Aquatic weed thriving on surface of Santa Rosa’s Spring Lake may have link to fires

A harmless aquatic weed is proliferating on Santa Rosa’s Spring Lake, possibly fertilized by runoff from fire retardant dumped during the October wildfires near the popular recreation spot for walkers, runners, bikers and anglers.

County parks officials say the floating plant is Azolla, also known as water fern, which often appears in the 72-acre lake in early spring, but to some people the current bloom evoked memories of the toxic algae on the Russian River that killed a dog in 2015.

“It literally looks like a brown carpet,” said Michael Schloss, who has lived near the lake for more than 50 years and walks around it nearly every day. “I’ve never seen anything like it.”

Azolla periodically sprouts in the lake, usually in isolated pockets and shallow, protected areas, said Bert Whitaker, county parks director. There’s a bit more of it than usual this year, but there are still large areas of open shoreline and good conditions for waterfowl and fish, he said.

“We are monitoring,” Whitaker said. “It does tend to migrate around the lake throughout the day, which is interesting.”

Earlier this week, the reddish-brown mass hugged the lake’s east shore, pinned there by the wind.

“We’ve never seen this much before,” said Judy McMahan, a longtime Rincon Valley resident walking the lake’s paved circumference trail with her husband, Ron.

A few days ago, the floating vegetation spanned the width of the lake, they said, showing smartphone photos of the scene. Azolla is capable of doubling its biomass in three to 10 days, experts say.

Rich Fadness of the North Coast Regional Water Quality Control Board said water fern commonly grows on small lakes and reservoirs and is “pretty innocuous.”

Schloss said he did some online research and concluded the Azolla bloom may have been caused by nutrients leaching into the lake from fire retardant released by Cal Fire aircraft during the October firestorm, which burned more than 60 percent of Trione-Annadel State Park abutting Spring Lake Park to the east.

Cal Fire dropped about 2 million gallons of the bright pink retardant on the North Bay wildfires in an effort to slow the flames that charred 137 square miles of Sonoma County.

The retardant is 85 percent water, 10 percent fertilizer and 5 percent other ingredients, such as color, thickener and corrosion inhibitors.

Fertilizer is included to promote post-fire growth in burned areas, Cal Fire Deputy Chief Scott McLean said, adding there is “nothing to substantiate” a connection between its use and the bloom in Spring Lake.

“These are what-ifs,” he said.

Dropped directly into a lake or stream, retardant can kill fish, amphibians and other species, and firefighting pilots in their drops are supposed to stay 300 feet from any body of water, said Timothy Ingalsbee, executive director of Firefighters United for Safety, Ethics and Ecology, an Oregon-based nonprofit.

Fire retardant is a likely factor in the larger Azolla bloom, he said, noting that fertilizer applied to Midwest cropland has leached into the Mississippi River, rendering the river’s mouth a “dead zone.”

Biological impacts from retardant typically go unnoticed because it is usually dropped on wildlands, Ingalsbee said.

“I don’t fault Cal Fire for attempting to stop the fire from coming into the community,” he said.

Joe DiTomasi, a retired UC Davis professor who calls himself a “weed scientist,” said fire retardant is “likely to exacerbate” any plant or algae bloom that normally occurs in a waterway.

But Azolla poses no health hazard, noting it has “been around longer than humans have.”

Azolla massed so densely on a stagnant pool in Putah Creek on the UC Davis campus a few years ago that sedges sprouted from it, making the floating vegetation look like a meadow, DiTomasi said.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com.