MENDOCINO CO., 7/29/25 — A flurry of posts on Facebook in mid-June alarmed Mendocino County locals about harmful algae blooms (HABs) caused by cyanobacteria in Lake Mendocino. The posts described an incident in which a dog may have died of neurotoxin poisoning after swimming in the lake. The poster was furious that no signage appeared at the lake about the dangers for pets.
Nothing on Facebook goes unchallenged, and this was no exception. For one, the danger of HABs is usually a subject for late August and September, not mid-June, according to historical data from the California Water Quality Monitoring Council. For another, Lake Mendocino has not been known for HABs, as opposed to nearby Clear Lake, according to the council’s data.
Facebook posters objected to the Facebook story, saying that their dogs swam in the lake daily, their horses drank from the lake, and no one had noticed dead deer, raccoons, or fish nearby. The dog owner responded that a veterinarian said the dog’s death was consistent with neurotoxin poisoning. (The Mendocino Voice could not confirm this claim.)
According to the Army Corps of Engineers, which manages Lake Mendocino, a an member of the public filed a formal complaint, reporting that HABs were found at the lake.
The Army Corps performed tests in late June and shared the report in early July.
“Last week, water samples were collected from the South Boat Ramp, North Boat Ramp, and Pomo-A Swimmers Area,” the Army Corps said. “The good news is that while some cyanobacteria are present, the levels found were low, and we didn’t detect any of the common toxin-producing cyanobacteria.”
Testing a few days later from the East Fork of the Russian River also detected no cyanobacteria. “Right now, we aren’t issuing any advisories, and the results we’re seeing are typical for this time of year,” the Army Corps concluded.
Cyanobacteria species are present in many water bodies
According to the Environmental Protection Agency, “Algae are natural and important primary producers in aquatic ecosystems. Some species, under certain conditions, can produce toxins that affect human health and the environment.”
Those conditions are heat, drought (lower water volume at higher temperatures), excessive nitrogen buildup from agricultural runoff, and other factors. To make it more complicated, only some cyanobacteria species can produce cyanotoxins that are dangerous to wildlife, humans, and pets.
Visible algae blooms floating above the surface of the water can be caused by rapid population growth of cyanobacteria. HABs have other impacts, such as reducing oxygen in the water body, impacting fish. Wildlife that drink from standing pools can be sickened or killed by neurotoxins. “Once these toxins are released, they can cause numerous health effects ranging from minimal (i.e., diarrhea, headaches, and skin irritation) to life-threatening (i.e., death),” the EPA said.
During the late summer, people need to be aware of the possibility of toxin-producing cyanobacteria and how the bacteria performs in different water bodies.
Last year, the Mendocino Voice interviewed Mike Thomas, who monitors HABs for the state’s North Coast Regional Water Quality Control Board.
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.
“A lot of what you are seeing is nontoxic filaments. Within that algae, there can be cyanobacteria… Swimming is generally OK,” Thomas said, describing the unsightly green algae in the Navarro River near Philo last year. “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. “In the planktonic bloom, toxins may be diffuse in the water column,” Thomas said, so clear water may still contain toxins.
What people and pets can do during late summer to reduce risk of illness from algae
If a body of water tests positive for HABs, state agency representatives will place public notices onsite. Check for warning signs and act appropriately based on what the sign advises.
People swimming in rivers or free-flowing creeks should stay away from floating mats of algae. As noted above, algae itself is natural. However, if swimmers notice mats, especially mats that look oily or slick, exercise extra caution. And of course, if people see dead fish or animals nearby, steer clear.
In general, a good rule is to rinse off after exiting waterways as soon as possible and try not to swallow any water from the creek or river.
When playing with dogs, specifically throwing balls or objects, keep to the area where water is visibly moving, not in pockets or swimming holes. Bring bottled water and a travel dish and encourage pets to drink on shore, discouraging dogs from drinking from the river.
In lakes and reservoirs, there may be no algae to alert swimmers and pet owners. If the water is warm, assume that cyanobacteria could be present. People and pets should not drink lake water and bring bottled water to enjoy instead.
Dogs could become sickened by licking their fur after swimming in water impacted by algae. Bring a gallon jug of water and after the last swim, empty the jug over dogs from head to tail, then rub them with a towel to clean off lake water. All this should go a long way to preventing heartache if pets –– or people — become ill.
The public can report suspected or confirmed algal blooms and any related human or animal illnesses to the California Water Quality Monitoring Council here.
The council, in partnership with the Army Corps of Engineers, monitors water conditions daily here, for bodies of water frequently used for recreation or other public uses.
An interactive map of testing results across the state is also available here.
Finally, the council has a list of safety tips and tools about HABs here.
So….avoid gross man made lakes? Got it