Harmful Algal Blooms

Green and yellow harmful algal bloom in a lake.

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What are harmful algal blooms?
People often refer to harmful algal blooms as
What are the signs of harmful algal blooms
What health effects of harmful algal blooms
How to keep your family safe from harmful algal blooms
Public Health contact information

What are harmful algal blooms (HABs)?

Harmful algal blooms happen when cyanobacteria grow in bodies of water like:
- Fresh water
- Brackish water (fresh and sea water mixed)
- Marine waters (ocean water)
Bacteria can grow quickly when conditions are good, and nutrients are present.
The algal create large blooms that may produce harmful toxins.

People often refer to harmful algal blooms as

Cyanobacteria
Blue-green algae
Golden algae
Algae

Bright green, blue, brown, or red water. It may look like paint floating on the surface.
Cloudy water
A very bad odor, like sewage or rotten eggs.
Dead plants or fish
Toxins can remain for days, even after the water looks clear.

Some harmful algal blooms release toxins that can poison humans and animals.

People exposed to these toxins may face:

Skin rashes from contact with contaminated water.
Diarrhea and vomiting occur if they drink the water.
Damage to the liver, kidneys, or nervous system occurs if toxin levels are high.
Some cattle and dogs have died after drinking contaminated water or eating dried algae from the shore.

Check for reported bloom:
- Visit the CDPH webpage to review the HAB Reports Map.
- Contact the waterbody manager at (530) 225-5661.
- Look for advisory signs.
Follow the recommendations on any posted advisory signs.
Visit the California Water Quality Monitoring website to report any suspected HAB or related illness.
Stay away from the water if you suspect a harmful algal bloom. If you're unsure, stay out.

Updated April 24, 2025, by the Communicable Disease Unit, sloomis [at] shastacounty.gov (sloomis[at]shastacounty[dot]gov)

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