of the pond is isolated from the upper layers due to these temperature differences.

Eventually, the bottom layer has no available oxygen and becomes saturated with toxic compounds like hydrogen sulfide and methane because of all the organic matter, including dead algae, dead fish, fish waste, dead plants and leaves from surrounding trees that accumulates at the bottom of the pond.

When ponds become extremely warm at the surface, which happens every summer throughout the southeastern states, and increasingly up into the Midwest, the fish get squeezed.

If they go too deep, they hit the cold, toxic bottom waters. If they go too shallow, they get stressed by the high temperatures.

“The smaller, deeper and more fertile a pond is, the more severe the layering effect becomes, and the higher the portion of ‘bad’ water at deeper depths,” he said. “I can’t count the times folks have told me, ‘My pond’s only a quarter acre, but it’s 20 feet deep.’ That’s when I try to explain that they have a ticking time bomb on their hands, because they’ve got 4 or 5 feet of ‘good’ water, and 15 feet of ‘bad’ water.”

When a pond has become stratified, the stage is set for a turnover. All that is needed is some disturbance strong enough to mix the bottom water with the upper layer of the pond, leaving the fish with little or no oxygen and exposed to toxic compounds. And Mother Nature has several ways to accomplish this.

“Heavy, cold rains in late summer or fall can often trigger turnovers because the cold water tends to accumulate on the surface and then sink in one spot,” he said. “This displaces some of the bottom water and starts the mixing process.”

An abrupt drop in temperature early in the fall can also have the same effect.

Strong winds also can be sufficient to disrupt stratification and cause turnovers in some ponds, and when tropical storms or hurricanes pass through the state, many ponds get a double whammy of wind and rain.

The telltale signs are a change in the color of the water, usually resulting in a gray or brown, murky appearance. Foul odors, especially a smell like rotten eggs, caused by hydrogen sulfide, are also common, he said.

Depending on how severe the turnover is, fish of all sizes and species may die over a short period of time.

“The options available to respond to a turnover are very limited once it begins. The pent-up oxygen demand from the bottom waters is more than any aerator can counteract,” Lutz said.

Fortunately, if temperatures cool off gradually during the fall, the temperature differences between surface- and bottom-waters will break down slowly over time and most ponds will avoid turnovers.

Lutz said even properly designed ponds will experience turnovers on rare occasions.

“Year-round mixing-type aeration can prevent stratification, but this requires significant expenditures for equipment and electricity,” he said. “This approach relies on bubble diffusors set on the pond bottom, or floating aerators that are modified to pull from deeper water depths.”

If you decide to install a system like this, do so with care.

If your pond is already stratified, then turning on a mixing aerator could actually trigger a turnover.

Either start the process gradually for a short time each day, or better yet wait until late winter or early spring to begin mixing your pond, Lutz said.

Algae in a pond.

Algae developing along the edges of a small pond. Drone photo by Johnny Morgan/LSU AgCenter

10/26/2023 2:03:09 PM
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