If you are at a lake and see a yellow “WARNING” sign, there are toxic algae in the lake.

warning sign
This lake is unsafe for people and pets.
  • Do not swim or water ski.
  • Do not drink lake water.
  • Keep children, pets, and livestock away.
  • Clean fish well and discard guts.
  • Avoid areas of scum when boating.

The yellow “WARNING” sign is posted when laboratory tests have found unsafe toxin concentrations in the water (above the Washington State Recreational Guidance values).

It is OK to boat or fish if you are careful and follow the advice above. But keep yourself, your children, and your pets out of the water! Keep your dogs from contacting the water – algae will stick to their legs, and dogs will lick the algae off of their fur.

If you are at a lake and see a red “DANGER” sign, there are very high concentrations of toxins in the lake.

danger sign
This lake is closed.
Keep out of the lake.
Do not swim, wade, fish, or boat.

The red “DANGER” sign is not common – so pay attention and keep out! This sign is only posted when laboratory tests have found very high concentrations of toxins in the water. There is a serious danger to people and pets.

Walking around the lake is OK, but make sure you can keep your children and pets away from the water at all times. Even a short exposure to the water could make people or pets sick. Keep your dogs from contacting the water – algae will stick to their legs, and dogs will lick the algae off of their fur.

Health risks

Cyanobacteria can produce many toxins, but two types of toxins are more common in Washington waters: microcystins and anatoxin-a. Microcystins are a group of toxins that affect the liver. Because microcystins are the most commonly-found cyanobacterial toxins in water, they are the toxins most responsible for human and animal poisonings. Microcystins are very stable and do not break down quickly in the environment.

Anatoxin-a is a potent toxin that affects the nervous system. Symptoms include lethargy, muscle aches, confusion, memory impairment, and, at sufficiently high concentrations, death. It is believed that a fast-acting algal neurotoxin such as anatoxin-a caused the deaths of two pet dogs at Anderson Lake, Jefferson County, in the summer of 2006.

Image for toxic algae bloom in Hicklin (Hicks) lake
Toxic algae bloom in Hicklin (Hicks) lake

See also Washington State Department of Health website for more information about cyanobacterial toxins.

Not all cyanobacteria blooms are toxic. Even blooms caused by known toxin-producing species may not produce toxins or may produce toxins at undetectable levels. Scientists do not know what triggers toxin production by cyanobacteria. However, toxicity is hard to predict in part because a single species of algae can have toxic and non-toxic strains. Also a bloom that tests non-toxic one day can turn toxic the next day.

Symptoms from sub-lethal poisonings differ with the kind of animal, nature of toxin, and quantity of toxin consumed. Pets and wildlife have died after exposure to toxic blue-green algae in Washington lakes. Any sudden, unexplained animal illness or death occurring near a water body containing a bloom should be suspect. People typically avoid swimming in or drinking cyanobacterial scums, so reports of human illnesses associated with blooms are much lower than reports of animal illnesses in Washington.

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