Hepatotoxins damage the liver. Microcystin, Cylindrospermopsin, and Nodularin are hepatotoxins. Symptoms of exposure to hepatotoxins include abdominal pain, vomiting, diarrhea, liver inflammation, hemorrhaging, lesions, acute pneumonia, and death (within hours to days after exposure). Human cases are often reported from drinking water as well as ingestion of untreated/raw water, contact during recreational activities, and hemodialysis with toxin-laden water.
Neurotoxins are nerve toxins and include Anatoxin-a, Anatoxin-a(s), and Saxitoxins. Symptoms of exposure to neurotoxins include tingling, burning, numbness, drowsiness, staggering, incoherent speech, gasping, convulsions, and respiratory paralysis leading to death (within minutes to hours after exposure). Human deaths have been associated with shellfish consumption (saxitoxins). However, humans can be exposed to these toxins via recreational contact as well. Animal deaths, especially dogs, have been associated with recreational exposure. There is limited data available on neurotoxin exposure via drinking water.
Dermatoxins are skin toxins and include Lyngbyatoxin-a, Aplysiatoxin, and Lipopolysaccharides. Symptoms of exposure to dermatoxins are similar to swimmer’s itch and include skin rashes and eye irritations.
Cyanobacteria cell walls contain an endotoxin (lipopolysaccharide) that can irritate any exposed tissue and are capable of eliciting an immune response, including gastrointestinal distress and fever, when exposed to the intestines.
The exact mechanism for HABs is not well understood. However, in general, algae blooms typically occur during periods of warmer temperatures and increased sunlight, an abundance of nutrients, and low flow conditions.
Many types of algae have an optimal growth temperature between 12°C and 15°C. However, the optimal temperature for most cyanobacteria growth is 25°C. When water is warmer than 25°C cyanobacteria can grow faster than other types of algae (ex. diatoms, green algae).
Cyanobacteria have adapted to flourish in variable and low light conditions unlike other types of algae. Some species of cyanobacteria, including the potential toxin producing Dolichospermum (formerly Anabaena), have gas vesicles that enable vertical movement through the water column. Adaptations such as gas vesicles allow movement toward optimal light concentrations and away from harmful light concentrations.
Low Wind/Low Flow Conditions
Water is more stable and less likely to mix during low wind and low flow conditions. With limited movement, lakes will thermally stratify; the top layer will heat up and without mixing the bottom will remain colder. The lack of mixing can lead to depletion of oxygen in the bottom layer, which can increase the release of nutrients from the bottom sediment.
Nutrients (Nitrogen and Phosphorus)
Typically, algae blooms, including cyanobacteria, are limited by phosphorus and/or nitrogen. An increase in the availability of nitrogen and/or phosphorus can lead to an algae bloom. Nutrients can wash into a lake or other waterbody from the surrounding environment. Many human activities including erosion from agricultural areas and lawns, deforestation, urban runoff, and sewage discharges can lead to nutrient enrichment of water and ultimately, an algae bloom.