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Beneficial Algae or Unsightly Pond Scum?

Algae are an important component of an ecosystem, but when algae populations grow out of control, they begin to decrease water quality. The summer months generate high water temperatures and abundant sunlight. This in combination with excessive nutrient overloads from decaying plant material, fish and livestock waste, dead bugs, and fertilizers encourage excessive algae growth.

Filamentous algae in a pond. Photo by Dr. Glen, NC State Extension.

Did you know that despite being in the plant kingdom, algae is actually not a plant at all? It does not have true roots, stems, or leaves. Algae can range in size from microscopic to a meter long and even resemble plant-like structures. Algae also serve as a food source for many organisms including fish, insects, mollusks, and zooplankton. The mere presence of algae indicates that you have an active ecosystem in your pond. Therefore, it is recommended that you tolerate some level of algae. Especially during the warmer months when the “bloom” of algae naturally occurs. It is suggested that algae cover no more than 20% of your pond. Greater than 20% pond surface area coverage of algae starts to develop foul odors, stagnant waters, and fish kills.

Filamentous Algae

While there are many types of algae, the one that I get the most calls about is filamentous algae. These microscopic algae link together to form threadlike masses. Filamentous algae is often referred to as “pond scum” or “pond moss” and can be controlled using the following methods:

  • Prevention: reduction of nutrient sources into the pond, utilizing floating wetlands, or adding straw

    Floating wetlands. Photo by Clemson Cooperative Extension.

  • Mechanical: manual harvesting of filamentous material and applying pond dyes to minimize sunlight penetration
  • Biological: Grass carp fish offer partial control
  • Chemical: herbicides include copper and endothall (Hydrothol), remember to always follow label instructions when applying any chemical treatments

There are many different species of filamentous algae, however, most of them can be control similarly as listed above. Some species such as blue-green algae are more difficult to control. If you have any questions on the type of algae present in your pond or how to control it, contact your local county extension agent.

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