Harmful algal blooms (HABs)

Bird flying over water
In this article:
  • Find answers to frequently asked questions about harmful algal blooms.
  • Learn about methods to prevent harmful algal blooms.
  • See reporting guidelines and procedures.
Bird flying over water

Harmful algal blooms (HABs) can affect all living organisms. HABs are found in both freshwater and marine environments and are caused by a variety of toxin-producing microalgae, including prokaryotic blue-green algae (also known as cyanobacteria) and eukaryotic microalgae. The toxins produced by HABs vary depending on the organisms. Toxicosis can result in acute or chronic disease in most vertebrates, depending on the type of toxin and amount of toxin absorbed.

The microalgae that create HABs are impacted by environmental changes. Changes in our environment and our way of life have increased the frequency of HABs and have brought a new focus to their impacts on animal populations.


Cyanobacteria, a type of microalgae, are ancient photosynthetic freshwater prokaryotic organisms found worldwide. These organisms can proliferate in a body of water under specific environmental conditions, such as warm water temperatures and availability of phosphorus and nitrogen. These proliferations are termed harmful algal blooms (HABs).

At least 95 cyanobacteria species have the potential to produce toxins, and cyanobacterial toxicosis has been documented in humans, companion animals, livestock, and wildlife. Cyanobacteria produce several types of toxins that can cause clinical diseases including respiratory, gastrointestinal, dermatological, and neurologic disease. In some circumstances, they can result in sudden death.

Cyanobacterial toxins
  • Hepatotoxins
    cylindrospermopsin, microcystins, nodularins

These toxins damage the liver, leading to depression, loss of appetite, vomiting, diarrhea, abdominal tenderness, and jaundice minutes to days after exposure.

  • Neurotoxins
    anatoxins, guanitoxin (formerly called anatoxin-a(s)), saxitoxins

Anatoxins are nicotinic agonists. Guanitoxin is a cholinesterase inhibitor that cannot cross the blood-brain barrier, and saxitoxins block sodium channels.  Each of these toxins disrupts nerve cell communications, and they can lead to respiratory paralysis. Many cases of neurotoxin ingestion result in death within one hour.

  • Dermatotoxins
    aplysiatoxin, lyngbyatoxin-a

These cause irritation to the skin and also can irritate the respiratory airways if inhaled.  This leads to coughing and wheezing.

Red tides and other marine HABs

Marine HABs are sometimes red, but green or clear water also can be highly toxic. These HABs are caused by several species of dinoflagellates and diatoms, which are single-celled organisms that can be found at different levels of the water column.

Red tide toxins
  • Brevotoxins: Neurotoxic shellfish poisoning (NSP)
  • Saxitoxins and their derivatives: Paralytic shellfish poisoning (PSP)
  • Domoic acid: Amnesic shellfish poisoning (ASP)
  • Okadaic acid: Diarrhetic shellfish poisoning (DSP)
  • Ciguatoxin: Ciguatera poisoning