Filamentous Algae

One of the most common aquatic weed problem in ponds is filamentous algae. Its presence can degrade water quality and recreational enjoyment. Excessive algae can cause an oxygen depletion leading to fish die-off when it decomposes in excessive amounts. Early and regular control measures will help reduce the problems associated with filamentous algae.

The Plant

Filamentous algae, also called "moss" or "pond scum," forms dense mats of hair like strands. Its growth begins on submerged objects on the pond bottom. As it grows, the algae gives off oxygen that becomes entrapped in the mat of strands. This gives it buoyancy and causes it to rise to the surface where it frequently covers large areas of the pond. Filamentous algae is often a persistent problem because it reproduces by plant fragments, spores and cell division.
Types of filamentous algae vary in texture and color. Microscopic examination is usually required to make an exact identification. Some of the more common forms are: Spirogyra is bright green and slimy to the touch; Cladophora has a cottony feel; and Pithophora is often referred to as "horse hair" algae because its coarse texture resembles that of horse hair and it may feel like steel wool.

Various methods of control are listed below and offer varying results depending on the characteristics of the lake and the species of algae. These methods are listed in order of least possible damage to the water system. The downside is that the least damaging methods are also the slowest to take affect. I recommend using chemical options as a last resort if at all.
Mechanical Control

Filamentous algae can be controlled by physically removing large floating clumps with a rake. This will prevent the algae from decomposing in the pond and consuming dissolved oxygen. Algae that has been removed can be piled for composting or used in a garden as mulch. This material dries quickly once out of water.

Steepening the sides of the pond to achieve a 3:1 slope will eliminate shallow water areas so that sunlight cannot reach bottom-growing algae. However, if the entire pond has filled in as a result of sedimentation or decaying vegetation, a dragline or dredge may be needed to deepen the pond.

I have also had some success in a few ponds with periodic increased water flow. The extra water movement breaks up the clumps and strands. Draining just a few CFS for a few hours can remove far more algae than you can rake in the same amount of time. The only downside here is that you may not have fresh water to re-supply your pond. Too much drainage will greatly hurt you in the long run so pumps and natural filters may be your only option here. Shoot me an e-mail if you have interest or knowledge in this arena.

Biological Control

Biological control involves disrupting plant growth by modifying the aquatic environment through natural manipulation, or it can mean introducing a living organism that is capable of controlling aquatic vegetation.

One method of biological control is maintaining a fertility level that fosters the development of a microscopic plant and animal population, which prevents sunlight penetration. This requires intense management and more time than the average pond owner may wish to devote to the pond. Sunlight penetration to the pond bottom where the algae begins to grow can also be reduced by introducing an inert dye (usually blue). I have never tried this option although a few private property managers swear by it. It also requires routine testing and maintenance with fertilizer or solutions.

The addition of triploid white amur (a vegetation-eating fish) as a biological control measure may have mixed results. Filamentous algae is not a preferred food, but will be eaten if no other vegetation is present. If other aquatic plants such as water milfoil or coontail are readily available, the filamentous algae may be ignored and continue to flourish.
Chemical Control

Copper Sulfate

Most species of algae can be controlled with very low concentrations of copper sulfate. It is available in crystalline nuggets the size of rock salt or as a finely ground "snow" grade (Figure 1). The recommended treatment rate is 2.7 pounds per acre-foot of water. [Acre-feet is a volume measurement of the pond. It is determined by multiplying average depth (feet) X surface area (acres). For more information on calculating measurements, consult Pond Measurements (Natural Resources Facts A-2), available from county offices of Ohio State University Extension.] When uniformly applied, this will result in a 1 part per million (ppm) concentration throughout the volume of the pond. For very hard water (more than 12 grains or 200 parts per million of hardness), this rate should be doubled.

The method of application will determine what size of copper sulfate crystals to purchase. The important principle to keep in mind is that actual contact of the copper sulfate with the algae is necessary in order to achieve satisfactory control. For best results, dissolve copper sulfate in water and spray it directly on floating algal mats or on the water surface above submerged algae. Finely ground, "snow grade" copper sulfate is best for this method as it dissolves easier. Mix the desired amount of copper sulfate with enough water to cover the area to be treated, and apply with a sprayer or bucket and dipper. Because copper is corrosive to galvanized metal, application equipment and mixing containers should be made of plastic or stainless steel.

In large ponds and when spray equipment is not available, it may be easier to treat with copper sulfate by placing the larger crystals of this chemical in a burlap bag and towing the bag through the water until all the crystals have been dissolved in the area to be treated.

One application of copper sulfate is unlikely to provide season-long control. Re-treatment may be necessary at 3-4 week intervals.

There are no water-use restrictions associated with the use of copper sulfate. When applied at the proper rate, the water may be used immediately for swimming, drinking, fishing, irrigation and livestock. However, since copper sulfate has a metallic odor, pond owners may want to suspend drinking, swimming and livestock watering uses for 12 hours.

Copper Chelate

Copper is also available in a chelated, or buffered, formulation, which is manufactured as a liquid or granule. This provides some advantages during application. The liquid form needs only to be mixed with water and sprayed out over the pond surface; there are no crystals to dissolve. The granular formulation consists of a clay granule impregnated with copper chelate. As the granule breaks down, the copper is released into the water. This formulation is especially useful when spot treatment is desirable. Granules are best suited for application early in the growing season because of the time required (2-3 weeks) for them to dissolve and release the chemical. There are no water-use restrictions associated with either formulation of copper chelate.
Special Precautions

Fish are extremely sensitive to Hydrothol 191. To reduce the hazard of a fish kill, start application at the shoreline and move outward so that fish can escape from treated areas. Select another product if fish toxicity is a concern.

Copper sulfate is corrosive to galvanized containers. Therefore, the solution should be mixed in wooden, earthenware, plastic, stainless steel or copper-lined containers. If a sprayer is not available, you may broadcast the solution with a plastic watering can or bucket and dipper.
If the algae is so abundant that it covers more than half of the total pond surface, a complete treatment may result in an oxygen depletion and fish kill. This hazard is greatest during very hot, overcast weather. When these conditions exist, treat only half the pond and wait 10-14 days before treating the other half.

Copper compounds applied at the recommended rates are lethal to fish eggs and some species of newly hatched fish. These materials should not be applied during spawning periods, unless it is desirable to destroy the eggs and the new hatch.

Reference

Lembi, C. A., S. W. O'Neal and D. F. Spencer. 1985. Pithophora. Aquatics 7:4, p. 8-9, 22.

Additional Pond Management Information

Pond Measurements; Ohio State University Extension Fact Sheet A2.

Controlling Filamentous Algae in Ponds; Ohio State University Extension Fact Sheet A3.

Chemical Control of Aquatic Weeds; Ohio State University Extension Fact Sheet A4.

Ohio Pond Management. Ohio State University Extension Bulletin 374.

Controlling Weeds in Ohio Ponds. 41-minute videotape. VT50.

Visit your county office of Ohio State University Extension for copies of these resources.

Disclaimer

This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this publication may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author assumes no liability resulting from the use of these recommendations.