String algae can harm ponds

Q. I have these mats of scum on my pond. I think its filamentous algae. If it is, how do I get rid of it?

Mark from Beloit

A. Without a sample we can’t know for sure, but it is most likely filamentous algae, commonly called string algae.

Filamentous algae are single-cell plants that form long threads (filaments) in the water. Filamentous algae grows on the bottoms of ponds and on submerged objects. They are a natural and important part of the pond ecosystem, producing oxygen and serving as food for pond animals.

In warmer months the filaments will break off, float and form mats on the surface of the pond. This is when most people notice them.

When the mats grow to cover a significant portion of the pond, problems can develop. Too much algae is unsightly, reduces photosynthesis in the pond and can cause stagnation. Large amounts of the algae can kill fish. This happens when the algae reduces pond oxygen either by loss of photosynthesis or when the algae dies and decomposes in late summer.

How much filamentous algae forms on a pond is directly related to the amount of nutrients in the pond water.

Filamentous algae do not have roots – they get their nutrients directly from the water. The more nutrients in the water, the more algae. These nutrients (phosphorus and nitrogen) can come from fertilizers, manure (wildlife, livestock, etc.), septic systems and storm runoff into the pond.

The best long-term control for filamentous algae is to reduce the amount nutrients entering the pond. Two of the best options for doing this are redirecting runoff and creating a vegetative buffer zone around the pond so those plants absorb the nutrients.

Short-term management options include physical removal using a rake or other tool. Be sure to wear protective clothing, such as gloves, when doing this. While not toxic itself, algae can trap toxins that may be in the water.

If the problem is not too bad, just let the algae run its course; it will be gone when the weather cools.

Biological controls include fish that feed on the algae. However, this algae is not a favorite food for pond fish in our area. They will eat it when nothing else is available. Lastly, chemical controls can be used. If you use chemicals, only herbicides approved for aquatic use by the U.S. Environmental Protection Agency are permitted. Be sure to read and follow the directions on the label. For information, visit go.osu.edu/filamentous.

This week’s answer by OSU Extension master gardener volunteer David Sprague. You can meet him at the Mahoning Plant and Pest Clinic. Call the clinic at 330-533-5538 to submit your questions. Regular clinic hours are 9 a.m. to noon Mondays and Thursdays.