On occasion, the water in a pond can have a bad smell (Sulphur-like) in spring and fall.
The water can become murky as toxic decaying matter that typically lies on the bottom of the pond can rise to the top.
Fish are either seen dead or gasping at the surface for air.
What this means is that the pond has turned over!
Pond turnover is a natural event where stagnant water in the pond becomes mixed, usually due to temperature changes in the surface water as the seasons change.
Pond turnover happens most commonly in spring and fall:
- In the Spring: The process of pond turnover happens as the water warms and reaches its daily most consistent temperature year.
- In the Fall: The cooler water eventually reaches a surface temperature that is equal to that of the water in the deeper levels of the pond. Once the surface water reaches this point, the entire pond will turnover and mix.
What Is Pond Turn Over, And How Does It Happen?
The process of pond turnover is natural and occurs on a regular basis in many small bodies of fresh stagnant water, such as backyard ponds and lakes.
Pond turnover usually means that the water has become murky and all the fish in the pond have died.
pond turnover may occur in those ponds that have the potential for biological mass development over time.
The biological mass can come from many sources; it can be from vegetation in a pond or vegetation along the shore. It can also come from the feces of fish, birds, cattle, and other wild animals or even from fish food.
These types of materials settle to the bottom of the pond to become a layer of decaying biological materials and produce and trap gases that are unsafe for aquatic organisms.
In the summer months, the heat from the sun warms the surface layer of water, which sits on a cooler, denser layer of water at the bottom of the pond. In essence, the water becomes stratified into layers due to the temperature gradient in the water.
Furthermore, due to the density gradient, two distinct layers are hard to mix.
Once the water on the surface cools down to a temperature similar to the bottom layer, typically in the fall, the water layers will decouple with temperature.
This causes the entire layer of decomposing matter to be lifted off the bottom and float to the surface, thereby mixing or “turning” over the entire column of water.
The water has no time to oxygenate, and the organisms living at the bottom cannot survive.
Resulting in Fish or any other animals typically die from a combination of lack of oxygen and toxicity.
The Annual Cycle Of Pond Turnover Process
Ponds typically experience four different phases in the circulation process of pond turnover in one year’s habitual cycle.
The stages are usually coherent with the four seasons of the year.
Summer Stratification
The summer stratification process creates three layers of water based on temperature, dissolved oxygen, and density.
Due to these very different properties, these layers do not easily interchange:
- The epilimnion is the uppermost layer of water. This layer is warm, light water because of its exposure to the sun and action of the wind, and it is always well oxygenated. As summer progresses, this layer will have rising water temperatures.
- The second layer is the thermocline. It is a transition layer where the temperature drops at least 0.5 degrees Fahrenheit for every foot of depth.
- The bottom layer is called the hypolimnion. The waters in this area are the coldest and densest. Temperatures in the hypolimnion layer often remain between 54-59 F, even during the hottest months of the summer. This layer is cut off from the atmosphere by the top two layers.
In this layer, organic matter decomposes, thus consuming all the dissolved oxygen.
By mid-July, this layer may be completely devoid of oxygen, and no animal or plant can live in this layer either.
Fall Turnover
As air temperatures drop below 50 degrees Fahrenheit in the fall, the surface layer of warm water (the epilimnion layer) cools down, becomes heavier, denser, and sinks downwards.
The fall wind then creates greater motion in the epilimnion layer which causes the warmer layer to mix with the thermocline layer and the cooler hypolimnion layer.
This stirring of the layers generates a complete mixing of the water columns, which is referred to as fall turnover.
Eventually, the free-flowing and circulating water in the pond is all completely oxygenated from the surface to the very bottom of the pond.
Because of this mixing, all the decaying particles and the gas that accumulated in the hypolimnion layer over the summer rises up to the surface of the pond.
Winter Stratification
With the onset of winter, the pond’s water will have reached a maximum density of 39 degrees Fahrenheit.
Over the deeper water, a membrane of colder and less dense water forms quickly.
This cold water then reaches 32 degrees Fahrenheit, and a film of ice forms on the pond surface.
This causes the water to break back into layers until the water located at the bottom is deoxygenated, just like in the summer.
We refer to this as inverse stratification as well.
Spring Turnover
A shift takes place in the spring.
Once the water in the pond warms and reaches its most uniform temperature and density of the year, the water column is determinedly mixed, providing both healthy levels of dissolved oxygen that aquatic organisms need, throughout the entirety of the column.
Factors Influencing Pond Turn Over Occurrence
Location of the Pond
In open and windy areas, ponds can turn over at any time of the year, only to stratify when the wind dies down.
Ponds located in deep narrow valleys or sheltered by forest land will stratify, and they may remain fairly stable throughout the summer.
Depth of the Pond
Another factor affecting pond turnover is the depth ratio of the water column that can be mixed by wind, and the depth established at the bottom of the water column.
This depth is critical to whether or not your pond can develop stratified layers of water.
Typically, shallow ponds do not stratify because there is not enough depth to create barriers in the water.
Also, shallow ponds tend to warm or cool to a uniform temperature, and high winds will continuously mix that water all year long, thus reducing turnover.
Deep ponds (more than 6 feet both depth) tend to develop distinct and stable columns of water that will eventually turnover on their own because of the layers that develop over time.
Temperature Changes
In summer, deep ponds form different layers due to temperature.
The water at the surface is heated from the sun and is less dense than the cold water below because it is transparent to sunlight.
Because the temperature is different, the layers do not mix and the water stays stagnant. As a result, a thick layer of decaying matter may develop on the bottom of the pond.
In the fall, the water surface cools down, and the temperature finally equalizes with the water on the bottom. This causes the water column to turn over, causing the bottom sediments to mix throughout the pond.
As a result, this process releases large amounts of sulfur and methane gas into the ecosystem atmosphere, creating very strong, septic odors.
Pond Size
Less turnover occurs in smaller ponds in the fall and spring than occurs in a more significant pond.
A small-scale pond can circulate for a matter of days. A large pond could circulate for weeks or longer.
How to Prevent a Pond from Turning Over
Because pond turnover is a natural process, it can be difficult to change it back once it’s started.
But there are some things that can be done to prevent fish mortality and future pond turnover.
Diffused Aeration
To prevent turnover, the only viable solution is to remove stratification, and thoroughly mix the pond all year long.
You can achieve this by utilizing a bottom diffused aeration system.
Diffuser aeration systems can drastically reduce the probability of pond turnover.
These systems are worked with a small air compressor mounted on the shore near the pond.
The air compressor can be connected as a 110v or 230v system, depending on the size required for the pond.
In some cases, they may instead be powered from a windmill.
The air compressor sends air, continuously, through a weighted tube located on the pond’s bottom, and out to an air diffuser located on the bottom of the pond itself, which then creates millions of tiny air bubbles that rise to the surface, along with significantly larger volumes of water and oxygen.
That water that the small bubbles bring up will help to reduce stratification.
So, the distinct two layers of water that were once in the pond are now mixed uniformly and oxygenated.
Having the oxygen levels increased will help prevent a massive fish die off and will at least mitigate the major problems associated with turnover.
Keep in mind: This will work best with deeper or larger ponds that will provide the benefit of good circulation. If pond is not deep, it likely will not work as efficient.
Water Circulators
This equipment will create directional flow while still supplying oxygen where diffused aeration doesn’t penetrate as effectively.
They can be installed at various depths and other locations.
Moving water will also prevent pond turning as it doesn’t stratify the water layers.
Stick with Shallow Ponds
Opting for a shallow pond, under six feet deep, is another undisturbed way to prevent thermal stratification.
The reason behind this is that shallow water tends to mix easier and thus to prevent thermal stratification, especially when aided by rain and wind.
Surface Aerators
This equipment is an excellent way to introduce oxygen into water, while also helping to get the thermocline layer around ten inches deep, depending on the size of the unit.
Surface aerators force water from below the water’s surface to the air and back into the water, while also adding oxygen.
Water jets exert various degrees of pressure to break the layers of water.
The additional oxygen will help the pond for the duration of the whole season, and the mixing will reduce the amount of water that will turn over.
If the water continues to turn over, the aeration system will help save the fish and the pond.
Final Thoughts
Warm, less dense water during summer is resting on cooler, denser water at the bottom of ponds.
Temperature layering weakens when the air temperature cools.
When the temperature levels out through the water column, the pond is able to mix.
Meaning the bottom water of the pond rises to the surface along with all the dead and decaying material, and the surface water sinks, and the pond has suddenly turned or flipped.