Toxic Algae

Several lakes in Snohomish County have experienced blooms of potentially toxic blue-green algae. Surface Water Management coordinates testing of blooms in response to citizen reports. When high levels of toxins are identified, alerts will be posted on this page and the lake public access sites.

Algae Scum Surrounding Boat at Lake Ketchum

Frequently Asked Questions

How can I be exposed to toxic blue-green algae?

Toxic algae refers to a particular type of algae found in lakes and streams called blue-green algae or cyanobacteria. This type of algae is a natural part of lakes, but sometimes it can grow rapidly or bloom. Lakes experiencing a bloom will look cloudy. The algae will begin to accumulate as a surface scum and often resembles blue or green paint. A handful of blue-green algae species sometimes produce toxins during blue-green blooms.

Should I be concerned about toxic algae?

When blue-green algae is producing toxins, high levels can cause serious illness in humans and animals who play in or ingest affected waters. Toxic algae exposure can kill pets, waterfowl, and other animals. Pets who drink lake water are at particular risk. Smaller children who may ingest water when swimming are also at higher risk.

What do I do if I see a blue-green algae bloom?

If you see a bloom of blue-green algae, please contact us to report it. Once a bloom has been reported, SWM will test the lake, and when needed, will post the lake as follows:

  • CAUTION if potentially toxic blooms are found (see sign below)
  • WARNING if unsafe toxin levels are found (see sign below)

Follow the sign advisories to not swim or water ski (especially in areas of scum), do not drink lake water, keep pets and livestock away, and avoid areas of scum when boating. Even if a lake has not been posted and you see a blue-green scum, please exercise caution as conditions can change rapidly. When in doubt - stay out.

Sign warning that toxic algae is present in a lake and urges precautions to avoid health risks.
Sign recommending staying out of lake if toxic algae blooms might be present

Signs posted in conjunction with the Snohomish Health District - Click on signs to enlarge

How can I be exposed to toxic blue-green algae? 

You or your pets can be exposed to cyanobacteria and cyanobacterial toxins by swimming or drinking water where cyanobacteria are present. Pets are at particular risk for drinking the water as they may prefer the taste of the algae scum. You may also be exposed by breathing air when in close proximity to scums of  cyanobacterial cells producing toxins. Wind surfing, jet-skiing, boating, or watering lawns adjacent to heavy scum accumulations are activities where this might occur.

What should I do if I am exposed to toxic blue-green algae and experiencing symptoms?

Blue-green algae can produce both nerve toxins (neurotoxins) and liver toxins (hepatotoxins). Call your doctor or veterinarian right away if you or your pets or livestock have signs of poisoning. For more information or to report exposure incidents after seeking treatment contact the State Department of Health. For health related inquiries you may also call the Snohomish Health District at 425-339-5250.

Can I eat fish from the lake when a toxic bloom is present?

 One type of toxin, microcystin, can accumulate in fish tissues, especially in the organs (liver, kidneys, etc.). Concentrations in the tissues would depend on the bloom severity where the fish was caught. Take caution when considering consumption of fish caught in a water body where major blooms occur. Before eating, remove the internal organs, which may contain more of the algae/toxin.

Is it safe to drink water from the lake during a bloom if it has been filtered?

The Snohomish Health District advises that lake water never be used as a drinking water source. The State Department of Health indicates "In addition, studies have shown that conventional water treatments such as coagulation, sedimentation, filtration, and chlorination may be ineffective for removal toxins associated with blooms of toxic cyanobacteria". For additional information contact the State Department of Health.

 How does the bloom impact wildlife?

Wildlife such as waterfowl, otters, and deer might also be affected by a bloom, particularly if they are consuming water with high concentrations of algal scum. Please report any incidents where you suspect wildlife may have been injured from the conditions.

What is causing the toxic algae blooms?

Blue-green algae is a normal part of the lake system and are always there in low numbers. Blue-green algae blooms often occur in late summer and early fall. While temperature and sunlight play a role in blooms occurring, often times excessive nutrients - particularly phosphorus - is the primary factor leading to blooms.

Frequent toxic algae blooms are one symptom of excessive phosphorus pollution and a sign of declining water quality. Phosphorus pollution flows into the lake from the surrounding areas, mostly from lawn fertilizers, poorly maintained septic systems, pet and animal waste, and runoff from roofs and driveways.

What can I do to help prevent toxic algae?

The best way to help prevent future toxic algae blooms at your lake is to reduce phosphorus coming into the lake. Even small reductions in phosphorus can make a big difference for your lake. Snohomish County has a program called LakeWise which helps residents take simple, voluntary actions on their properties to reduce phosphorus pollution. Join your neighbors and find out more today.