Blue Green Algae in Ponds : Signs & Solutions, How to control Cyanobacteria in a Pond.

Cyanobacteria are the bacteria that make up blue-green algae. When scums die, they usually appear green but can also turn bluish. Cyanobacteria are photosynthetic prokaryotes that are not real algae but bacteria. They love water that is quiet, warm, and nutrient-rich. When present in large quantities, blue-green algae represent a serious threat to water quality (known as an algal bloom). Large concentrations of blue-green algae cause taste and odor problems, and many blue-green algal species create chemicals that are harmful to higher creatures.

According to some estimates, there could be up to 7,500 different blue-green species. Given the proclivity of algae to take on different forms in different situations, this estimate could be excessive.

Several blue-green algae are;

  • Gloeocapsa: is a very simple blue-green that grows as single cells or in small clusters of cells.
  • Oscillatoria: is a common blue-green that grows in filaments.
  • Nostoc: is a type of blue-green that forms in colonies as large as a plum.
  • Anabaena: is a type of blue-green capable of using atmosphere nitrogen to make compounds that can be used by higher plants.
  • Chamaesiphon: is a type of blue-green that produces endospores. 


Bloom refers to an accumulation of algae cells to the point where they color the water, build scums, cause unpleasant tastes and aromas, damage fish populations, and impair water quality. Decomposing algae can also deplete oxygen levels, resulting in fish deaths.

Blue-green algae species may become dominant and overabundant in water if: 

  • nutrient levels, particularly phosphorus and nitrogen are sufficient to support the population growth
  • the ratio of nitrogen to phosphorous concentration is low
  • water is still and turbulence is low (lack of mixing)
  • weather patterns are stable for a week or so
  • weather is warm (although blooms can occur in cooler weather too).

Blooms of blue-green algae can last for several weeks, if not months, depending on the weather and flow conditions. Blooms may be reduced or prevented by cooler, windier weather or increased flow.

The cells tend to become ‘leaky’ when the bloom dies. Toxins will be released into the surrounding water if the bloom comprises species that produce toxins. Some toxins can last for up to three months after being discharged before degrading.


It’s nearly impossible to discover blue-green algae in a pond early. Because these organisms are so little. The capacity of blue-green algae to develop at any depth of water makes it difficult to spot, posing a dilemma for pond keepers, particularly rookie pond keepers.

The few signs of blue-green algae in ponds are:

  • Dead fish 
  • Unpleasantly scented water
  • Water will have scum (paint or growth mat like)
  • Water looking like pea soup

The change in pond watercolor is the easiest method to tell if you have blue-green algae in your pond. If you notice a change in the color of your water, you should have it tested for deadly cyanobacteria as away.

There are a few quick fixes for blue-green algae blooms that can help prevent additional damage. This allows you to gain control of the situation.

  1. Keep the animals away from the pond until it is diagnosed or removed.
  2. Scoop out all excess debris and start the damage control process.
  3. Use a liquid additive that will balance your pond’s ecosystem while removing blur green algae.
  4. Decrease the sunlight available to the blue-green algae: there are various ways to decrease sunlight available to the algae. Ways such as;
  5. Spreading wheat straw in a thin layer across the surface of the pond to shade the algae.
  6. Adding water-soluble dyes to the water reduces the amount of sunlight that can get to the pond.
  7. Installing submersed aeration. Adding oxygen at the bottom of the pond will act as the glue that will help bind the nutrients (phosphorous) to the iron in the sediment and keep it inaccessible to the blue-green algae.

Bloom frequency and intensity may be reduced in the long run by reducing the overall amount of nutrients in a water body. However, changing the nutrient contents in a body of water might take a long period.



Physical Management Options

Only by replenishing the pond water can floating, blue-green algae be managed mechanically or physically. The algae in the pond will be diluted by water exchanged from a well or other source that does not have an algal bloom. Unless their ponds are very tiny and they have wells nearby, this is not a viable choice for most pond owners.

Non-toxic dyes or colorants, like fertilizer, restrict or reduce aquatic plant development by limiting sunlight penetration. Dyes, on the other hand, do not boost the natural food cycle of the pond and suppress it.

Non-toxic colors and other products include, but are not limited to, the following:

  • Aquashade
  • Blue springs
  • Crystal blue

Biological Management Options

Fore biological control of cyanobacteria, a variety of naturally occurring species such as viruses, bacteria, fungus, actinomycetes, and protozoa are available. Isolation from environmental samples, characterization of anti-cyanobacterial activity, microcosm and large-scale field tests, and finally the creation of a biological control lake management strategy are all part of the process of developing these organisms into biological control agents. Actinomycetes (e.g., Streptomyces exfoliates, which acts by producing a lytic agent) and protozoa are the two opponent groups studied in-depth (Nuclearia delicatula and Nassula tumida, mode of action by predation). A variety of biological and Physico-chemical parameters influence the efficiency of biological control agents in the lake environment. To improve their performance, they can use a variety of tactics.


Always follow the recommendations and precautions on the product label, since the label is the law. To see the label, click on the product’s name. Read the label to see if there are any restrictions on how much water you can use.

The following active compounds are effective in the treatment of blue-green algae:

  • Copper Complexes
  • Alkylamine salts of Endothall 
  • Sodium Carbonate Peroxy-Hydrate

Copper Complexes

Because of its availability and inexpensive cost, copper, sometimes known as “bluestone,” is probably the most widely utilized algae treatment. Depending on how finely copper sulfate is ground, it comes in a variety of forms. Larger crystals melt more easily than smaller crystals. Copper sulfate is difficult to employ in extremely hard water because it binds to calcium, precipitates out of solution, and renders copper ineffectual as an algaecide.

All copper compounds can be hazardous to fish if used at levels higher than those recommended on the label, and they can also be poisonous in soft or acidic conditions when used at recommended levels. It is best to evaluate the alkalinity of the pond water before applying copper and adjust copper treatments to the alkalinity levels.

The following are examples of common trade or product names: 

  • Cutrine plus
  • K-tea
  • Captain
  • Clearigate

Alkylamine salts of Endothall

Endothall alkylamine salts are available in liquid and granular forms. It’s a herbicide that comes into touch with the plant.

The following are examples of common trade and product names:

  • Hydrosol

NB: Hydrosol can be toxic to fish.


Any chemical control strategy has the risk of oxygen deprivation following treatment due to the decomposition of dead plant material. Oxygen deprivation in the pond can kill the fish. If the pond is badly infested with weeds, it may be possible to treat the pond in portions and let each section degrade for two weeks before treating another section (depending on the herbicide used). Aeration over several days after therapy, especially at night, may help manage oxygen depletion.