Toxic algae warnings issued in areas along the Navarro River near Philo • The Mendocino Voice
MENDOCINO Co., 8/23/24 – On Friday, Mike Thomas, who has the cumbersome title of “Freshwater Harmful Algal Bloom (FHAB) Monitoring and Response Program Coordinator” for the state’s North Coast Regional Water Quality Control Office, posted “toxic algae” signs along the Navarro River in Hendy Woods State Park and at some campgrounds near Philo.
There are two types of signs that can be placed near rivers or streams. The signs Thomas put up are the more severe warnings. The signs were put up after Thomas conducted tests on Aug. 9. “We found some cyanobacteria. There were some antitoxins, in relatively small amounts.” Although Thomas thought the levels were low, he knew he couldn’t come back every week to take samples. “Out of an abundance of caution, we recommended posting the toxic algae warning,” he said in an interview Friday afternoon.
Cyanobacteria can produce toxins that can be dangerous to humans, pets and wildlife, including fish. Neurotoxins, for example, affect the nervous system, while other toxins can damage the liver or cause diarrhea and respiratory problems. Unfortunately, in recent years, some pet owners have lost dogs to neurotoxin exposure. Death for dogs exposed to neurotoxin exposure can occur within minutes to hours.
Thomas did the testing ahead of Labor Day weekend, when Mendocino County welcomes visitors to its campgrounds, rivers and beaches.
Is it dangerous to swim in the Navarro?
Thomas explained that harmful algal blooms manifest in two ways: planktonic, as is common in lakes and reservoirs, and benthic, in rivers and streams. Thomas described the unsightly green algae in rivers: “A lot of what you see is nontoxic filaments. There can be cyanobacteria in that algae. We’ve actually seen some bacteria out there (in the Navarro). Swimming is generally OK. We recommend that people avoid green patches and floating mats. The majority of the toxins are in the mat itself. If you’re swimming in clear water in rivers, you’re generally OK.”
For lakes and reservoirs, however, that advice changes. “Plankton blooms allow toxins to diffuse through the water column,” Thomas said, so clear water can still contain toxins. Thomas just tested Lake Pillsbury and Lake Mendocino, but the results are not yet in.
Other rivers in the county for which Thomas has issued algae warnings include the South Fork of the Eel, the East Fork of the Russian and the Standish-Hickey State Recreation Area.
Thomas said they conducted a study last year, testing the Navarro every week. “This time last year we published a paper,” he said. “Mainly due to the abundance of cyanobacteria, but the toxin levels were never very high.”
Cyanobacterial blooms can occur for a variety of reasons, but the most likely factors are high water temperatures, stagnant water, and high nutrient levels from agricultural fertilizer runoff.
