Filamentous algae are colonies of microscopic plants that link together to form threads or mesh-like filaments. These primitive plants normally grow on the surface of hard objects or other substrates under the water but they can break loose and form floating mats. Filamentous algae are important because they produce oxygen and food for the animals that live in the pond, but they also can cause problems such as clogs and stagnancy. Filamentous algae do not have roots; rather they get their nutrients directly from the water, meaning that their growth and reproduction are entirely dependent on the amount of nutrients (i.e. fertilizer) in the water. Because stormwater ponds collect water flowing from yards and roads in the community, they often grow an abundance of algae as a result of the many sources of nutrients in residential and commercial developments. It is not uncommon for stormwater ponds to develop large floating mats of algae during the warm months of the year in response to fertilization of lawns and animal wastes in the watershed.
Are clumps of filamentous algae unhealthy? Should the water be clear?
No, not necessarily. Most filamentous green algae do not produce toxins that are harmful to humans. The algae are growing in response to nutrients that have washed into the pond, so excessive growth of algae may indicate that there are other pollutants that also have washed into the water. If the source of nutrients is pet or animal waste, it is likely that bacteria and other pathogens are living on the algae mats. Residents should not handle algae harvested from stormwater ponds because there may be harmful bacteria associated with it. If homeowners do handle algae, they should wash their hands thoroughly or use sanitizer. Algal growth serves as Nature's way of capturing nutrients and contaminants that otherwise would be carried downstream to impair rivers and beaches, so having some algae in a stormwater pond helps the pond do what it is designed to do, capture contaminants and protect water quality. The clumps are unsightly, but they are not themselves a threat to your health.
Can too much algae become a problem?
Yes. When mats of filamentous algae grow to the extent that they cover large areas of the pond surface, they limit the exchange of oxygen between the water and the atmosphere, and they prevent photosynthesis from producing oxygen in the water. As a result, ponds that are largely covered in algal mats are more likely to have fish kills and noxious odors due to lack of oxygen. Also, large algal mats can contribute to areas of stagnancy, clog outfalls, and contribute to localized flooding. Excessive algae growth contributes to increased sedimentation and will accelerate the time table for dredging.
Should I try to eliminate all algae? How much is too much?
No. Homeowners and HOAs should tolerate some algae during the warmer months (June - September) of the year because it is a natural part of the aquatic ecosystem and the annual cycle of plant growth. On the other hand, ponds that have algae covering more than 20% of the surface area are more likely to develop stagnancy, noxious odors and fish kills, so it is recommended that filamentous algae be controlled to prevent it from covering more than 20% of the pond surface. During droughts when water levels are very low and temperatures are very high, it is not advised to treat ponds for excessive algae because of the increased threat of water quality impairment and a fish kill.
How do I control filamentous algae?
Use Integrated Pest Management...
How can homeowners or HOAs prevent and control algae blooms?