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The Russian River tested clean this week for a toxin related to blue-green algae that prompted cautionary signs at 10 popular beaches last month and in each of the past two summers.

The river remains open to swimming and other recreation. But warning signs urging visitors to avoid ingesting river water will remain at 10 popular beaches between Cloverdale and the river mouth as a precaution against exposure to the neurotoxin involved, the Sonoma County Department of Health Services said.

The weekly sampling suggests the threat, already minimal, could be diminishing. But precautions can only be lifted after several weeks pass without detection of the neurotoxin called anatoxin-a, county health personnel said.

State guidelines developed for static bodies of water like lakes require at least two weeks of negative test results for blue-green algae, formally called cyanobacteria, before the signs can come down, Sonoma County Health Officer Karen Milman said Thursday.

In a long swath of water with a moving current, there’s greater potential for harmful blue-green algae to relocate and be present in an area that hasn’t been tested, indicating a more conservative approach may be prudent.

In addition, county environmental health personnel monitoring the Russian River have seen growth of blue-green algae that still could release toxins at some point, as well as filamentous green algae in which cyanobacteria can intermix, Milman said.

Besides testing, “we want to take into account other things,” she said.

County environmental health personnel posted the caution signs July 27 after routine testing of river samples revealed very low levels of anatoxin-a, a potentially dangerous substance naturally produced by certain kinds of blue-green algae, which have proliferated in fresh water lakes, ponds and streams around California and the rest of the planet in recent years.

Anatoxin-a attacks the nervous system, causing symptoms that may include staggering, fatigue, convulsions, paralysis or even death.

Dogs and small children are particularly susceptible, given their size and potential for ingesting contaminated water. Studies indicate the odor of blue-green algae actually attracts dogs, who may eat it.

Two dogs died after frolicking in the Russian River near Healdsburg in 2015 — at the height of California’s historic drought — prompting widespread awareness of the potential for harmful algae blooms in warm, slow-moving water.

But after plentiful rain this past winter, water quality officials say, abundant runoff could have increased concentrations of nitrogen, phosphorous and other substances from fertilizer and other sources that feed toxic algae blooms.

The levels of anatoxin-a detected first at Cloverdale River Park Beach, Del Rio Woods Beach in Healdsburg and Patterson Point beach, located just downstream of Monte Rio, were extremely low, averaging 0.15 micrograms of toxin per liter of water.

Warnings against swimming and other water recreation aren’t required until at least 20 micrograms per liter is present.

But the state requires public notifications even when the smallest concentrations of anatoxin-a are present and a beach remains open to recreation, which has been the case with the Russian River in recent weeks.

Health officials urge visitors to avoid algae in the water and refrain from drinking, cooking with or washing dishes with river water. In addition, they advise bathing after exposure to river water, monitoring children closely and preventing dogs and other animals from drinking river water or eating algae scums.