What are Harmful Algal Blooms؟

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General Information

Though many types of algae can form blooms, freshwater harmful algal blooms have the ability to produce toxins that are dangerous to other organisms such as humans, dogs, and livestock. Most harmful algal blooms occur in warm, slow moving, eutrophic waters in mid June through late September and are formed by cyanobacteria (aka blue-green algae) which are now known to be photosynthetic bacteria. Algal blooms can also be caused by haptophytes, dinoflagellates, green algae, raphidophytes, euglenophytes, diatoms and cryptophytes, but though they can be a nuisance, they do not produce toxins like cyanobacteria do and have not been linked to any adverse human health effects in the United States.


Photo Credit: Erika Phillips

It is possible for blooms to appear quickly and form floating mats of various colors, however, not all blooms form mats on the surface. Some remain in the water column and discolor the water. While it is estimated that most blooms are toxic, it is difficult to predict exactly when or even if a bloom is producing toxins. A bloom of a certain type of cyanobacteria may or may not produce toxins and the toxicity may change throughout the duration of the bloom. It takes a few days for blooms to be sampled and tested for toxicity, and by that time the toxicity may have changed. Furthermore, the harmful effects of blooms may even occur when a bloom is not visible.

Blooms that occur in drinking water sources can produce compounds that lead to toxicity as well as taste and odor problems of the water. The majority of the time, taste and odor compounds, not toxicity, are the largest problem in terms of cyanobacterial contamination in drinking water. Toxicity can occur in drinking water even without taste and odor compounds, though. In Wisconsin, most drinking water comes from groundwater, not surface water, so drinking water problems caused by harmful algal blooms are rarely a concern.

Some of the techniques that can be used to control or remove blooms, such as algaecides, can increase the toxicity of the bloom. They may also be detrimental to organisms other than cyanobacteria and are generally not recommended. The most effective method to reduce blooms is community effort to reduce the Phosphorous loading of fresh water bodies. Though cyanobacteria are found in some of the oldest fossils and are natural to aquatic habitats, harmful algal blooms are increasing in frequency in the United States and across the world and are becoming more of a problem.


Cyanobacteria are prokaryotes that are usually as large as eukaryotic algae and can be either filamentous or non-filamentous. Most cyanobacteria stay near the surface of the water and have mechanisms such as gas vesicles to control their buoyancy. They can adjust buoyancy with light and nutrient levels during the day, but at night they may float to the surface, causing it to look like the bloom appeared rapidly. Also, on days when there is little mixing due to wind, the buoyant cyanobacteria can float to the surface and form blooms with high population densities because they are all on the surface instead of being spread through the water column. Because of this, when mixing is weak, cyanobacteria have the advantage over other phytoplankton.

The genus cyanobacteria is very large and diverse. They can occur in all types of water (marine, estuarine, fresh), and their critical light intensities also vary greatly. Cyanobacteria colonies are usually too large for zooplankton to eat, and they are generally not nutritious which leads to little top-down control. In addition, most also form a mucilage sheath that makes them unpalatable to organisms such as zooplankton and zebra mussels, which can help their population to grow to bloom numbers. Because blooms of one type of phytoplankton prevent other algae from growing, consumers and predators may starve during cyanobacterial blooms due to their lack of palatability.

Nitrogen and Phosphorous are necessary for cyanobacterial growth, but Phosphorous is usually the limiting nutrient because they are generally poor competitors for Phosphorous in comparison to other phytoplankton. However, Planktolyngbya, Anabaena, and Aphanizomenon are strong competitors for Phosphorous in lakes that contain more organically bound Phosphorous. This is because these types of cyanobacteria produce the enzyme alkaline phosphatase that releases Phosphorous from small organic Phosphorous compounds. The Nitrogen fixing cyanobacteria Anabaena, Nodularia, Nostoc, Gloeocapsa, Trichodesmium, and Synechococcus are most often poor Phosphorous competitors. This is why waters that are nutrient rich due to fertilizers high in Phosphorous, untreated sewage, wastewater, or agricultural runoff are more likely to experience blooms.

Some cyanobacteria produce heterocysts that alow them to fix nitrogen and/or resting cells called akinetes that allow them to survive through unfavorable conditions. The WHO says that 100,000 cells/mL is a moderate human health risk, but there are currently no standards for cell or toxin concentrations in the United States.

Cylindrospermopsis raciborskii is a type of cyanobacteria that isn’t native to Wisconsin but it was probably brought to the state by waterfowl. It usually blooms in late July through late September and stays in the water column. It can produce multiple toxins including Cylindrospermopsin, but that hasn’t been found in any Wisconsin lakes.

Table of Common Cyanboacteria: (some values may not apply to entire genus)

Diameter (µm)
Colonies, Filaments
Carbon Content (pg)
C/Cl a weight ratio*
Cell Volume (mm3)
Anabaena 4-14a Blue-green to yellow-green Yes Yesf Filamentsa Gas vacuolei 7.6l 50 116-399j Saxitoxins, anatoxins, microcystins, cylindrospermopsins
Aphanizomenon 5-6a Pale blue-green, blue-green Yes Yesa Filamentsa Gas vacuoleb 3m 50 32-89j Saxitoxins, cylindrospermopsins, anatoxin-a
Aphanothece 1.0 × 1.5c Green, brown, blue-green Yes Yesg Coloniesb Not buoyanth *0.2 50 2.1j  
Microcystis 3-5a Blue-green, grayish, yellowish No Noa Coloniesa Gas vacuolesa 291.3n 50 19-182j Microcystins
Oscillatoria 7d Blue-green, brownish, pinkish No Noh Filamentsa Some have gas vacuolesh 2m 50 1.9×10-8 m Anatoxin, aplysiatoxins, microcystins, saxitoxins, lyngbyatoxin-a
Cylindrospermopsis 2.5-4 x 2.5-16e Pale blue-green, olive green No Yesb Filamentsb Gas vacuolesb *0.2 50 15j Cylindrospermopsin, saxitoxin
Gloeotrichia 8-10a Olive green, yellow-green, brown, blue-black Yesa Yesb Coloniesa Gas vacuolesa *0.2 50 **2.52-3.66 μm3 Microcystinsk

* General for marine phytoplankton – Fukuda, R., H. Ogawa, T. Nagata, and I. Koike. 1998. Direct determination of carbon and nitrogen contents of natural bacterial assemblages in marine environments. Appl. Environ. Microbiol. 64:3352-3358.
**General for cyanobacteria – C. K. Lockert, K. D. Hoagland, and B. D. Siegfried. 2006. Comparative Sensitivity of Freshwater Algae to Atrazine. Bulletin of Environmental Contamination and Toxicology. 76:73-79.
a – Common Cyanobacteria Species. (accessed 5/13/10).
b – Komárek J. & Hauer T. (2010): – On-line database of cyanobacterial genera. – Word-wide electronic publication, Univ. of South Bohemia & Inst. of Botany AS CR,
c – Dwivedi, R.K., Shukla, S.K., Shukla, C.P., Misra, P.K. and M.K. Seth. 2008. Cyanophycean Flora Of Southern Himanchal Pradesh, India. Ecoprint. 15:29-36.
d – Prescott, L.M., Harley, J.P., Klein, D.A. Microbiology. Third Edition. Dubuque, IA: Wm. C. Brown Publishers, 1996. pp. 37-41.
e – Valério, E., Chambel, L., Paulino, S., Faria, N., Pereira, P. and Rogério Tenreiro. 2009. Molecular identification, typing and traceability of
cyanobacteria from freshwater reservoirs. Microbiology. 155:642-656.
f – Anabaena variabilis ATCC 29413. (accessed 5/13/10).
g – Singh, P.K. 1973. Nitrogen fixation by the unicellular blue-green alga Aphanothece. Archives of Microbiology. 92:59-62.
h – Micro*scope. (accessed 5/13/10).
i – Allaby, Michael. Anabaena. (1998). In A Dictionary of Plant Sciences. Retrieved from
j – Appendix 4: Biovolumes Explained. (accessed 5/13/10).
k – Carey, C.C., Haney, J.F., Kathryn L. Cottingham. 2006. First Report of Microcystin-LR in the Cyanobacterium Gloeotrichia echinulata. Environmental Toxicology. 22:337–339.
l – Gilbert, John T. 1990. Differential Effects of Anabaena Affinis on Cladocerans and Rotifers: Mechanisms and Implications. Ecology. 71:1727-1740.
m – Fanuko, N., and Marko Valčić. 2009. Phytoplankton composition and biomass of the northern Adriatic lagoon of Stella Maris, Croatia. Acta Bot. Croat. 68:29-44.
n – Tóth, L.G., and Kenji Kato. 1996. Development of Eodiaptomus japonicus Burckhardt (Copepoda, Calanoida) reared on different sized fractions of natural plankton. Journal of Plankton Research. 18:819-834.
o – Huisman, J., Matthijs, H.C.P., and Petra M. Visser. Harmful Cyanobacteria. Dordrecht, The Netherlands: Springer, 2005.



The toxins produced by cyanobacteria are the main concern during harmful algal blooms. They are produced within the cells and remain there until the cells break open, which may lead to the unpredictable toxicity of blooms. Normal water treatment processes can remove cyanobacteria and their toxins, but sometimes further steps are necessary when cell or toxin concentration is high. The extracellular toxins are also more difficult to remove than the toxins within the cells.

In aquatic environments, the toxins are broken down by heterotrophic bacteria, but during times of slow mixing, the toxins can build up because the bacteria can’t get to the toxins as quickly. The World Health Organization standard for Microcystin-LR in drinking water is 1.0µg/L, but there are no standards for recreational waters in the United States.

Microcystins are the most commonly occurring toxins. There are at least sixty microcystins known now and the toxicities can be vastly different. Nodularin is very similar to microcystins and has the same effects. Anatoxins are the cause of paralytic shellfish poisons are very poisonous. Lyngbyatoxin a and aplysiatoxins are both tumor promoters, and lyngbya toxin a causes seaweed dermatitis. Cylindrospermopsis is most harmful to the liver, but it may damage other organs as well.

The toxins produced by cyanobacteria can also harm other aquatic organisms. The toxins produced by Scytonema hofmanni and Fischerella muscicola can inhibit photosynthetic electron transport. Anabaena flos-aquae prodices a toxin that paralyzes the green alga Chlamydomonas, and microcystins inhibit carbon fixation in other phytoplankton.

Toxins Produced
Anabaena circinalis Saxitoxins
Anabaena flos-aquae Anatoxins, Microcystins
Anabaenopsis Microcystins
Aphanizomenon Saxitoxins, Cylindrospermopsins
Cylindrospermopsis Cylindrospermopsins, Saxitoxins
Hapalosiphon Microcystins
Lyngbya Aplysiatoxins, Lyngbyatoxin a
Microcystis (M.aeruginosa, M.viridis) Microcystins
Nocularia Nodularin
Nostoc Microcystins
Phormidium (Oscillatoria) Anatoxin
Planktothrix (Oscillatoria) Anatoxins, Aplysiatoxins, Microcystins, Saxitoxins
Schizothrix Aplysiatoxins
Trichodesmium Not yet identified
Umezakia Cylindrospermopsin

From Cyanosite –

Toxin Types
Neurotoxins Anatoxin-a, Anatoxin-a(s), Saxitoxin, Neosaxitoxin Affects central nervous system, causes seizures, paralysis, respiratory failure, and death
Hepatotoxins Microcystins, Nodularins, Cylindrospermopsin Affects liver, causes hemorrhaging, tissue damage, tumors, liver cancer, and death
Dermatotoxins and Gastrointestinal toxins Aplysiatoxins, Lyngbyatoxin-a, lipopolysaccharide endotoxins Affects skin and mucous membranes, causes rashes, respiratory illness, headache, and stomach upset
Cytotoxins Cylindrospermopsin Affects liver and other organs, causes chromosome loss, DNA strand breakage, and organ damage



Exposure to cyanobacteria and cyanobacterial toxins can occur from:

  • Drinking water that contains cyanobacteria
  • Drinking untreated water
  • Engaging in recreational activities in water experiencing a bloom
  • Inhaling aerosolized cyanobacteria or toxins such as the spray from jet skiing and boating, and using contaminated water to water lawns or irrigate golf courses
  • Consuming cyanobacterial supplements containing mycrocystins
  • Dialysis (this has only occurred in Brazil)


Effects of Harmful Algal Blooms


  • Harmful algal blooms negatively impact the food web by decreasing the amount of nutritious, edible phytoplankton that zooplankton and other primary consumers need to survive. These organisms may then starve, leading to decreased food for secondary and higher order consumers. Increased cell concentration can block sunlight from primary producers under the water’s surface as well, leading to decreased food and oxygen levels. When the cells in the bloom begin to die it can also lead to decreased dissolved oxygen levels that can be lethal to other aquatic organisms and cause fish kills. Low dissolved oxygen can be made worse by overcast days and warmer temperatures.


  • Decreased recreational use and aesthetical value of waters due to toxicity, mats of algae, and the smell when cells begin to die are only some of the problems associated with harmful algal blooms. Cyanobacterial blooms can contaminate drinking water with taste, odor, or toxic compounds. The toxins produced during blooms are possible carcinogens to humans and current research is studying the link between certain cyanobacterial toxins and neurological disease. Harmful algal blooms have been known to kill waterfowl and livestock, and dogs have died after eating mats of cyanobacteria or licking their fur after swimming in bloom infested waters. In some cases, humans have also died after exposure to harmful algal blooms.

sourceGLEON Harmful Algal Bloom Working Group Resources

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