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Filamentous Algae: Understanding Control and Prevention in Ponds

by Joe Cadieux on December 15, 2022

What is up my friends?

Filamentous algae:

Otherwise known as:

Horsehair, cotton candy, pond scum, moss (though there are aquatic mosses that can be a big issue too)…

The nuisance forms of filamentous algae form heavy matts and/or slimy balls, and/or stick to rocks and wave in the streams and ponds edges.

The Filamentous algae that is prolific enough to become a problem are all colonial organisms… That is, they are many individual, multi-celled (most of the time) micro organisms living together.

There are many thousands of genera of green and blue-green (cyanobacteria) algae.

These are photosynthetic organisms. They excrete oxygen as a waste product (thanks for that btw) this process is also why they become an issue when they become overabundant.  All filamentous algae begin there lives attached to the bottom.  As the grow, the oxygen (and other gases) begin to accumulate underneath the matts as they form.  The collecting gases and buoyant biomass get to a point where it peals off of the bottom and floats to the surface.  This does two things…. It creates those floating matts we all love, and it makes way for the next generation to begin its life on the vacated bottom rocks/sediment.

The above information is important because it tells us why controlling them can be a challenge.  Filamentous algae can grow very quickly because it is a simple organism and can assimilate available nutrients at high rate because 100 of its surface area is capable of fueling growth.  This allows for rapid regrowth after treatments.  If even one cell is left alive, it will begin growing and multiplying immediately.

This also makes any treatment a temporary endeavor. Algae are not plants, they do not have roots or a fixed location, and therefore cannot be controlled systemically.  That is: one treatment will not control them for an entire season.  If there are enough nutrients and sunlight, the algae will regrow.

As I am wont to say…. Every pond is different.  Water chemistry, temperature, weather, biodiversity, and nutrient load all play a huge part in how ponds fare through the season.  So the rule of thumb is that there are no rules of thumb.  There are generic guidelines about dosage and a multitude of products out there to help control this scourge, but it will take some tinkering to find out what works for your pond and what will not.

Filamentous Algae Control:

If one allows the problem to get out of hand, algae control requires much detail and attention to potential changes in the water column.  Whether you have a back yard watergarden or a 3 acre pond….. Large algae blooms must be dealt with carefully.  If the bloom covers more than half of the pond, or inundates sensitive areas (special habitat,  spawning fish, or unique cover)  you should not treat the entire area at one time. As the heavy product volumes necessary to gain control and the associated oxygen deficit from the breakdown of the dead and dying algae will potentially consume too much oxygen in the water leading to a fish kill and other ecological degradations.

During times of large algae blooms treat 1/2 the area or less and wait several days for the algae to break up, then treat the next section.  This will ensure the survival of your fish and allow the ecosystem to rebound to a more stable state.

Once the matts emerge on the surface, know that the next generation is not far behind. All algaecides are contact killers and are most effective when broadcast on top of existing mats and churned into water where algae is regrowing from the bottom.  This way, you are controlling both the matts you see and the matts that would float to the surface next week.

Watergardens are easier as they have built in mixing apparatus that will transport the algicide to the problem. but it is still important to apply to areas that do not mix well to get a complete treatment.  Pleased not use copper based algicides on your koi, they will not thank you for it. (That is code for “they will die….and it’s all your fault”) use the oxidizers like (Algaefix)

Large ponds…. unless you are raising trout (see koi from above) the chelated copper algicides are the way to go. Chelated copper algicides do not persist in the environment and with not bioaccumulate.  So please dose according to the label and you should be fine.

In both cases, whether it a watergarden or large pond, if the algae hasn’t turned brown and gone away with in 4-5 days, treat again because it didn’t work.  Increase your dose, or try a different algicide.  Each pond is different, and if yours has difficult conditions then a professional like myself should probably come out and take a look.  At the very least, give me a call/comment/email…. tell me what the problems are and I’ll steer you in the right direction.

Also in both cases, be sure to fire up your bacteria and enzyme regime a couple of days after the treatment; just about all algicides also kill the beneficial bacteria you have growing in the pond.

This is a large discussion that I have not explored fully here, but I hope that I’ve given you some idea in how to proceed and make you favorite waterbody pleasing and usable for the duration of the season. please contact me with questions so I can get you the answers your are searching for.


Thanks for taking the time….


Senior Biologist



http://grotonlakes.com/lake-weeds/algae/ I https://informationtips.wordpress.com/2015/05/26/cyanobacteria-photo-gallery-of-green-and-blue-green-algae/ I http://selftution.com/plant-kingdom-plantae-examples-classification-characteristics/