Control
There are a variety of options for controlling pond algae once it is identified. Algae growth is stimulated by light penetration in water and the availability of nutrients needed for plant growth, such as carbon, nitrogen and phosphorus. Light penetration to the soil of the pond bottom occurs in the shallow areas. These shallow areas are where growth of pond algae and weeds typically starts. Carbon and nitrogen are generally abundant nutrients in ponds. The lack of available phosphorus is usually a factor that keeps algae blooms at bay. Ponds that receive loads of nutrients, especially phosphorus, tend to experience chronic problems with algal blooms.
One method of controlling pond algae is to deepen as many shallow areas of the pond as possible so that light does not penetrate to the soil of the pond bottom. Water depths of three feet or more will help to control the start of aquatic weed and algae problems in ponds.
Controlling the amount of nutrients carried into the pond during periods of heavy rain can also assist in controlling algal blooms. Reducing the use of phosphorous-rich fertilizers close to the pond and/or planting a buffer strip of high grasses or shrubs around a side of the pond with a steep bank or drainage area can help to reduce the amount of nutrient laden run-off entering the pond. Diversion trenches to redirect run-off around the pond banks can be used in some situations.
More recently, the use of triploid grass carp, also called white amur, as a means of biological control for pond algae and weeds has received attention. Grass carp are native to the river systems of northern China and southern Siberia and were first imported to the U.S. in the early 1960s. Grass carp are herbivorous, feeding entirely on soft plant material that may include filamentous algae.
The effectiveness of grass carp for long-term pond algae control is questionable. Kentucky State University has reported success in algae control using young grass carp less than four inches in length. However, this research report notes, "Filamentous algae may be controlled by grass carp 2-4 inches in length in ponds without predators. Larger fish may consume filamentous algae, but it is not a preferred food and effective control may be unpredictable." Because grass carp are not a fish species native to West Virginia, the purchase and stocking of grass carp in ponds requires an importation permit from the West Virginia Division of Natural Resources. Pond owners should contact their local DNR office for information.
Serious infestations of aquatic algae may require chemical treatment for adequate control. Aquatic herbicides used to treat algae are called algicides and are primarily copper-based compounds, such as copper sulfate, copper chelate communes or the chemical Endothall. Care must be taken in the use of algicides as with the use of any aquatic herbicide to protect nontarget species, such as fish, from toxicity due to use of the chemical. The use of copper sulfate in soft water with alkalinity measures of less than 50 parts per million is especially risky with respect to toxicity to trout. Several aquatic herbicides including the algicides also have restriction periods for different uses of the water, such as watering livestock or irrigation, following the use of the chemical. The label for the chemical should clearly state the restricted activities and time of the restriction periods. An important rule of thumb in using aquatic algicides and herbicides is to read the label carefully and use only the specified application rate. Questions regarding the use of aquatic algicides and herbicides can be addressed to your county Extension Service office.