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Home > Resources & Publications > Newsletters & Magazines > Chenier Ecology > 2010 > 3-10

Resources & Publications:  Chenier Ecology

March 2010

Filamentous Algae Control

One of the most common aquatic weed problems encountered by fish pond owners is pond scum. The proper term is actually filamentous algae.

Filamentous alga usually becomes a problem in clear water with shallow areas, which allows sunlight to reach the bottom or near the bottom. This promotes the growth of filamentous algae and eventually other aquatic weeds. In most instances the pond owner is not aware of the filamentous algae growth until the mats “overgrow” and begin to die back. When this happens, the dead alga begins to decompose releasing gases. These gases form bubbles in the algae mat and float it forming a slimy green mass on the surface. In extreme cases the decomposing mats can consume enough oxygen to cause fish kills. These unsightly mats also make fishing difficult.

The best method to controlling filamentous algae is prevention. If possible, during construction, deepen pond edges so that the slopes and areas less than two feet deep are minimized. Once soil particles settle out of the water following construction and bank edges have established vegetation, ponds usually clear up. This is the critical time in preventing aquatic weed problems. When water clarity reaches 18 inches of visibility, it’s time to either fertilize the pond to promote planktonic algae bloom or dye the water.

Since winter time fertilization is ineffective, early spring is the best time to fertilize, once waters have reached 60-65F. Liquid fertilizers specially formulated for ponds work best when sprayed onto the surface during bright sun. These pond formulations, usually high in phosphorus, will have ratios such as 0-34-0 and are sometimes called superphosphates.

Planktonic algae not only prevents sunlight from penetrating shallow, clear water areas, but it also jump starts the food chain in ponds. These microscopic plants undergo photosynthesis, which releases oxygen into the water and provides food for microscopic organisms, insect larvae and larval fish. Larger fish feed on these, thus driving the food chain up to the desired fish species we often harvest for food. Research has shown that fertilized ponds produce two to three times the fish as unfertilized ponds.

Another alternative to controlling filamentous algae is dye. Several manufacturers sell pond dyes under a variety of trade names and they come in several colors including blue, green and black. Dyes are recommended in excessive weed growths where fertilizers may exacerbate the problem. These dyes are fairly inexpensive and last for several months so long as the pond does not receive heavy runoff and flow through.

Copper sulfate is the recommended herbicide for controlling algae. However, caution must be used not to treat the entire pond in one application since killing large masses may consume dissolved oxygen. Also, copper can be toxic to fish so be sure to follow label recommendations and rates.

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