Looking at the forecast for this weekend and planning what I’ll write ahead of the coming weather, I’m betting I may get some calls about someone’s pond ‘turning over’ and having a fish die-off.
Many a pond owner may have experienced the unsettling phenomenon. We get nice cool front that takes the summer heat away for a little while and brings a nice rain. Then the next day, you find dead fish floating. It’s a phenomenon that happens occasionally with pond owners. It’s something called “pond turnover”, and understanding it can save both your fish and your peace of mind.
During summer, ponds can become thermally stratified. That means warm, oxygen-rich water sits on top, while cooler, oxygen-depleted water settles below. As noted by AgriLife Extension’s experts, this layering is common in deeper ponds (typically over 8 feet) and sets the stage for a sudden catastrophe.
A turnover event occurs when the cool, oxygen-poor depths get mixed with the warm surface layer—usually triggered by a cold rain or a lightning storm that chills surface water quickly. The result? Oxygen levels in the entire pond crash, and fish—with nowhere to escape—often die.
It matters now, and most summers in East Texas, as we get have hot temperatures as well as frequent thunderstorms. Since most ponds often don’t have an mechanism to create oxygen-rich bottom stirring, the combination can make turnover events more likely—and more devastating.
Some may ask, how might they avoid this potential problem? The continuous use of an aeration system is a solution. Research emphasizes that mechanical aeration—especially systems with bottom diffusers—can disrupt stratification and prevent turnovers. Surface fountains may look pretty, but they only oxygenate the top layer and do not stop mixing issues.
Don’t start a bottom diffuser now—it can actually trigger the turnover you’re trying to prevent.. Bottom diffuser aerators should be started much earlier in the year and continuously used to keep any stratification from occurring in the first place. Think of the month of May as a great time to start using a bottom diffuser.
A much wiser course of action would be to closely monitor your ponds after a summer cool-front and resulting storm. Get out to inspect your pond early at dawn and look for gasping fish near the surface. Monitoring with a dissolved oxygen meter—especially after weather shifts—can alert you early to dangerous oxygen dips. Sunrise is always going to be the time when you pond has the lowest oxygen level during any day of the year. The reason is because of the added influence of vegetation in your pond.
Plants produce oxygen during the day but consume it at night. That’s why oxygen levels are always lowest at dawn. Dense populations of vegetation in the pond will create larger demands on the oxygen in the water.
If you have recently had a vegetation die-off from natural causes or even from a management effort to properly reduce excessive vegetation, that decaying organic matter will consume oxygen as it rots. Natural algae die-offs can also trigger oxygen crashes that mimic turnover events.
When looking at your pond’s vegetation coverage, research suggests keeping vegetation below 10–15% of surface area, and consider targeted control of problematic weeds to keep your pond balanced.
Above all, know your pond. If your pond is shallow (say, under 6 feet), turnover is less likely. Any pond that has seen a turnover event once could likely see it again, so assume the risk unless proven otherwise.
If you have an over-abundant supply of fish for your pond, you may simply run out of oxygen from an overstocking problem. It doesn’t take a cattleman to see thin, starving livestock in a pasture devoid of grass. But “seeing” too many fish in water that is devoid of oxygen is not an easy observation. A great way to keep a proper stocking rate of fish is to simply go fishing and keep everything of edible size. Have an annual family or neighborhood fish fry to avoid any problems with oxygen depletion, no matter the cause.
Wrapping it up, here are three things pond owners should do. First, inspect your pond at dawn especially after summer storms. Look for ‘gasping’ fish. Second, keep vegetation under 10–15% of surface area to reduce oxygen demand. And last, harvest fish regularly to avoid overstocking and oxygen crashes.
