Surface Water Management Division
Algae are microscopic organisms similar to plants that can be found in all freshwater lakes. Algae are a natural and essential component to lakes because they serve as the base of the aquatic food chain. Usually, algae have little impact on the ability of people and animals to recreate and enjoy lakes. However, certain types of algae are capable of producing toxins that pose a threat to the health of humans and animals that come into contact with the toxins.
Tell me more about toxic algae
Algae are primarily categorized into groups based on their color – for example green or golden-brown algae. One group of algae is known as blue-green algae or cyanobacteria. These algae are actually a type of bacteria that produce blue-green pigments that allow them to conduct photosynthesis like other types of plants and algae. Blue green-algae are extremely common in freshwater environments and, in most cases, are not toxic. However, there are a few species of blue-green algae that are capable of producing toxins, particularly when their populations rapidly expand to form dense algae masses or algae blooms. The species that most frequently cause toxic blooms are Anabaena, Aphanizomenon, and Microcystis (sometimes referred to as “Annie”, “Fannie” and “Mike”). The WA Department of Ecology has more information on toxin-producing algae.
A bloom of Anabaena that was found to be toxic in 2008.
Is it possible for my lake to have toxic algae?
Blue-green algae (cyanobacteria), including the toxin-producing species, can be found in lakes at any time of year, although their presence is often undetectable. However, in the right weather and water conditions, most often in late summer and early fall, algae blooms can form. It is typically during blooms that blue-green algae may produce harmful levels of toxins. When an algae bloom occurs, the water becomes cloudy, the lake color may change, and a scum of dying algae may appear on the lake surface such as in these bloom examples. Blooms of toxin-producing algae are typically bright green or blue in color and are often mistaken for paint floating on the surface.
Blue-green algae bloom that was tested and found to be toxic in 2007.
It should be noted that not all algae blooms are caused by blue-green algae and not all blue-green blooms are toxic, although blue-green blooms may be unsightly and produce bad odors. Blue-green algae blooms are most common in lakes that receive excess nutrients from sources such as lawn fertilizers, road runoff, soil erosion, and poorly-maintained septic systems.
My lake is experiencing an algae bloom - how do I know if it is toxic?
If your lake is experiencing an algae bloom, it does not necessarily mean that the lake water is toxic. First, the bloom may be caused by another type of algae, such as filamentous green algae. Next, even if the bloom is caused by a toxin-producing blue-green species, it may not actually be producing toxins. The only way to be certain if a bloom is toxic is to test the water for toxins. It is always best to assume that a bloom may be toxic and take the appropriate precautions until tests can confirm whether a bloom is toxic
If you want to have a bloom tested, please contact the Snohomish County Lakes Program (contact information below). We can facilitate sampling and testing of the lake through the Washington Department of Ecology’s Algae Monitoring Program. To help keep you and other lake users informed, Snohomish County Surface Water Management, in conjunction with the Snohomish Health District, will post the public access sites of lakes that are experiencing toxic algae blooms. If a blue-green algae bloom is observed, the lake will be posted with a “Caution” sign. If the bloom has been verified to have harmful toxin levels, a “Warning” sign will be posted. You may click on the signs to the right to see the enlarged versions. In addition, you can also sign up for email notification if you wish to be notified of toxic algae problems at your lake.
What should I do if my lake is having a toxic algae bloom?
Because blue-green (cyanobacterial) toxins can be lethal to animals and humans in relatively small amounts, caution should always be taken when a bloom occurs. If you see what you believe to be a blue-green algae bloom in your lake, the Washington Department of Health, recommends the following:
- Stay out of the water; Do not swim or water ski
- Do not drink the lake water, even if it is treated
- Keep pets and livestock away from the lake
- Clean fish well and discard guts
Continue being cautious even after a bloom is no longer visible as certain types of toxins can be persistent in the water for days or weeks after a bloom has dissipated.
How could a toxic algae bloom impact my family's health?
According to the Washington Department of Health, toxins produced by blue-green algae may affect humans, pets, waterfowl, and other animals. These toxins have the potential to affect the liver, the nervous system, or exposed skin. In order to be affected, people, pets or wildlife have to be exposed to the toxin by drinking or playing in water with a toxic bloom. Pets may also be affected by licking algae from their fur after swimming in a lake that is experiencing a toxic bloom. Please see the Washington Department of Health for extensive information on the health effects and appropriate response to exposure to toxic algae blooms. If in doubt about possible impacts from exposure to toxic algae, contact your doctor or veterinarian.
Why do toxic algae blooms occur and how can I prevent them?
The exact reason for algae blooms is not always clear. Factors that can contribute to blooms are excessive nutrients, warm temperatures, and sunlight. Although algae blooms are natural, their frequency can be increased by the addition of excessive nutrients into the lake. In many cases, excessive lake nutrients are caused by human activities in and around the lake and can be prevented. Sources of nutrients include: fertilizers, pet wastes, and erosion from construction and land clearing. Nutrients may also directly enter the lake through poorly-maintained septic systems. Find out what you can do to reduce the potential for toxic algae blooms in your lake.
Where can I find more information on Toxic Algae?
Algae and Lakes
Algae Bloom Examples
Snohomish County Algae Projects
Links to Additional Toxic Algae Resources
Gene Williams, Senior Planner, 425-388-3464 extension 4563
Marisa Burghdoff, Water Quality Specialist, 425-388-3464 extension 4639
Jen Oden, Water Quality Specialist, 425-388-3464 extension 4352
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