Clear Lake is not so clear these days

Pull up the satellite map from the San Francisco Estuary Institute, and it colors each lake by how much cyanobacteria the satellite can see - black for absent, up through blue and green and orange to a deep red at the top of the scale. Clear Lake glows red almost from one end to the other.

The map reads phycocyanin, the pigment cyanobacteria produce. It can't tell you where the water is actually toxic - only where the bloom is. And the bloom is nearly everywhere. On the north shore, you don't need a satellite. You can smell it.

If you live in Sonoma or Mendocino County, this isn't only a Lake County problem. The same cyanobacteria drive the toxic-algae warnings that turn up most summers on the Russian River - Sonoma County posts alert signs at the swimming beaches - and the advisories Mendocino County has put out on the Navarro and the East Fork. Clear Lake is the extreme case. That's what makes it the best place to learn what the stuff is, when it's dangerous and what to do about it.

For that, you want Sarah Ryan. She's spent 25 years with the Big Valley Band of Pomo Indians, has run its environmental program since 2006, and has read this lake longer than almost anyone.

Green algae or cyanobacteria?

"Not all of it is a problem," Ryan said. Green algae are part of the food web, the same family as seaweed in sushi. The algae that can turn toxic are cyanobacteria, often called blue-green algae.

There's a test anyone can do at the water's edge.

"If you put a stick in it and lift it up, and it hangs on the stick and it's fibrous, then it's green algae," Ryan said. "But if you put a stick in it, lift it up, and it pops off, and it's slimy - that's cyanobacteria. And that's the stuff that can be toxic."

Is it toxic right now?

Some of it, in some places, is toxic right now. The toxin the tribe tracks most closely is microcystin, which attacks the liver. The state considers water safe for recreation below 0.8 micrograms per liter. In 2024, Big Valley's highest single reading was more than 16,000, at Lily Cove in the lake's Lower Arm.

Last year was calmer, with a peak of 1,376 anywhere on the lake, still way past the danger line but a fraction of the year before. This year's bloom is building now.

The county posts warning signs at monitored sites, keyed to what the tribe finds. Caution means keep animals out and don't eat fish or shellfish from that spot. Danger is worse.

"At the danger level, you shouldn't even be in a boat," Ryan said, "because the toxins aerosolize - all the mist and spray that comes up."

The fish carry a separate warning, and it has nothing to do with the bloom. Because of mercury that builds up in their tissue, the state's fish advisory tells women 18 to 49 and children to eat no bass from Clear Lake, and everyone else to hold it to one serving a week. But on top of that, the tribe's testing has found the bloom's toxin, microcystin, in carp and mussels above the level the state allows for a single fish meal.

The other danger: No oxygen

A big bloom doesn't only add toxins. It sends the lake's oxygen level up and down every day. In the afternoon sun the cyanobacteria photosynthesize and pump the water full of oxygen - Big Valley's sensors read well above normal at midday. After dark that stops, and the oxygen crashes. Warm summer water holds less of it to begin with, and when the bloom dies off, decay burns through still more.

The regional water board standard says dissolved oxygen should never drop below 5 milligrams per liter. Below that, fish suffocate.

"We are always below that level of dissolved oxygen," Ryan said, "and that is harmful to fisheries."

The crash is worst in the Oaks Arm, where the tribe's sensors have read below the line at the surface since late June and near zero at the lake bottom. Where oxygen runs that low, fish don't last - and they've been dying on the lake for weeks.

The other danger: No oxygen

Clear Lake is especially vulnerable to cyanobacteria because there's about a century of food sitting on the lake's bottom. Decades of fertilizer, eroded soil and septic runoff have loaded the lakebed with the nutrients cyanobacteria feed on.

Add hot, still summer water, which holds less oxygen to begin with, and the bloom takes off. It even arrives in a set order, Ryan said. Nitrogen-fixing cyanobacteria show up first and take the surface. As they die, they clear the way for Microcystis, which is a big toxin producer. Watching that handoff is part of how the tribe predicts when the toxins will spike.

The tribe that stepped in

For most of the past decade, the steadiest eyes on Clear Lake have belonged to the Big Valley Band. The tribe started its cyanotoxin program in 2014, after a dog died on the lake and the county still wasn't doing regular monitoring. It runs on roughly $75,000 to $100,000 a year, mostly federal money, with a couple of people working part time.

"It's a very lean program," Ryan said. "So we just have to bust our butts."

Ryan came to the work sideways. She started with the tribe in 2001 and found years of water data nobody had made sense of. When she became environmental director in 2006, she went looking for a yardstick.

"I went to the county and said, do you have a list of the standards for Clear Lake - what ranges should it be in?" she said. "And they said no. And I said, well, then what are we comparing this to?" So she built the comparison herself, measuring the lake against the state's standards for each use the water is supposed to serve.

When the neighboring Elem Indian Colony had to drop its own sampling around 2020, Big Valley took over those sites - some of the most toxic on the lake - for no extra money. The tribe puts everything online at bvrancheria.com, with a site-by-site toxin tracker and real-time sensor readings anyone can pull up. That openness wasn't always welcome, Ryan said, but the point was never to assign blame.

"It's not the county's fault," she said. "We're all in this together." And, she added, "it's better to know."

The tribe pushes this hard because its members live on the shore and depend on the water. "These are resources the tribe uses for their sustenance, their heritage, their cultural traditions that they like to pass on to their kids," Ryan said.

What's being done

A longer fix runs through the Blue-Ribbon Committee for the Rehabilitation of Clear Lake, created by Assembly Bill 707, the 2017 law authored by Assemblymember Cecilia Aguiar-Curry. It has funded a round of pilot projects - erosion control, wetland restoration, nutrient studies - and Big Valley is running several of them right now, using tools like lidar surveys to find where soil is washing into the creeks and traditional ecological knowledge to decide what to replant.

Because they're pilots, Ryan said, the test is whether they can prove they work. "We need to see that as successful," she said. "And we need to know how we’re going to measure it."

In the meantime, her advice for anyone at the lake is simple. Stay out of water that looks scummy and keep pets and animals out of it. Don't eat fish or shellfish from posted sites. And don't try to clear up a bloom yourself with bleach, hydrogen peroxide or store-bought pesticides - it only makes a fragile system worse. If mats of small floating plants are shading part of the water, leave them, because they block the sunlight cyanobacteria need and eat the same food it does.

For Ryan, that's the whole point of putting it all out there. "The more that we're all discussing it, the more it increases our environmental literacy," she said, "and then we can manage the resources."

Big Valley posts current toxin readings and recommended advisories with real-time sensor data on its EPA page. The statewide satellite bloom map is at fhab.sfei.org, and the state's fish-consumption advice for Clear Lake is at oehha.ca.gov.

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