Harmful algal blooms (HABs) are not limited to marine environments, they are also found in freshwater. These HABs are commonly the result of cyanobacterial -- or blue-green algae -- blooms. Bloom conditions that favor freshwater HABs include: increased nitrogen and phosphorous loading (high nutrients), slow moving water, stability in the water column, and high dissolved organic matter. Bloom decay commonly occurs rapidly, often leading to oxygen-deprived conditions. Over 40 species of freshwater cyanobacteria have been implicated in toxic blooms.

HABs are commonly found in freshwater systems throughout the U.S. and also in estuaries where fresh and saline waters meet.
Humans and other animals are exposed to these toxins through skin contact, inhalation, water and food consumption. The most common route is drinking water. Affected animals include ducks, geese, grebes, gulls, songbirds, fish, cattle, and dogs.
Freshwater toxins are diverse and result in a variety of symptoms that depend on the toxin composition of the algae. Symptoms may range from gastroenteritis and neurological disorders to liver damage and respiratory failure.
  • Anatoxin-a: Toxin affects muscle contraction by mimicking a neurotransmitter (i.e., acetylcholine) that causes ion channels to remain open. Muscle cells contract until they fail from exhaustion. Symptoms include staggering, decreased movement, abnormal breathing, convulsions, and death due to respiratory paralysis.
  • Anatoxin-a(s): Toxin binds to the enzyme responsible for deactivating acetylcholine, leaving the ion channel open. In domestic animals, symptoms include hypersalivation, mucous nasal discharge, diarrhea, and tremors.
  • Saxitoxins: Structure and behavior are the same as those found in marine algae.
  • Microcystin & Nodularin: Like okadaic acid, they inhibit the proteins (phosphatase 1 and 2A) that control sodium secretion; however, these are selective for liver cells. These toxins have also been demonstrated to promote liver tumors. In 1996, microcystins caused 26 deaths in Brazil when contaminated reservoir water was used at a dialysis clinic.
  • These are examples of species that produce these toxins:
  • Anatoxins are produced by Anabaena flos-aquae (one of the most toxic cyanobacteria), Aphanizomenon flos-aquae, and Oscillatoria sp.
  • Saxitoxins are produced by Anabaena circinalis, Aphanizomenon flos-aquae, and Lyngbya wollei.
  • Microcystins are produced by Microcystis spp., Anabaena spp., Nodularia spp., Nostoc spp., and Oscillatoria spp.
  • Nodularins are produced by Nodularia spumigena.
  • Image credits: Cyanosite, Roger Burks (University of California at Riverside), Wayne Carmichael (Wright State University), Isao Inouye (University of Tsukuba), Susanne Kratzer (Stockholm University), and Mark Schneegurt (Wichita State University).