Filamentous Algae in Your Pond

Posted: April 11, 2013

These algae colonies begin their growth in the late winter and early spring on the bottom of the pond as warmer temperatures and sunlight activate the spores and surviving cells.

Despite what seems like a slow start to spring this year, I have already gone out on several pond visits and noticed that filamentous algae is well on its way to causing problems in some ponds.

Many different species of filamentous algae exist, but all have a similar appearance and growth habit. These algae colonies begin their growth in the late winter and early spring on the bottom of the pond as warmer temperatures and sunlight activate the spores and surviving cells. Most filamentous algae growth begins in less than 3 feet of water where sunlight penetrates to the pond bottom. Single cells reproduce and join together into long hair like strands or colonies that grow toward the water surface. By mid-summer, these strands form large mats that trap gases, break loose from the pond floor and float to the surface. These floating mats normally begin to appear in July and may cover the entire pond by late summer. However, you may see these mats earlier than that, even within the next month.

All types of algae are important to pond and lake ecology because they serve as food sources for a variety of life.  However, filamentous algae frequently reach nuisance levels. Their abundant growth can result in a number of management concerns, including aesthetics, swimming nuisance, and interference with fishing. Abundant algae can also cause fish kills in late summer and fall as the dying algae consume oxygen from the pond water.

Long-term control of overabundant aquatic plants is best accomplished by reducing or redirecting nutrient sources from the pond. This can be done by reducing fertilizer applications near the pond, maintaining septic systems properly, redirecting nutrient-rich runoff away from the pond, and maintaining vegetative buffer strips around your pond.  Combining and using multiple management methods is recommended for control of filamentous algae.

Mechanical control of filamentous algae usually involves netting or raking the algae mats from the pond surface. If this method is used, it is important to dispose of the algae mats away from the pond edge to prevent nutrients from reentering the pond as the algae decays. While this method is labor intensive and time consuming, it can be very effective on small ponds.

The use of barley straw has been shown to reduce filamentous algae growth in some ponds but results are very inconsistent. Barley straw does not kill existing algae colonies. Barley straw must be added to the pond in loose bales anchored at the water inflow in late winter or early spring to prevent algae growth.

When used carefully according to the label instructions, aquatic herbicides can be safe and effective management tools but certain things must be kept in mind.

Positive identification of the target plant is a must.

You should carefully measure the pond area and/or volume.

You must complete and submit a two-page permit which is required for any chemical application to any private pond or lake.

Common herbicides can be purchased at home and farm supply stores, hardware stores, or from various online suppliers. Most algaecides will cost between $50 and $200 to apply per acre of pond area.

Follow the herbicide label carefully. The herbicide label gives specific instructions on when and how to apply the chemical. An aquatic herbicide should not be applied to the entire pond during one application. Killing all of the pond algae with one treatment may cause oxygen depletion and fish kills due to the decay of the large amount of algae. Instead, treatments should be restricted to one-half or one-third of the pond at one time with a two-week delay between treatments.

It is best to treat filamentous algae early in the summer when they first appear on the pond bottom and before they form large floating mats. This will reduce the amount of plant material that must be killed and, thus, reduce the chance of a fish kill due to low dissolved oxygen from decaying algae.

If you are interested in learning more about pond management, check out the Penn State Water Resources web site or call the extension office.