Algal Bloom

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An algal bloom describes the development of a high concentration of planktonic algal cells that forms rapidly when water conditions are suitable. This can occur in a freshwater or marine environment.

There is usually one or a small number of phytoplankton species involved, and this may cause the water to appear coloured. Most blooms are green or blue-green. 'Red tides' usually occur with species of dinoflagellate, and 'brown tides' with diatom blooms.

Factors leading to the appearance of blooms include:

temperature: relatively warm
high light levels
excess of nutrients: runoff of fertilisers, feed wastage in ponds


Harmful Algal Blooms (HAB) are used to describe blooms that lead to the release of toxins. These toxins can lead to fish kills, usually from an osmotic imbalance due to increased gill permeability.

The toxins can also become concentrated in fish and shellfish rendering them toxic to humans.

Apart from producing toxins, algal blooms can be detrimental to fish in several other ways:

Algae produce oxygen during the daylight house as a by-product of photosynthesis, and this is a major source of oxygen in fish pond waters. At night or during cloudy weather, oxygen production ceases or is very reduced, but algae still consume oxygen, resulting in a deficit in the waters. Low dissolved oxygen in the water can lead fish to suffocate or be more susceptible to disease.

In waters with a low or moderate buffering capacity, algal blooms can lead to wide fluctuations in pH and if it reaches 10 or above this can depress fish growth and health.

If large numbers of the algae die off due to competition for space and nutrients, this can lead to high ammonia concentrations which can affect fish appetites and growth rates and even lead to death.

If the algal bloom is dense, this may limit photosynthesis and oxygen levels at the bottom of the pond and may lead to a buildup of toxic gases and waste products. Fish will be increasingly stressed, have a lower production rate and may even die because of this.


The ideal situation would be to keep algal blooms at an intermediate level where oxygen production would be maximal during daylight hours in relation to what is needed at night. However the regulation of algal bloom levels is still very difficult and imprecise.

Chemicals to control the blooms, such as the use of copper sulphate, is usually ineffective as algal decomposition can lead to rapid oxygen depletion, and the copper can accumulate to toxic levels in ponds.

Excessive feeding contributes to the problem by providing excessive nutrients to the algae. Underfeeding may help control the problem of algal blooms but is usually not economical for most commercial producers.

Lack of oxygen can be compensated for by the use of mechanical aerators. This practice should be proactive and is of little use once oxygen depletion is present.

Dissolved oxygen levels should be checked every night during the warmer months.

It may be possible to anticipate when a bloom will die back and deplete the oxygen by noting any changes in the colour and odour of the bloom.

Biological methods, such as filter-feeding fish, would be the ideal methods of control, but have been unsuccessful to date.

Algal bloom can also cause toxicity in mammals.

Algal Bloom Learning Resources
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Test your knowledge using flashcard type questions
Ornamental Fish Q&A 18


Moksness, E. (2004) Culture of cold water marine fish John Wiley and Sons

Roberts, R. (2001) Fish Pathology Elsevier Health Sciences

Brunson, M. (1994) Algae Blooms in Commercial Fish Production Ponds Southern Regional Aquaculture Centre publication

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