Toxic algae warnings posted at areas on Navarro River near Philo

MENDOCINO Co., 8/23/24 — On Friday, Mike Thomas, with the unwieldy title of Freshwater Harmful Algal Bloom (FHAB) Monitoring and Response Program Coordinator for the state’s North Coast Regional Water Quality Control Board, posted toxic algae signs along the Navarro River at Hendy Woods State Park and a couple of campground retreats near Philo. 

There are two levels of signs that may go up near a river or creek, and the ones Thomas posted are the more severe warning. The signs went up after Thomas did testing on August 9. “We found a little bit of cyanobacteria. There were some antitoxins, comparatively low.” While Thomas felt the levels were low, he knew that he could not come back weekly to sample. “Out of an abundance of caution we recommended posting the toxic algae alert,” he said in an interview later Friday afternoon.

Cyanobacteria can produce toxins that can be dangerous to people, pets, and wildlife, including fish. Neurotoxins, for example, impact the nervous system while other toxins might damage the liver or cause diarrhea and respiratory problems. Sadly, some pet owners have lost dogs to neurotoxin exposure in past years. Death in dogs exposed to a neurotoxin load can occur within minutes to hours.

Thomas was testing ahead of Labor Day Weekend, when Mendocino County will welcome visitors to its campgrounds, rivers, and beaches.

Is it dangerous to swim in the Navarro?

Thomas explained that there are two ways in which harmful algal bloom manifest: planktonic, common to lakes and reservoirs, and benthic, in rivers and creeks. Describing the unsightly green algae in rivers, Thomas said, “A lot of what you are seeing is nontoxic filaments. Within that algae there can be cyanobacteria. We did see some bacteria out there [in the Navarro]. Swimming is generally OK. What we recommend is that people avoid green spots and floating mats. The bulk of the toxin load is within the mat itself. If you’re swimming in clear water in rivers you’re generally fine.”

This advice changes when it comes to lakes and reservoirs, however. “In the planktonic bloom, toxins may be diffuse in the water column,” Thomas said, so clear water may still contain toxins. Thomas just tested Lake Pillsbury and Lake Mendocino, but the results have not come back yet.

Other rivers in the county that Thomas has posted for algal alerts are the South Fork of the Eel, the East Fork of the Russian, and in Standish-Hickey State Recreation Area.

Thomas said that last year they were doing a study that included testing on the Navarro every week. “We did post last year around this time,” he said. “Mainly due to the abundance of cyanobacteria, but the toxin levels never got very high.”

Cyanobacteria blooms can occur due to a variety of causes, but warm water temperatures, still water, and high nutrient levels from agricultural fertilizer runoff are the most likely factors.