Farm Ponds, Fish kills and Oxygen

By Duane Friend

Thousands of farm ponds dot the landscape across rural Illinois. They are scenic, functional, and affected by environmental factors, such as temperature, oxygen content, nutrients, and biological activity. For instance, take the case of changing temperatures within a pond.

Water reaches its maximum density, or weight, when its temperature is 39 F. As temperatures either drop below or rise above 39 F, water density lessens. This is important because water density has a major effect on the stratification, or layering, of water. And stratification can sometimes affect oxygen levels available for fish and other aquatic life.

To understand how this can happen, take a look at the seasonal stratification process.

In early spring, the different layers of water mix. Stronger winds this time of year allows for atmospheric oxygen to dissolve into pond water. But as surface water warms, it decreases in density or weight. A layer of warmer, less dense water forms on top, while the cooler, denser water forms a layer near the bottom of the pond. This layering of water temperatures is called stratification.

As summer progresses, so does stratification. The warmer water remains on top, while the cooler water stays below. In between the two layers, a transition zone forms during the summer. This “thermocline” zone is characterized by a rapid change in temperature. As water warms, its ability to hold oxygen goes down. A daily oxygen cycle also occurs, with highest pond oxygen content during the day and lowest occurring around sunrise. A summer fish kill can occur when plants in a pond start to die and decompose, which uses up more oxygen. This is fairly common in July and August.

During fall, stratification disappears, allowing surface and deeper waters to mix. This is sometimes called a fall turnover and may be characterized by a temporary change in water color or cloudiness. As water mixes, sediment and organic material from the bottom is stirred up, causing the water to become a muddy brown. A fall turnover brings low oxygen water to the surface, and may lead to a fish kill, but typically it doesn’t.

Stratification returns in winter. A layer of colder water (near freezing) forms on top, just beneath the ice cover, while slightly warmer water (near 39 F) stays close to the bottom. This pattern is the typical way in which stratification plays out over the seasons.

If a pond is covered by ice and snow in winter, dissolved oxygen content can plummet, leading to winter fish kills. Snow cover is the main culprit, decreasing sunlight that can penetrate ice and keep small amounts of algae alive. The algae produce enough oxygen to allow fish to survive. Ice cover for long periods also stops oxygen dissolving from the atmosphere into the water.

A winter fish kill is different from a summer kill in that fish will be long gone by the time the ice melts. In many cases people will not realize a kill has taken place until a fishing line is thrown in and little to no fish are biting. Summer kills are easily seen and unfortunately smelled.

To prevent a winter or summer fish kill, an aerator may be used. There are many types of aerators on the market. Diffusers or agitators are two main types that can be used, with each having advantages and disadvantages.

Agitators move large amounts of water at the surface, providing good conditions for atmospheric oxygen to dissolve into the water. Diffusers work from the bottom, adding oxygen as air bubbles work their way up to the surface.

For more information on pond management, contact your local Extension office, IDNR fisheries biologist, or NRCS/SWCD office.

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