Filamentous Algae - Conditions and Control

filamentous algae on boat oar

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. 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 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 collection of animal wastes in the watershed.

Are clumps of filamentous algae unhealthy?  Should the water be clear?
No, not necessarily. Most filamentous 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 in 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. 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.

Pithophora covering pondCan 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 due to lack of oxygen. Also, large algal mats can contribute to areas of stagnancy and noxious odors, clog outfalls, and contribute to localized flooding. Lastly, excessive algae growth contributes to increased sedimentation and filling-in of the basin.

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 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, 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...

  1. Prevention: The only way to prevent chronic regrowth of algal mats is to reduce the nutrients washing into the pond (see section below).

  2. Physical controls: Mechanical/manual harvesting and raking can provide immediate short-term control but are not feasible long-term maintenance strategies. Applying aquatic dyes prevents sunlight from penetrating to the plants and slows the rate of growth. Aquatic dyes must be applied early in the season and reapplied regularly throughout the growing season to be effective.

  3. Biological controls: Blue Tilapia (Oreochromis aureus) is a fish that eats filamentous algae and provides good control through the growing season.  Tilapia are tropical fish, which need to be restocked each year due to winter die-back. Triploid Grass Carp do not provide very good control of filamentous algae. 

  4. Chemical control: Several herbicides are labeled for control of algae in ponds. State law requires that these compounds only be applied to stormwater ponds by licensed applicators. Homeowners and HOA board members should not apply herbicides to stormwater ponds. Ponds that are experiencing excessive algal growth are more likely to have a fish kill when treated with an herbicide because of the rapid death and decay of the algae. Licensed applicators should be able to determine if it is feasible to treat a pond for algae without harming the aquatic environment.floating wetlands

  5. Technology: Floating wetlands may be placed in ponds to absorb excess nutrient in the water and reduce algal growth. Floating wetlands use native wetland plants contained in a synthetic matrix that floats to pump nutrients out of the water. Diffusion circulators can assist with managing nutrients in the water and prevent stagnancy, and they do not clog like fountains and irrigation pumps.

How can homeowners or HOAs prevent and control algae blooms?

  1. Soil test before fertilizing to prevent over-fertilizing.
  2. Sweep/blow fertilizers off of roads and driveways after spreading to prevent it from washing into storm drains.
  3. Do not apply fertilizers on the bank slopes of ponds.
  4. Do not discard grass clipping or yard waste in ponds or ditches.
  5. Pick up pet waste and throw it in the trash.
  6. Do not wash cars in driveways - the soaps break down into fertilizers.
  7. Do not feed fish, turtles, or waterfowl because it is essentially fertilizing the pond.
  8. Plant shoreline wetland plants to filter runoff and absorb nutrients.
  9. Install floating wetlands to utilize nutrients from the water.
  10. Consider aquatic dyes to suppress algae growth.