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Cynobacteria (Blue-Green Algae)

What are blue-green algae

Algae are naturally occurring organisms usually present in low numbers in freshwater lakes, ponds, and streams throughout the world. Algae are a natural component of our waterways. Conditions such as high nutrient levels and sunlight levels, combined with surface waters that are warm and undisturbed, can cause these organisms to reproduce rapidly, forming visible colonies, referred to as algal blooms. Blue-green algae are technically known as cyanobacteria and are able to photosynthesize, creating a green, yellow, blue-green, or brownish color. Some algal blooms may look like blue-bright green paint floating on the surface of the water, or may form mats, foam, and/or scum on the surface. As the algae die, a foul, musty-smelling odor is released.

Are blue-green algae blooms harmful?

Nearly all species of blue-green algae (cyanobacteria) produce dermatoxins that under certain conditions can cause skin irritation, rashes, and/or gastrointestinal distress; sensitivity to these toxins varies widely from on individual to another. A few species of cyanobacteria can produce more harmful toxins, and ingestion of large quantities of these toxins can occasionally cause more serious health effects. 

Do's and Don'ts 

  • Do avoid contact with water where algae mats or floating scum are visible or where surface water is discolored.
  • Do rinse off (yourself, children and pets) after swimming in any lake, pond, or stream regardless of the presence of visible algae.
  • Do obey posted signs for beach closures.
  • Do report any large algal blooms by calling the Water Resources Division of the Department of Public Works at (707)263-2341
  • Don't drink untreated surface water, regardless of whether visible blooms are present, as it may contain harmful bacteria and parasites. IMPORTANT: Boiling the water WILL NOT remove algal toxins.
  • Don't wakeboard, water-ski, or jet-ski over algal mats.
  • Don't allow children or pets to wade in, swim in, or drink water where algae or algal mats are visible
  • Don't use algaecides or disinfectants to kill the cyanobacteria - when the cells die, the toxins are directly release into the water.
  • Don't irrigate crops, lawns, or gold courses with water that looks or smells bad

Download Cyanobacteria FAQs (English) or Cyanobacteria FAQs (Spanish) for more information.
Or visit the County of Lake Environmental Health webpage