Harmful blue-green algae found at Riverfront Park near Windsor

“Danger” signs were posted last week at Riverfront Regional Park near Windsor after water regulators identified potentially harmful blue-green algae in Lake Wilson, one of three former gravel-mining pits that serve as the centerpiece of the county-owned park.

Visitors are being urged to avoid water exposure, meaning boating and fishing are now off-limits. They also are instructed to keep pets from drinking the water or swimming in any of the park’s three lakes to avoid potential risks, Sonoma County health officials said.

The kind of blue-green algae, or cyanobacteria, present in Lake Wilson is naturally occurring but can produce a dangerous neurotoxin called Anatoxin-a. Pets and small children are most vulnerable to the substance because they are most likely to ingest it inadvertently or even on purpose.

There are no restrictions in place for the Russian River and no indication of problems there, officials said.

Swimming is prohibited year-round in Riverfront Park’s three lakes, but people commonly use them for fishing, paddleboarding and kayaking, prompting health officials to act “out of an abundance of caution” and post the warning signs, Deputy County Health Officer Karen Holbrook said Monday.

The water also is a lure for dogs, who could be at risk of illness or even death if they ingest Anatoxin-a. Studies indicate dogs are drawn by the smell of blue-green algae and desire eating it.

“We put up red signs that say, ‘Stay out of the water until further notice,’?” Holbrook said, “so that means boats and other watercraft should stay out. And we don’t want dogs to go in, obviously. They gravitate toward these algal blooms.”

Health-related limits on water recreation are part of a new reality in Northern California and in much of the nation amid warming temperatures and a rising incidence of blue-green algae blooms, often fed by fertilizer-heavy runoff and other nutrient sources, experts say.

Lisa Bernard, a planner with the North Coast Regional Water Quality Control Board, said the increased prevalence of blue-green algae blooms also likely reflects the fact that “our awareness is more focused on this.”

“We’re seeing it more now, because we’re looking for it,” she said. “But I do think that we’ll continue to see increases as temperatures continue to warm.”

Blooms are particularly likely where water is stagnant. They generally increase in frequency during late summer, as the weather heats up and still water grows warm.

Lake County health officials reported more than a dozen blooms in Lake County and one in Upper Blue Lake over the past week, for instance. Warning signs were posted at five locations on Clear Lake, including Clearlake Oaks, where very high levels of a blue-green algae-related toxin called microcystin were detected, warranting a danger sign, county officials said.

The North Coast water board also recently issued public guidance urging people to be aware of the potential risks associated with blue-green algae and to exercise caution around freshwater lakes and streams where toxins have been known to occur.

Those include the Eel River and its south fork in Mendocino County, the Van Duzen River and the Trinity River in Humboldt County, and Lake Pillsbury in Lake County.

The Russian River has had its own problems, with blooms occurring in each of the past three years. In 2015, two dogs died after ingesting Anatoxin-a amid a particularly persistent series of blooms.

The problem was more isolated and limited during the summers of 2016 and ’17. The water in the river this year shows no indication that conditions are ripe for a bloom, water officials said.

“The river is in better shape, the water quality is better, than it’s been in the last five or six years,” said Don McEnhill, executive director of Russian Riverkeeper, a nonprofit stewardship and advocacy organization.

He said the problems at Riverkeeper Park have no bearing on what’s happening in the Russian River, which is cooler than in recent years and flowing.

“Those gravel pits, they have a totally different chemistry than the river,” McEnhill said. “There is zero circulation. There is no current.”

The Russian River is subject to robust county and state monitoring, where the other water bodies are not.

Since the 2015 outbreak, the agencies have kept close watch on the river each summer, beginning the week after Memorial Day. In addition to visual inspections, they record water quality markers like water temperature, acidity levels and oxygen diffusion to determine when conditions might support a blue-green algae bloom.

If and when that point arrives, it triggers the beginning of routine sampling at 10 popular swimming beaches between Cloverdale and Patterson Point in Villa Grande for signs of dangerous toxins.

So far, no testing has begun because river conditions are good, officials said.

“We don’t at this time have any evidence of any harmful algal blooms along the Russian River,” Holbrook said.

Local and state health officials recommend avoiding algae scums and patches, closely supervising children and pets, and washing everyone down after water play. Untreated river water should never be used for drinking, cooking or washing dishes.

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