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Understanding pond stratification

Stratification is a common phenomenon in ponds, but also an integral part of pond management.

Radio interview source: Bill Lynch, Extension Aquatic Specialist, Ohio State University

Click here to listen: Understanding pond stratification

If you swim in your pond, you may notice the water at the bottom is cooler than at the surface. This is called stratification, and it's important for a healthy pond. The pond may develop a green tinge and seems to bubble and churn. It's going through a turnover that mixes different thermal layers together. When those layers separate again, it's called stratification.

Bill Lynch is an aquatic specialist at Ohio State University. He says thermal stratification is a common phenomenon in ponds and small lakes. It happens when surface waters warm quickly in early summer, and there's little wind to mix up the water. "And so we get this warm upper layer that has oxygen in it, that sits on a cold layer," Lynch says. In most ponds and small lakes, that cold layer loses its oxygen due to it's not getting any new oxygen from above because those layers don't mix, and all that bottom muck uses up the oxygen to finish its decomposition processes."

Ponds under eight-feet deep usually don't stratify because breezes keep the water mixed. Deeper water almost always stratifies. While stratification is normal, Lynch says a sheltered pond can experience a turnover fish kill, which happens when the two layers suddenly become mixed. "If your bottom layer is very large compared to the upper layer and everything mixes, you don't have enough oxygen in the upper layer to compensate, and you lose your fish," says Lynch. "The protection naturally against that kind of fish kill is that the upper layer is bigger than the lower layer in terms of volume. The best way to prevent stratification is that we install some sort of a bottom aeration system, a bubble system."

As surface waters cool in the fall, the upper layer becomes thicker, pushing the oxygen deeper and the pond turns over. During the winter, the cool upper layer becomes less dense, causing the pond to stratify again. However, the colder water is near the surface rather than at the bottom.  

 

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