By Bob Robinson – Ponds and lakes in the northern United States have in the past experienced what I will term a “fish kill” related to lack of dissolved oxygen present in the water. Oxygen is essential to the metabolism of all aerobic (air breathing) organisms. Oxygen typically enters lakes at the surface via diffusion from the air, by wave action or by photosynthesis from aquatic plants. Luckily, there are farm pond maintenance tactics you can perform to help dissolved oxygen levels. More on that in a bit.
A combination of thick ice and heavy snow accumulation can in some instances be cause for concern in lakes/ponds. If your body of water has a high concentration of organic material on the bottom, is relatively shallow, or is heavily infested with rooted and floating plants in the summer, there is a chance that severe winter conditions could result in a fish kill because of lack of oxygen. All lakes are in an ever-changing mode of succession. In simple terms, lakes are slowly converting back to land because of the accumulation of organic material on the bottom. The rate of succession is something that can be controlled or stopped completely with proper management.
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Shallow lakes are probably the most likely candidates for winterkills. But deeper small lakes have experienced fish kills in the winter due to lack of oxygen. Many reservoirs were created by flooding land by placing some type of dam in a river system. Most of these types of lakes will have a higher than normal amount of decaying vegetation on the bottom because they are essentially flooded lowland. They also are typically quite shallow. Heavy ice and snow cover do not allow sunlight to penetrate which means there will be no photosynthetic activity to produce oxygen. So instead, oxygen is consumed as plants die off and carbon dioxide is produced.
Farm Pond Maintenance Tactics to Help Dissolved Oxygen Levels:
- Physically remove as much aquatic vegetation as possible as often as possible throughout the year. Keep in mind that some structure is necessary for shelter to keep small fish away from predators. Chemically treating lakes with herbicides is typically a short term solution and does not get rid of the nutrients that are the reason the plants are there in the first place.
- Keep runoff from flowing into the pond by creating berms around the entire perimeter.
- When it comes to farm pond design, build ponds deeper with an average depth of something like 10 feet. Shallow ponds allow for more shallow vegetation to grow, which can die off in the winter months. Whenever there is an accumulation of snow greater than four inches or so, shovel or plow as much as you can off the ice.
- Make sure that you have a properly working septic system or if you are using an old outhouse, that the bottom of the pit is not close to the water level (build it up if you have to).
- Refrain from using soap if you bathe in your lake. Soaps can contain phosphorous which is one of the limiting nutrients for plant growth.
- Be careful if you fertilize and use lake-friendly type fertilizer. Do not fertilize prior to any heavy rains. It is better to fertilize when it is dry and lightly water your lawn to allow it to slowly soak in and not run off into the lake.
- Do not clear vegetation on land all the way to the shoreline. This edge vegetation will trap some overland flow runoff and filter it prior to it arriving in the lake.
- Keeping ducks on a lake means more droppings. The nutrients they can drop into the water can be a significant food source for unwanted plant growth. Try to keep the number of waterfowl on your lake controlled.
Another farm pond maintenance approach is to keep a small area ice free to allow oxygen transfer from the air into the water. An open area as small as a few percent of the overall water surface is typically sufficient to prevent a winterkill. Keep in mind that the saturation level for oxygen in water is temperature dependent and that colder water holds more oxygen. Because fish are cold-blooded, their metabolism is slowed down in winter, so only a small amount of oxygen is required in the winter months to satisfy the oxygen demands for fish. On average, throughout the year all the living critters in a lake will not consume more than approximately 15% of the oxygen. The rest of the oxygen demand comes from plants and decomposing organic material.
Farm Pond Maintenance Methods to Keep Areas Ice Free
- Pump warmer water onto the Surface – this will only work if the ice is relatively thin. If the ice is relatively thin, you probably will not be experiencing a big problem with low dissolved oxygen.
- Use equipment for farm pond maintenance in the winter:
- Wind Aerators / Circulator: There are two types of aerators that fall into this category. The first has two sets of blades. The first fans tick out of the water to catch and harness the wind energy and the second are blades that are under water that mix and move the water. This is an interesting approach because it does not require powder. It is very limited because it does not work on days when there is no wind. The second type of wind aerator actually uses a diaphragm type compressor attached to the wind blades of a windmill and pumps air into the pond bottom via an airline and diffusers that rest on the pond bottom. Once again this will only work when the wind is blowing and the amount of air that is generated by these types of pumps is usually not significant enough to get to depths greater than about 10 feet with enough air to be considered effective.
- Chainsaws: Cutting holes in the ice might work in an emergency situation but would get rather old if it had to be done on a consistent basis.
- Solar-Powered Air Pump Systems: These types of systems pump air into the pond bottom and cause circulation to the surface via a diffuser. Obviously they sound like a neat way to go and do not cost any electricity to run. Problems in the past have been relatively high initial costs vs. the resultant benefit. In order to get the proper amount of air to the pond bottom you will need a compressor that will pump at least three cubic feet per minute of air into one diffuser that is resting in a pond 15 feet deep. That compressor will require a large solar panel and some type of electricity reservoir for when the sun is not shining. Also, in the past the DC motors that have to be used with solar power have failed over short periods of time because they were not designed to operate continuously throughout the year.
- Electrical Air Compressor: The basic operating principle here is to create an airlift pump design. The air compressor pumps air into some type of diffuser that causes the water to be lifted to the surface where it can keep an area ice free and absorb oxygen. This type of system does not work well in shallow ponds of depths of 10 feet or less. The main reason is that the bubbles will rise at a foot per second and are not in contact with the water for a decent amount of time which results in less entrainment of water to the surface. Also, it is critical that the airline used be either buried below the frost line or always pointing downhill. The heat of compression causes internal condensation and could result in a freeze up if the line is not buried or going down hill. Recently I have seen some non-harmful anti-freeze type material released into air lines to keep them open. A positive note about this type of aeration is that there is no electricity in the water. The compressors will make some noise so put them in a building where the noise could be muffled.
- Circulator Motors / De-icers: This type of device employs a motor and shaft with a prop which looks similar to a prop from a trolling motor. It can be operated in a horizontal or vertical plane to either move water up from the bottom or to circulate water in a horizontal fashion. The key is that you do not want to splash water into the air because you will super-cool the water and run the risk of creating a giant ice cube out of your pond. These types of devices can either be hung by two ropes attached to your dock, a dock mount apparatus or by a float. 120 volt power is required to run these units. They will probably not address depths greater than 18 feet or so. Other types of aerators that might be considered include fountains and agitators. Again, anything that splashes water into the air should be avoided in the winter months. Aspirators have been used in some type of de-icing application with limited success. Basically an aspirator has a motor outside of the water attached to draft tube and a propeller that rests in the water. The unit drafts air into the prop and causes directional flow. These types of devises can work but are not as efficient as either diffused air or circulators because 1) They suck in cold air and mix it into the water, and 2) Thrust is compromised in order to bring in air and consequently the efficiency drops off a bit.
De-icing equipment can also be used to allow for wet storage of docks and boats in the winter months. These units operate by directing the flow of warmer water from the bottom to the surface to keep areas ice free.
Keeping an area ice free in your lake also acts as a refuge for water fowl. Predators such as stray cats/dogs, wolves and coyotes will walk out on the ice but will not go in the water after the birds. Pushing water from the deeper part of the lake back towards shore can keep the shoreline open for livestock if so desired.
The area of water that can be deiced by any given farm pond maintenance technique is a function of water depth, air and water temperature and depth of the working unit. Each body of water must be looked at closely to determine which method of de-icing is most appropriate.
Originally published in 2003 and regularly vetted for accuracy.