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Signs at Russian River beaches warning of the potential for harmful blue-green algae in the water were being taken down Thursday, after tests failed to detect the presence of algae-related toxins in recent weeks.

Only highly diluted concentrations of an algae-produced toxin were found in the river this summer even when tests sporadically came back positive, health officials said.

But there has been no detection of the neurotoxin, Anatoxin-a, since a single sample collected Aug. 21 in Forestville revealed a very low density of the substance, according to lab results.

Visual monitoring and inspections of the river additionally indicate that water conditions are no longer favorable to the growth of blue-green algae likely to produce harmful toxins, Sonoma County Health Officer Karen Milman said.

North Coast water regulators have reported noticeably less algae and more consistent water temperatures just in the past two weeks, so “things are looking good,” Milman said.

With that favorable outlook, the county is suspending two months of regular water testing at the river’s 10 most popular public beaches, in addition to removing the caution signs posted seven weeks ago, Alonso said.

The warnings advised visitors to not ingest river water, to steer clear of algae patches and keep pets out of the river. It’s not clear how much, if at all, the signs may have affected visitorship to the Russian River during what has been the hottest summer on local record.

Spiking temperatures jammed river beaches with people seeking relief from the heat, particularly over the recent Labor Day weekend, when the mercury again topped 100 degrees throughout the region.

Monte Rio Fire Chief Steve Baxman, chairman of the community’s parks and recreation board, said the only noticeable decline in attendance at Monte Rio Beach occurred in the wake of the July Fourth holiday, when health officials announced elevated levels of fecal bacteria in water samples from the area and closed the beach for several days.

Otherwise, “it’s been business as usual,” Baxman said Thursday.

Anatoxin-a is one in a variety of toxic substances produced in certain species of naturally occurring blue-green algae, formally known as cyanobacteria.

It attacks the nervous system and can cause sudden, violent, even fatal symptoms in high doses.

Dogs and children are considered especially vulnerable, given their relative size and lack of discrimination about what they put in their mouths.

Anatoxin-a was blamed for the deaths of two dogs who swam in the Russian River two years ago and likely consumed harmful algae directly. It has been linked to past incidents around Northern California, as well.

The 2015 outbreak caught county officials and the public by surprise, though harmful blue-green algae blooms have proliferated around the globe in recent years.

Milman said she feels the early detection protocol developed since then in conjunction with the North Coast Regional Water Quality Control Board has worked. The process calls for monitoring water conditions and conducting weekly sampling once conditions are favorable for harmful blue-green algae.

“We’re always looking at how to do things better, but I feel that this year went pretty well,” Milman said.

State guidelines call for public notification any time Anatoxin-a is detected in recreational waters, even in the very smallest amount. But only densities of 20.0 micrograms per liter are required to trigger health advisories warning beach-goers to avoid contact with the water.

Six water samples collected from the 10 Russian River beaches since July 17 have revealed Anatoxin-a, according to sampling results.

The very highest level of Anatoxin-a was 0.25 micrograms per liter of water, about 1/80th the threshold articulated in state guidelines.

The toxin dilutes and degrades over time once released, experts say.

Larry Laba, owner of SOAR Inflatables and Russian River Adventures, a high-profile watercraft vendor in Healdsburg and one of those who lost a pet in 2015, said he has seen no indications of blue-green algae on the river for a month or more but has kept signs up at his place of business so that customers area aware of the potential problem.

“There’s always some people who, still, show some concern and decide not to come, but that’s really few and far between,” Laba said. “Really, all and all, blue-green algae has had little impact this year on our business.”

You can reach Staff Writer Mary Callahan at 707-521-5249 or mary.callahan@pressdemocrat.com.