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THE HAPPY little lake that could kill you looks as harmless as a watercolor painting. Anderson Lake’s 60 acres of glassy green water is surrounded by marshes, rolling grass fields and quiet trees, all nestled within a Washington state park west of Chimacum in Jefferson County. The closest “civilization” is an RV park about a mile up the road.

The critters here include the usual sorts: Deer, beaver, migratory waterfowl and schools of rainbow trout, stocked for Port Townsend-area anglers who have plied these waters for decades. But in a mystery that has confounded scientists for the better part of a decade, the placid waters also now host an alarmingly deadly life-form.

Anatoxin-a, which attacks the central nervous system of mammals, emerges here in potent levels with algae blooms every spring. In the past decade, the lake’s toxic soup has arrived as regularly — and, disarmingly, perhaps every bit as naturally — as the blossom of Northwest skunk cabbage.

The toxin is one of several produced by “cyanobacteria” or blue-green algae — the sort often seen fouling lakes ranging from tranquil ponds to concrete-jungle oases, such as Seattle’s Green Lake. Numbers of outbreaks — or at least their diagnoses — are on the rise across the country, according to the Environmental Protection Agency.

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Many of these blooms, telegraphed by thick, green (occasionally brown or scarlet-red) surface scum, are harmless. But for reasons unknown, they sometimes produce poisonous toxins. The most common in these parts is microcystin, which attacks the liver. Drinking water to more than 400,000 people in and around Toledo, Ohio, was shut off this summer because of a microcystin outbreak in Lake Erie. Mycrocystin also has closed Seattle’s Green Lake to public access several times in the past decade.

More rare, but far more lethal in small doses, is anatoxin-a, which mysteriously thrives in sleepy little Anderson Lake.

How potent is it? Bacteriologists once referred to the substance as “VFDF” — short for “Very Fast Death Factor.” Depending on the dose, its potency and the size of the victim, anatoxin-a can kill a person in less than five minutes.

“If you were to tip over in your kayak and get a mouthful, that would be pretty bad,” says Jefferson County health specialist Michael Dawson, one of many people shuffling pieces of the Anderson Lake algae puzzle.

While toxin-related animal deaths appear to be on the rise, reports of human illness remain infrequent, with many cases likely undiagnosed. But the potential is frightening, especially given the strong suspicion that climate change is boosting the blooms. In recent years, the focus of that ominous potential has shifted toward obscure little Anderson Lake, where water samples six years ago revealed a level of anatoxin-a that literally leapt off the charts.

One microgram of anatoxin-a per liter of water is considered safe to animals and humans. A 2008 Anderson Lake sample registered 172,640 micrograms per liter. It was, by far, the highest level of anatoxin-a ever recorded, in any body of water, anywhere on the planet. Another test in 2009 registered 144,000 micrograms. Newer testing methods have not replicated those astonishing highs. But tests still routinely show toxins at up to 1,000 times the safe limit.

Few lakes in the U.S. are routinely tested for harmful algal blooms, or “HABs,” in lab-coat lingo. Anderson Lake’s outbreak, in fact, was discovered in a typical way — by accident, after the events of a single frightening week alerted health officials to a murky scientific puzzle which, eight years later, they have yet to solve.

ON MEMORIAL DAY weekend, 2006, Virginia Johnson, a veterinarian at Hadlock Veterinary Clinic, got a frantic call: A dog and its owner, a local physician, had been out for a routine summertime romp at Anderson Lake. The dog, a 4-year-old Walker coonhound in perfect health, waded in the water and lapped some up.

About half an hour later, while walking around the park, the dog lapsed into a seizure and died. Johnson, puzzled by the sudden death of an otherwise healthy dog, suggested a necropsy, but the owner declined. A sheriff’s report was posted, but the lake remained open.

Days later, two other dogs that had ingested Anderson Lake water were stricken. One exhibited seizure symptoms and lapsed into a coma, but eventually recovered. The other died on the way to a clinic. Blood samples from the dogs eventually pointed to anatoxin-a. The three pets had become, in what unfortunately is an oft-repeated pattern, “sentinels” for the Anderson Lake outbreak.

In the absence of expensive, regular water testing at most lakes, animals are usually the first victims. Even if they don’t imbibe, dogs are particularly prone to licking the toxin off their coats.

Compounding the problem: Because many blooms are not toxic, people rarely think to keep animals away. The telltale blooms, which can resemble thick paint in the water, are often obvious. But only lab work can detect poisonous toxins.

Blue-green algae has survived for an estimated 3 billion years by being spectacularly persistent — and elusive. Quick to appear and driven by wind or current, waterborne clouds of the blooms can constantly move around a lake. Even knowing where a bloom is gives little insight into what it’s doing. Some strains that produce toxins do so steadily, fouling water for months. Some produce them one day, then stop the next. The only way to know is to test.

The mantra that health officials try to get the public to chant is simple: “When in doubt, stay out.” Once alerted to an illness, health officials usually order a test and post warning signs until the results are known. A positive for toxins usually creates an upgraded warning about water exposure — or, in cases such as Anderson Lake, complete closure.

That’s what happened at Anderson Lake in 2006, and has happened nearly every spring since. Anderson Lake’s bloom usually starts in May, which creates tension with local anglers who start fishing for trout in late April.

Over the past eight years, fishing has been allowed off and on, depending on regular test results. A 2013 study of fish from this lake and eight others indicated that toxins ingested by fish tend to accumulate in the organs, not the flesh. No dangerous traces of anatoxin-a, a very fragile compound, were found in tested fish. The county hasn’t had any reports of health effects from people eating fish, Dawson says, “but it certainly makes me nervous.”

SO FAR, most studies of Anderson Lake’s potent bug have produced more questions than answers:

Algae blooms need only sunlight, water and a nutrient to thrive. They typically occur in lakes in developed areas, where nutrients like phosphorous or nitrogen are delivered by runoff from fertilizers, seepage from septic systems or agricultural effluent that carries a combination of both. But isolated Anderson Lake doesn’t fit that definition, raising a longer list of questions:

Why does this lake produce so much algae? Why are toxins resulting from that algae so consistently virulent here? Did anatoxin-a emerge here only recently and, if so, why? Why is it not found in nearby lakes? Is the particular cyanobacteria producing this toxin some sort of super bug, or is it similar to anatoxin-a found elsewhere? In short: What makes Anderson Lake so special? So deadly?

A few tantalizing clues exist:Before it became a state park in the 1960s, the land at the south end of Anderson Lake was home to a dairy farm. Cattle likely had access to the lake for water. Cow manure is rich in nutrients for algae.

“Once it’s in there, it tends to stay,” recirculating throughout the lake’s ecosystem, Dawson says.

That effect could be enhanced by the lack of flushing action at the lake, which is about 30 feet deep. Water comes in and out mostly through seepage. But countless lakes in the region share that profile, and none produce algae with toxins as potent as Anderson Lake.

Other possibilities: A student researcher taking sediment cores from the lake for a study of megaquakes recently detected vivianite, a mineral that could leach phosphorus into the lake naturally, Dawson notes.

More answers might be coming soon. Researchers at Oregon State University are wrapping up a study of the genetic structure of anatoxin-a found in Anderson Lake. Learning whether it’s a unique strain might be useful in understanding the problem, if not devising a treatment plan.

Treatments for blue-green algae outbreaks include seeding with alum, which has proved successful at killing the blooms — but only for a few years — at places such as Green Lake and a number of other state waterways.

At a cost of about $1 million to treat a lake like Anderson, Dawson laments, “That’s a pretty bad cost/benefit ratio.”

THE POTENTIAL debacle, of course, is much bigger than little Anderson Lake. Other signs of the stealthy toxins are all around, and they are not new. Joan Hardy, a toxicologist with the state Department of Health, has been following cyanobacteria around Washington for 20 years.

The most common she sees is microcystin, the liver toxin that also can kill animals and sickens humans, but typically only through long-term ingestion. Hot spots for that bug in the past decade include Lake Spokane, Kitsap Lake, Waughop and Ohop lakes in Pierce County, Lake Cassidy in Snohomish County, and Black Lake in Thurston County.

Anatoxin-a, deadly in much smaller doses, has been detected in about a dozen state waterways on both sides of the Cascades, the largest being Rufus Woods Reservoir on the Columbia River. Anatoxin-a has been found in Tacoma’s Clear Lake in winter months. Cranberry Lake, a popular swimming hole in Deception Pass State Park, showed nearly 900 micrograms per liter in August.

Algae blooms tend to proliferate from July to October, but they’ve been observed in Washington lakes covered by ice.

The blooms have been a rare but deadly plague to pet owners. The first recorded cyanobacteria pet fatality in Washington was a hunting dog killed in Spokane County in 1976. Other dog deaths have been reported in Moses Lake and the Potholes in Grant County. Cats have died after exposure to American Lake in Pierce County.

A recent study reported nearly 100 dog deaths nationwide in the past decade. The number is likely much higher, the study concluded.

Toxic algae found in water troughs at a feeding site killed 100 elk in New Mexico last year, and algae blooms regularly foul the Great Lakes.

The animal-poisoning problem even extends offshore: In the past decade, dozens of otters have died of liver failure in California’s Monterey Bay. Mycrocystin was traced to a stream connected to a freshwater lake with algae blooms. A Department of Health/Washington State University study aims to determine how much migrating freshwater microcystin can accumulate in low-food-chain creatures, such as mussels, in Puget Sound.

Nationally, the Centers for Disease Control and Prevention recently spent five years tracking human illness from the toxins in 10 states, including Washington. Congress this summer authorized $82 million for additional federal research.

OUR STATE, by necessity, has become a leader in the fight. Hardy credits the stepping-up of state scientists as well as county health officials, particularly around Puget Sound.

“We’re doing a lot with not very much money,” she says. The state dedicates roughly $270,000 a year to the problem, most from boat-license fees.

Health officials in Pierce County were early to jump on board. King County is taking a lead on testing and publishing results. The state Department of Ecology does field testing.

Because frequent testing is prohibitively expensive, the state has developed a “passive” monitoring system, which relies on citizens. People who spot algae blooms are encouraged to call their local health department.

Positive samples result in action ranging from posting warning signs to closing off a lake, depending on the toxin and its potency.

Washington has had no widespread illness outbreak, partly just by luck, Hardy believes.

Our future coexistence with cyanobacteria is as murky as the stuff itself. The EPA cites “mounting evidence” that outbreaks are related to climate change, but definitive studies are lacking. Health officials warn that, even without climate change, the problem is likely to worsen simply by increased population.

No one is under the illusion that a magic bullet will rid the world of an ancient bacteria that has survived far worse foes than us — namely, ice ages and asteroids. A victory would be mitigating damage.

“The goal is prevention,” Hardy says.

Back at taped-off Anderson Lake, on a warm July day with not a single person in sight, county health official Dawson crouches on the front line, peering into the clouded waters and pondering the invisible, deadly bug.

“It’s been in here, certainly, for decades,” he says. “How long? We don’t know, but 2006 certainly was the wake-up call.”

The call has not been heard by the population at large, algae watchers say. There’s a chance, of course, that in the end, Anderson Lake might stand as little more than an historical blip on the public-health radar. The killer toxin that reigns here now might not be replicated elsewhere. Or it might disappear as readily as it arrived.

But in this little-understood field, the opposite appears just as possible. The otherwise-forgettable lake might go down in history in a tragic way no one drawn to its peaceful edges could have imagined: Like the dogs that drank here and died, unwitting sentinels of what’s to come.

Ron Judd is a Pacific NW magazine staff writer. Steve Ringman is a Seattle Times staff photographer.