Pond Scum Wipe-Out!! Control of Nuisance Algae in Ponds
The arrival of spring often presents a problem for pond owners as the beautiful, clear green water of their ponds disappears beneath a blanket of lime green, slimy, stinky pond scum. “Pond scum” is actually a popular term for aquatic algae, which tend to “bloom,” or grow excessively in the early months of spring, spreading over the surface of the pond in unsightly and odorous mats of green and brown.
Aquatic algae are primitive plants, having no true roots, stems, or leaves. Pond algae can be found either floating on the pond surface or attached to other plants, bottom sediments or other hard surfaces. There are thousands of species of aquatic algae, but for simplicity they can be classified in three categories: microscopic algae, filamentous algae, and attached-erect algae.
Microscopic algae, also called phytoplankton, are tiny, free-floating algae that give the pond water its characteristic green color. Microscopic algae are the primary producers of dissolved oxygen in pond water. The presence of a healthy level of microscopic algae in a pond is important for maintaining good water quality and health of the aquatic organisms in the pond, such as fish. Microscopic algae can undergo excessive blooms during mid-summer months, rising to the surface of the pond as a layer of yellow-green or reddish scum. A sudden die-off of microscopic algal blooms, caused by a change in water temperature or a stretch of several overcast days, can deplete dissolved oxygen levels in ponds to a critical level for the survival of aquatic organisms.
The clue for the pond owner in the control of microscopic algae is to look for a change in the color of the water that might signal that a bloom of microscopic algae is taking place. This color change would be from the clear green water of the healthy pond to a bright, pea-soup green.
To quickly check the density of microscopic algae in a pond, nail a light-colored object, such as a coffee can lid or aluminum pie pan, to the bottom of a yardstick. Place the yardstick in the water and observe the depth at which the light-colored object disappears. In a healthy pond, the light-colored object should be visible at a depth of 24 inches. If the object disappears before a depth of 24 inches is reached, a bloom of microscopic algae is taking place in the pond. If sight of the light-colored object is lost in less than 10 inches of water, the bloom is heavy and the pond owner may want to seek advice about control of microscopic algae.
Attached-erect algae are a less common problem for pond owners, but excessive blooms of submerged attached-erect algae may occur across pond bottoms causing difficulties for anglers or swimmers. Attached-erect algae, commonly called stonewort or muskgrass, is often mistaken for more advanced pond plants because it resembles a higher plant with leaf-like structures arranged about a long stem-like structure. Attached-erect algae have a gritty texture due to surface calcium deposits. A positive identification of attached-erect algae is important for chemical treatment because chemicals used to treat many submerged aquatic plants often do not provide good control of algal species.
Green filamentous algae are the last category of pond algae and the one which gives pond owners the most headaches. Many species of green filamentous algae are tolerant of cold water temperatures and undergo blooms in early spring. Typically in West Virginia, ponds having recurring problems with filamentous algae begin to exhibit algal blooms as early as March, although some blooms in late February have been reported.
Blooms of filamentous algae begin in clear water in shallow areas where sunlight can penetrate the water to reach the soil of the pond bottom. Algal cells join together in long stands resembling green hairs, which grow in fur-like clumps along the pond bottom and edges, breaking off and floating to the surface to form dense mats. Sudden die-offs of dense blooms of filamentous algae can create serious water quality problems, not to mention unattractive and odorous conditions as the dead algae decays.
Control of pond algae begins with an identification of the type of algae causing the nuisance in the pond. This identification can usually be made through the WVU Extension Service or through the West Virginia Division of Natural Resources. To collect a sample of microscopic or filamentous algae for identification, use a jar with a lid that seals tightly to collect a sample of the algae and some of the pond water. This sample should be refrigerated until delivered to an office for identification. If the alga is a submerged attached-erect species, the pond owner should wrap several branches of the alga or aquatic plant in a wet paper towel and seal the sample in a sealable plastic bag. This sample should also be refrigerated until delivered to an office for identification.
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.
Finally, the pond owner should be aware that using algicides to treat blooms of aquatic algae is a very effective and quick method. In other words, algicides typically kill algae fast, usually within a day or two. If a pond is heavily infested, the pond owner should treat the algae in sections rather than treating the whole pond at one time. Treating in sections will prevent a sudden kill of all the algae, which would cause a significant drop in the amount of dissolved oxygen available in the pond, not to mention the mess left by a pond full of dying algae.
For additional information regarding management of ponds in West Virginia, contact your county Extension office.
Aquatic Pest Control Category 5. Indiana Commercial Pesticide Applicator Training Manual. Lembi, C., Purdue University publication # PPP-5. 1997.
How to Identify and Control Water Weeds and Algae. Fourth ed. Schmidt, J.C. ed. Applied Biochemists, Inc., 1987.
Written by: Aggy Spicer, Aquaculture Specialist, WVU Extension Service. 1997.