Surface Water Management Division

Filamentous Algae

Floating green mats are a common site on the surface of lakes and ponds during the summer.  In most cases, these mats are caused by filamentous green algae.  Filamentous algae provide an important food source and cover for aquatic insects and fish.  However, large accumulations of the algae may also be a nuisance to lake users.

Surface accumulation of filamentous green algae.

What are filamentous algae?

Most types of algae are individual cells that are free-floating.  Filamentous algae, however, are colonies of algae that join together to form long strands.  As the colony grows, the strands develop dense hair-like mats.  The algae provide valuable food and cover for aquatic insects and fish.  Filamentous algae are primarily from the group Chlorophyta, commonly known as green algae.  A few of the most common types of filamentous green algae in local lakes are Spirogyra and Ulothrix.

A microscopic view of Spirogyra, a common filamentous algae.

How do I recognize filamentous algae?

Filamentous algae may be found attached to rocks, aquatic plants, and other substrates.  As it grows up towards the water's surface, it appears as a billowy green cloud.  Large mats of algae may also float to the surface in mid to late summer.  Surface mats are typically green in color, but may appear yellow or brown as the algae begin to decay.  The texture varies by species, but is typically slimy and stringy.   Filamentous green algae may be confused with surface scums of potentially toxic blue-green algae.  However, filamentous algae mats do not break apart easily when agitated and are stringy in texture.  You can compare the two types of algae by looking at these algae bloom examples.

Surface accumulations of filamentous green algae.

 

Is filamentous algae harmful to my lake?

An extreme example of a filamentous algae bloom covering the surface of a lake.

All types of algae, including filamentous algae, are a normal and important component of the health of the lake.  Excessive growth of filamentous algae can, however, become a nuisance to lake users.  Dense mats of filamentous algae may be unsightly and impede swimming, fishing, or other lake recreation.  Decaying mats of algae mats may also cause odor problems and in smaller water bodies lead to oxygen depletion.  Filamentous green algae, however, do not produce toxins and do not pose a health threat to you and your family.

 

How can filamentous algae problems be avoided?

Excessive growth of filamentous algae is usually a sign of high nutrient levels in the lake.  Therefore, the primary prevention strategy is to control the sources of nutrients washing into the lake. Using no-phosphorus fertilizers, maintaining septic systems regularly, diverting roof and driveway runoff into grassy areas rather than directly to the lake, disposing of pet wastes in the garbage, and planting a buffer of native plants next to the lake shore are the best actions to help prevent nutrients from reaching the water. Find out more about how to reduce nutrients and prevent algae in a lake near you.

Related topics & additional resources

Algae and Lakes

Toxic Algae

Algae Bloom Examples

Snohomish County Algae Projects

Additional Algae & Lake Resources

 

Contact

Gene Williams, Senior Planner, 425-388-3464 extension 4563

Marisa Burghdoff, Water Quality Analyst, 425-388-3464 extension 4639

Some of these documents are in pdf format. To read these files you'll need to have Adobe Acrobat Reader installed on your computer.