Pond Turnover – What it is and How to Stop It

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Pond turnover is a phenomenon that would go largely unnoticed if it weren’t for the sometimes awful consequences. Mild turnover can leave your water cloudy for a couple days.

Severe turnover can kill thousands of fish, and leave your property smelling like a month-old rotting egg.

What is pond turnover? How does it affect my pond? How to I prevent turnover from occurring?

The purpose of this blog is to answer these questions-helping the average homeowner to find a solution to their pond turnover problem.

What is Pond Turnover?

Pond turnover is a term used to describe the mixing of the stagnant, or ‘stratified,’ waters in a pond.  Stratification is an occurrence whereby the water separates into three distinct layers-like a layer cake-each with its own different temperature and dissolved oxygen levels.

The upper level, known as the epilimnion, is composed of warm, lighter water that has high dissolved oxygen content – this is where fish tend to dwell. The middle layer of a pond is known as the thermocline; in this ‘transitional’ layer, water temperature and dissolved oxygen level become lower with depth.

Finally, the lowest layer of a pond is referred to as the hypolimnion. This layer is the coldest and most oxygen deprived of the three, as it is sheltered from any atmospheric conditions by the above two layers. Pond turnover is when weather conditions favor the mixing of these layers.

As the epilimnion is cooled by the fall air to a temperature below that of the lower layers, it becomes denser and sinks-mixing the layers together into a uniform temperature and density. The exact opposite occurs after winter-and the cycle continues annually. Sometimes cold rains and heavy winds can provoke an additional mid-summer turnover, as well.

How does Turnover Affect My Pond or Lake?

Organic matter like dead plants and fish, leaves, etc. sink to and accumulate in the lowest level, the hypolimnion, because gravity naturally pulls it there. Unfortunately, this layer has little to no oxygen that would help aerobic, or oxygen breathing, bacteria break down the material. Instead, anaerobic, or non-oxygen breathing bacteria do the job. Not only are these bacteria seventeen times slower than their aerobic counterparts, they also release toxic gasses like methane, hydrogen sulfide, and ammonia as they digest.

This toxic gas gets trapped in the hypolimnion, and is released all at once when the levels mix. So, how does this turnover affect your pond? It does four things:

  1. The toxic gasses released poison the fish, causing a fish kill
  2. Aerobic bacteria are given access to organic matter once locked at the bottom, quickly sucking all the oxygen out of the water and suffocating your fish
  3. The odor of the released gasses fills the surrounding area, including your home, with rancid odors
  4. The mucky hypolimnion clouds your water, causing your pond to appear very dirty

How Do I Prevent Turnover From Occurring?

Two things need to happen in order to prevent the ill effects of pond turnover. The first thing you need to do is mix the layers of your pond to keep them from stratifying. The second thing you need to do is provide oxygen throughout the entire pond, promoting the consumption of organic matter by aerobic bacteria.

Not only will the air avoid the production of toxic gasses, but it will also ensure that your fish are given ample air to breathe. Both of these are important to avoiding fish kills and noxious odors.

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