What is filamentous algae and why is it dangerous to my pond?

What is filamentous algae and why is it dangerous to my pond?

Filamentous algae sounds nasty and looks nasty too. Filamentous algae is the type of moss you see on some ponds that has long, intertwined threads or filaments. The filaments can grow together to form a mat that seems like an icky, green wet wool or a mat of human hair.

Most people would recognize filamentous algae better by the simpler terms “pond moss” or “pond scum”.

Filamentous algae in threads.


It starts out growing on the bottom of your pond in shallow water or attached to structures in the water like docks, rocks or plants.

Pond moss offers no real value to recreational ponds, and instead can be very hazardous, toxic and a yucky, ugly nuisance to swimmers and sport fishing. It cannot be directly used/eaten by aquatic animals. The best thing we can say about it is that in some non-recreational lakes, the algae can be somewhat beneficial but only when it’s kept at a low level. Pond moss can make for decent cover for aquatic animals such as snails, some insects and scuds. (Scuds, aside from being popularized in the 1st gulf war as short-range missiles, are also a tiny form of crustacean that look similar to shrimp.)

A scud. Ain't he a pretty little guy?

A scud. Isn’t he a pretty little guy?

Typically however, filamentous algae can be a huge detriment to the life of a pond. It reproduces at an amazing rate; you might not see any and then all of a sudden have a huge algae bloom. As the algae grows and grows it gives off oxygen. The oxygen gets trapped in the weave of intertwined threads of algae and whole bunches of it rise to the surface.

Algae has a short “shelf life” however, and as it dies off it is consumed by decomposer bacteria. This bacteria gives off dangerous CO2 (carbon dioxide). If there’s enough dead algae and enough bacteria, the CO2 levels could be dangerous. Low oxygen levels can lead to fish kills.

The pond can quite literally choke itself out and become a dead zone.

So it’s a good idea to keep an eye on just how much algae is in your pond. There are ways of controlling or completely getting rid of filamentous algae which we talk about in other articles.

If you’d like more information, don’t hesitate to contact us with questions!

Call us (1-877-772-MUCK) or email us!