What is Filamentous Algae?

Of all the problematic algae species in ponds, the most feared and most ambitious is filamentous algae. This algae species can be relentless, quickly reproducing and clinging to any surface in your pond. This is also the most well recognized algae species in pond care. Any pond owner can identify filamentous algae by its characteristic hair-like mats. Worst of all, it is ugly and makes your pond look horrible!

At first glance, you may think hair algae would be single-cell loners in your pond much like planktonic algae. However, they are unicellular, but the cells enjoy the company of other filamentous algae cells. As a result, this algae grows together in thick, hair-like strands that cling to pond surfaces. There are several species: Spirogyra, Cladophora and Pithophora.

Spirogyra is easily identified by its bright, almost neon-green color. At the microscopic level, this algae resembles a DNA helix. In the pond environment, it’s very slimy and usually has thick bubbles of protein (i.e. scum) surrounding it. Cladophora is another very common filamentous algae that resembles cotton. If you look closely at the shallow parts of your pond, you’ll probably find Cladophora. Cladophora resembles tiny plant-like stalks that undulate and look “hairy”. Pithophora is the third most common filamentous algae that grows in very long thick strands. Pithophora is also called “horse hair” algae because it resembles thick long hair and feels like wool.

You’ll first notice filamentous algae in the shallow parts of your pond. If you have a waterfall, or several pond ledges, they will establish itself in the bottom shallows first. In small populations, filamentous algae are quite harmless and will not become prolific as planktonic algae. Out of control breeding occurs when you have high nitrates and waste in your water, which serves as food. Unchecked, filamentous algae can grow quickly and make your pond look like a mat of green scum.

As the algae spreads to the bottom of your pond, it produces large amounts of oxygen as it grows. This oxygen gets trapped in the thick mats, and eventually these mats float to the surface. This is why ponds with string algae problems have thick floating mats. Left unchecked, they can quickly cover your entire pond surface.

In order to prevent an explosion, you should stock your pond with plenty of plants and/ or have a good filtration system in place. Plants compete with algae for oxygen and and nitrates while also providing shade. All of these things prevent algae from overrunning your pond. Another important control measure for filamentous algae is mechanical removal of floating mats that may emerge from time to time. Just by raking these mats from the surface of your pond, you can instantly cut your population in half. Then, of course, there are algaecides on the market that can make quick work of this pest. Just be sure to follow directions accordingly.

If you find that your pond is quickly becoming overrun with filamentous algae, you can quickly reduce the population by vacuuming your pond floor. This will cut down the breeding population and reduce waste at the bottom, which cuts down the food supply. With a little bit of elbow grease, filamentous algae is easily controlled.