Until recently, this surface film was considered a form of “blue-green algae”. Blue-green algae are actually classified as bacteria and are correctly called cyanobacteria. As bacteria, they are mobile and can rise and sink through the water column. They have chlorophyll and can produce food for themselves through photosynthesis. Possible colors range from yellow to red to violet to green to deep blue, blue-green, gray and black.

What is Cyanobacteria doing to my lake?

Probably the oldest living forms of life, cyanobacteria are not well understood. They exist everywhere. Nevertheless, nuisance blooms are increasingly troublesome throughout the Southeast. Cyanobacteria blooms are unfavorable due to their appearance, odor and ability to produce toxins. Fortunately the toxins are rarely associated with deaths of fish and livestock drinking the water. The blooms are not very stable and often crash in a relatively short period of time.

Unlike desirable plankton, cyanobacteria do not contribute significant amounts of dissolved oxygen. Cyanobacteria reproduce relatively slowly, about 1 time per day. Dead decomposing cells consume available dissolved oxygen. They are also not good oxygen producers like green plankton algae. The slow recovery of low oxygen levels contributes to occasional fish kills. Excessive blooms are not likely in lake and ponds we manage. As a result of our routine management, you are not likely to see fish kills and toxicity. We usually see short-lived thin surface films that are simply unattractive. 

The factors to consider when maintaining control over cyanobacteria:


Agitation: Flushing, mixing and turbulence all reduce cyanobacteria growth. If water flushes through the lake faster than the nuisance can bloom, problems are reduced. If mixing can overcome the ability of the bacteria to maintain its position in relation to light and available nutrients, growth is reduced. Shear turbulence can disrupt colonies and break up filaments. Aeration with fountains, and even diffused bubbles, can significantly reduce surface film formation.

Shading: With limited light available, photosynthesis is inhibited. The concentration of the bloom and the composition of the species present are affected by the amount of light available and the depth of light penetration through the water. Shading can be effective for reducing numbers. Lake colorants are one way to provide shading.

Temperature: Warm water temperatures, above 80oF., are associated with cyanobacteria blooms. Warm water is less dense than cooler water. Therefore, the warm water actually “floats” causing stratification allowing surface water to warm dramatically. Through contact with the air at the surface of the water, nitrogen and carbon dioxide can be used by cyanobacteria as food sources.


Excess nutrients: High levels of nitrogen and phosphorus are associated with cyanobacteria blooms. If the nutrients are reduced for months and years, blooms have been shown to diminish.

pH: Low pH (acidic water) tends to inhibit growth of cyanobacteria. When the pH is above 8.0 problems increase. Regular additions of lime can buffer the pH and reduce high pH conditions.

Salinity (or salt content): Although cyanobacteria occur in salt water, research shows that the species can be controlled by 1 – 5 parts per thousand of salt. Therefore, each acre-foot of water would require 270 – 1,350 pounds of salt.

Iron:  Apparently, cyanobacteria are capable of out-competing algae for limited levels of iron. When high nitrogen and phosphorus levels exist in the lake, adding iron may encourage the growth of desirable green algae. More research needs to be conducted using this method.


Fish : Tilapia are fish that eat algae and will grow while eating cyanobacteria. However, the fish are tropical and will die when water temperatures drop below about 50oF. Therefore, they have to be stocked each spring. If you stock about 50 – 100  fish per acre in the Spring they will effectively control the growth of some algae. Tilapia reproduce abundantly. However, availability is limited and predators may eat small fish. The fish cannot be stocked until the water temperature is consistently above 70oF in the Spring.

Beneficial bacteria concentrates: Although results are somewhat inconsistent, beneficial bacteria concentrates may compete with cyanobacteria for available nutrients. Revive is the product we use. It is a dry powder with 25 billion viable microorganisms and 18 billion spores per gram. A bran carrier contains a “conditioned” media that serves as nourishment when the powder is mixed with pond water. Supposedly, this allows the bacteria to double every 20 minutes. Furthermore, the spores germinate to the vegetative, active growing form.

Barley straw: The EPA has not registered barley straw as a pesticide to control algae growth, however, it has been found to be effective. The popularity of using barley straw to “clarify” ponds has significantly increased over the last several years; initiated in Europe and spreading to the United States.

The surface film types of cyanobacteria actually represent a very small volume of material in lakes/ponds. If you strain it out it would only be a “handful”. It is simply very noticeable. The methods described have been successful in controlling the volume of cyanobacteria, however it is very difficult to eliminate the species altogether. It might also be the case that continual chemical treatment for cyanobacteria will simply make it persist longer. If you are not too unwilling to live with the appearance of cyanobacteria for a week or two, it is likely it will die off.   

The season is almost over and the problem will be going away as the water cools and the sun gets lower in the sky. Depending upon your lake’s characteristics, our strategy for next year includes:

Preparation for Upcoming Year

  • Add barley straw twice per year.
  • Apply the Revive beneficial bacteria/enzyme concentrate at 3 pounds per acre-foot in March and at least 1 pound per acre-foot each month through September. Treatments lost to flushing will be re-applied. The beneficial bacteria should consume the nutrients required by the cyanobacteria.
  • Add colorants as necessary to limit light penetration.
  • Depending upon size, stock 50 - 100 blue per acre in May. The fish will be filtering the cyanobacteria from the water and growing.
  • Monitor stratification of the water column. Aeration with diffused air or fountains is always advantageous. If fountains are not mixing the lakes, it might be beneficial to add a sleeve so the fountain water intake is close to the lake bottom.
  • As a last result, algaecides can be used at a higher rate.  

Additional options:

  • Liming the lakes - Adding lime should buffer the water chemistry and reduce the high pH levels often found during the afternoon.
  • Reducing the amount of fertilizer entering the lakes
  • Dredging shallow ponds to increase pond depth

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