Filamentous algae are a common concern among pond owners. Sometimes referred to as pond scum or incorrectly as moss (moss is a different division of plants), filamentous algae include hundreds of species; many are true algae, while several are cyanobacteria. Depending upon the species, they can resemble mats of wet wool, hair, cotton or slime that are usually green, but can become yellowish, grayish or brownish. Filamentous algae occur naturally in most surface waters.
A healthy pond ecosystem should have filamentous algae. Filamentous algae are eaten by gadwall, lesser scaup, channel catfish and other organisms. They provide substrate and cover that support aquatic insects, snails and scuds (amphipods), which are important foods for fishes, ducks, amphibians and other organisms. However, filamentous algae can become problematic, especially in ponds with excessive shallow, clear water and nutrient inputs (particularly phosphorus and nitrogen). When over-abundant, filamentous algae interfere with fishing, swimming, boating, irrigation, fish production and pond attractiveness.
A temporary bloom of abundant algae frequently does not warrant control. A narrow strip floating around the margin of a pond usually does not need control. In waterfowl ponds, even more extensive growth can be acceptable. The best way to prevent filamentous algae problems is to build a pond properly and divert nutrients and/or filter nutrients with robust vegetation because limiting shallow water and nutrients minimizes opportunities for algae growth. Where nutrient reduction is impractical in ponds with environments conducive to excessive filamentous algae growth, management options include herbivorous fish, chemical algaecides (such as copper sulfate, Cutrine®-Plus, GreenClean®, Reward® and many others), water dyes or pond fertilization to limit light penetration, removal of algal mats by raking or netting, and deepening shallow portions of ponds.
We found herbivorous fish was the least expensive option to provide long-term control in ponds and troughs that consistently grew excessive filamentous algae. Deepening a pond is the only option that lasts longer, but is considerably more expensive. Although many publications claim grass carp do not control filamentous algae well, diploid grass carp always controlled it in Noble Research Institute ponds when healthy fish 8 to 14 inches long were stocked at 12 per acre and were contained with appropriate parallel-bar barriers across the spillways. Control usually lasted 20-25 years. Unfortunately, such stocking rates also controlled all submersed aquatic plants and controlled several emersed aquatic species. Lower densities of grass carp, sometimes as low as five or six per acre, frequently controlled filamentous algae, but we did not work enough with lower stocking rates to confidently identify control thresholds. I believe there seldom was a need to stock more than 10 per acre.
As of Jan. 1, 2010, diploid grass carp are no longer legal to stock into Oklahoma ponds and never were legal for Texas ponds. Pond managers who stock grass carp in either state are required to stock triploid grass carp. Triploids have an extra set of chromosomes, which make them sterile. There is some scientific debate whether triploids are as effective as diploids.
Personally, I would try triploid grass carp before implementing another more expensive, shorter duration management option.
Goldfish can be an effective option for reducing filamentous algae in livestock troughs that do not go dry. Grass carp do not work well in troughs because they tend to jump out. Goldfish are not a good option for earthen ponds because they usually increase clay turbidity (muddiness), which has negative repercussions for fish production, duck habitat and pond attractiveness.
In summary, many filamentous algae problems can be prevented with proper pond design and management, the mere presence of filamentous algae is not necessarily a problem, and several options exist to manage ponds with excessive filamentous algae.