PORT TOWNSEND — Anderson Lake has been closed to fishing and other recreation five days after the lowland lake fishing season opened.

Elevated levels of the potent nerve toxin anatoxin-a were detected in a water sample taken from Anderson Lake on April 25, it was announced Friday.

The toxin level is 1.30 micrograms per liter, which is just above the state recreational threshold of 1 microgram per liter.

The bloom of cyanobacteria in the lake near Chimacum contains toxin-producing species of algae, including Dolichospermum (formerly Anabaena) and Aphanizomenon.

Anatoxin-a can result in illness and death in people and animals when ingested.

Washington State Parks has posted “Danger — Keep Out of Lake” signs at access points to the park alerting visitors of the potential hazards.

Recreational activities such as fishing, boating and swimming are prohibited until the toxins dissipate and the lake is deemed safe for recreation.

The 410-acre Anderson Lake State Park remains open for other recreational activities such as horseback riding, hiking and birdwatching. A Discover Pass is needed to park there.

Jefferson County Public Health has monitored local lakes for cyanobacteria since 2007 after two dogs drank the water and died in 2006.

The 60-acre Anderson Lake has had closures every year since then. In 2019, a dog splashed briefly in the lake and died soon afterward.

The lake made history in 2008 with the highest levels of anatoxin-a ever recorded, 172,640 micrograms per liter.

Anderson Lake was monitored monthly this winter, and toxins were not detected, but starting in April, bloom activity and toxin level have been steadily rising, said Michael Dawson, water quality manager for Jefferson County Public Health.

Crocker, Gibbs, Leland and Tarboo lakes have all been assessed for cyanobacteria activity this year and researchers found no evidence of blooms forming.

The blue-green algae found in Anderson Lake is naturally occurring and found in freshwater lakes across Washington state. However, sometimes those algae blooms produce toxins, which are released into the water.

Although researchers know algae growth spikes in the summer when sufficient nutrients such as phosphorus are present, they don’t understand what sparks the production of toxins from some species.

The cause of the high amount of toxins in Anderson Lake isn’t understood as yet. One theory is that runoff from a dairy farm that operated near the lake for more than 50 years contributed a high amount of phosphorus to the lake, which fueled the growth of the blue-green algae.

To check the status of Jefferson County lakes and learn more about toxic cyanobacteria monitoring, consult the JCPH website at www.jeffersoncountypublichealth.org/723/Lake-Status or call 360-385-9444.

For fishing seasons and regulations, see the state Department of Fish and Wildlife website, www.wdfw.wa.gov/fishing.

Information on visiting Anderson Lake State Park is available at www.parks.state.wa.us/240/Anderson-Lake.

No toxic blue-green algae has been reported in Clallam County, where health officers do not test for toxins. Instead, they visually monitor lakes for signs of algae bloom.

Algae blooms in Clallam County lakes should be reported to the Clallam County Department of Health and Human Services’ environmental health division at 360-417-2258.