Each year, especially those with extreme temperature changes, I get a number of pond owners call who have had a pond “turn-over”, with dead fish the end result. The weather we have been experiencing over the past few weeks, cool then hot, then cool again, have had some pond owners experience foul odors, murky water and dead fish.
It is oxygen depletion that is the cause of the most serious problems, especially the dead fish. Here’s how these turn-overs happen.
During hot summer weather, surface water becomes less dense as it absorbs heat and floats over a cooler, more dense layer of water. This layer prevents the oxygen produced in the warmer layer from mixing with the lower cooler layer. Since these two layers may not mix for weeks at a time, especially in deeper ponds, oxygen is eliminated near the bottom of these ponds.
Biological and chemical activities that take place in the lower, cooler layer is what uses and depletes the available oxygen. Next, a cool snap or a thunderstorm with wind and hard rain can cool the warm surface water making it heavy enough to sink and mix with the oxygen deficient bottom layer. The net result is a dilution of the oxygen and an increase in the demand for oxygen from dissolved minerals and decaying organic matter.
To complicate these problems –the algae usually die at the same time. When a die-off occurs, the green water often becomes streaked with gray, black or brown. The color of the water may eventually become totally brown, gray, black or even milky. A distinct foul smell may also be noticeable, often like rotten eggs. “Turn-overs” cause the most catastrophic fish kills in ponds, with 100% losses.
Some ways to help avoid this problem.
First, keep the nutrient level down so that plankton blooms don’t become excessive. Risk of turn-over is considerably less when visibility of a white object (disc) in the pond is 16-18 inches. You can hook a white object like a large pickle jar lid to a stick and mark the stick off in inches. This can be used to check the water.
Secondly and very helpful is the installation of an aerator that can be run at night and during extended periods of cloudy days. An aerator is good insurance against fish kills. An area of oxygenated water near the aerator will help the fish survive until the pond recovers from a low oxygen period.
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