PORT TOWNSEND — Anderson Lake, which opened for the fishing season Saturday, was closed Thursday after tests results showed the water contains a high level of a quick-acting nerve toxin.

“They will close it today,” said Greg Thomason, Jefferson County environmental health specialist, on Thursday afternoon.

“The park remains open, but the lake will be closed today.”

State park Ranger Kinnan Murray made the determination to close the lake to fishing and other recreational use Thursday after the county received the most recent tests results, said Lori Bond, office assistant at Anderson Lake State Park.

“The signs are going up, and it is closed today,” Bond said.

The 410-acre park itself remains open. Only the 70-acre lake within the park, which is between Port Townsend and Chimacum, is closed.

“You can horseback-ride, ride your bikes, anything you want to do — except water recreation,” Thomason said.

“They got only five days of fishing.”

The lake was thick with boats last weekend as people flocked to it to fish for trout on the first days of the statewide lowland fishing season.

Thomason recommended that State Parks close the lake Thursday after receiving test results from King County Environmental

Labs of samples taken Monday

The results showed that Anderson Lake water contained 1.52 micrograms per liter of anatoxin-a, a nerve poison that can quickly cause convulsions and stop breathing.

The safety threshold for the toxin is 1 microgram per liter.

“It jumped up five times what it was last week,” when 0.30 micrograms per liter was found in the water, said Thomason, adding that the level had been steadily increasing this month.

The toxin is created by certain types of blue-green algae.

Blue-green algae flourishes in warm temperatures when sufficient nutrients, such as phosphates, are present, but researchers don’t understand why some species of blue-green algae begin to produce toxins, nor what makes them increase.

Since 2006, Anderson Lake has been closed during parts of the warmer months because of dangerous levels of toxins.

“There’s a bloom going on there now,” Thomason said Thursday.

A Discover Pass is needed to visit state parks.

It can be bought at Anderson Lake State Park, as well as at others, and costs $10 for one-day use or $30 for an annual pass, good for one year from the date of purchase.

Tests results found that the anatoxin-a levels remained low in Leland, Gibbs and Crocker lakes.

“Leland is still good at 0.14 [micrograms per liter]. Gibbs is less than detectable, and Crocker is 0.02 [micrograms per liter], Thomason said.

The levels of microcystin, another type of algae-produced toxin historically found in East Jefferson County lakes, could not be detected in samples from any of the lakes, Thomason said.

Microcystin can cause skin irritation and, if ingested over a period of many years, can result in liver failure.

Caution signs remain at Leland, Gibbs and Crocker lakes because they contain species of algae that are known to produce toxins at times.

The county’s yellow caution sign warns lake users to refrain from drinking lake water and from swimming or boating in areas of scum.

It also urges users to keep pets and livestock away from it and clean fish well and discard guts.

Gibbs is south of Port Townsend. Leland is north of Quilcene.

Crocker Lake is off U.S. Highway 101 near the intersection with state Highway 104.

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 by phoning 360-417-2258.

Anyone who observes an algae bloom at a lake is urged to phone the Jefferson County Public Health Department at 360-385-9444.

For more information about lake quality in Jefferson County, visit the environmental health website http://tinyurl.com/6z64ofy.

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Managing Editor/News Leah Leach can be reached at 360-417-3531 or at leah.leach@peninsuladailynews.com.