.Increase in Algae Blooms a Concern at Once-Toxic Pinto Lake

How scientists are working to keep water safe at Watsonville lake, and around the county

With the brassy stutter of a rusty wheel whizzing downhill, an American coot announces itself from the shores of Pinto Lake as Kendra Hayashi scoops water from the side of the dock. Today, the sample in her glass bottle looks mostly clear, as does the liquid buoying the bird.

But three weeks earlier, she says, the lake’s surface bloomed with potentially toxic algae. The coots and mallards didn’t seem to notice. “When it’s super green out and you see them motoring through, you’re just like, ‘Ugh!’” Hayashi says.

Hayashi manages a lab at UC Santa Cruz that monitors microcystin—a toxin produced by algae in the genus Microcystis—in Watsonville’s Pinto Lake. Though it once had among the highest measured levels of microcystin of any freshwater body on earth, the lake hasn’t had a toxic event in years, thanks to local cleanup efforts.

Some of that progress, however, has started to stall. And as the climate crisis creates conditions for more frequent algal blooms—layers of algae that grow out of control in lakes and other bodies of water—keeping Pinto Lake safe may require more effort and funding, potentially stretching local agencies thin.

Blooming All Around

The challenge extends beyond just one lake. Algal blooms occur throughout Santa Cruz County around early fall, when weather remains hot but rain has yet to come. Those conditions leave freshwater bodies lower, warmer and more stagnant, allowing algae to thrive.

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2022, however, saw a plethora of blooms. This likely owes to a heavy September storm that brought enough rain to wash extra nutrients into lakes and ponds, feeding the algae, but not enough to flush the algae out.

At one point in September, says county water resources manager Sierra Ryan, her staff responded to calls about five blooms in the span of a week and a half. 

Ryan says only one of these blooms, in a water body on private property, included toxin-producing algae—and even then, her team wasn’t able to detect any actual toxin when testing the water. But it’s impossible to tell the difference between toxic and non-toxic blooms just by looking, so the county posts warning signage at blooming lakes and ponds to encourage caution. Pets who swallow microcystin can die within 15 minutes. For humans, ingestion causes breathing difficulty and stomach distress. Skin contact can produce a rash.

Ryan doesn’t want people to panic; she just wants them to be careful. “If you come in contact with the water, you clean yourself off. You keep your pets away,” she says. And don’t drink it. “Other than that, there isn’t really a risk involved.”

But the recent blooms don’t exist in a vacuum. To Ryan, this year feels like part of a trend. “Every year we seem to see more water bodies with blooms,” she says.

As climate change brings hotter, drier conditions to California, that trend will likely continue. “All of these organisms producing toxins in the freshwater generally like really warm conditions,” says Dr. Raphael Kudela, professor and chair of Ocean Sciences at UC Santa Cruz. Kudela, who heads the lab monitoring microcystin in Pinto Lake, says those algae grow best at 25 to 30 degrees celsius. As climate change heats up California, lakes like Pinto edge closer to that range.

“If it’s kind of stagnant, if it’s hot and dry and you’ve got a nutrient source, then it’s just going to keep getting worse and worse and worse,” Kudela says. “That’s basically describing climate change.”

At Pinto Lake, the climate trend coincides with another: a chemical treatment designed to keep the algae at bay has begun to approach its limit.

Taming the Green Tide

In 2017, the City of Watsonville treated Pinto Lake with alum, an aluminum-based compound commonly used to purify drinking water and treat wastewater. Rather than killing algae, alum cuts off their food, binding up the nutrient phosphorus in the sediment of the lakebed.

The treatment came after years of high toxicity. Because it sits on a natural phosphatic deposit, Kudela says, Pinto Lake always had high nutrient levels. But decades of extra nutrient input from leaky septic tanks and fertilizer runoff from farms and lawns painted the lake green with scum. Kudela describes the blooms’ scent as sweet-but-rotting, “like if you’re composting grass.”

The World Health Organization sets the limit for microcystin in drinking water at 0.8 parts per billion. Watsonville cordons off Pinto Lake City Park at 10 parts per billion—the city’s threshold for a “toxic event.”

In 2007, the lake reached 2.9 million parts per billion. “It was the second-highest measured toxin anywhere in the world,” Kudela says. At that point, “probably even just touching it wouldn’t be a great idea.”

Back then, even breathing near Pinto Lake might have had consequences. Some algae, including Microcystis, can emit toxins into the air, especially if kicked up by boats, jet skis or powerful winds. Though research hasn’t proven a causal relationship, a strong association has emerged between long-term exposure to airborne algal toxins and liver disease—including a hot spot in the Bay Area. While no specific data exists for Watsonville, Kudela says, microcystin likely wafted on the breeze during Pinto Lake’s most toxic years.

Riding the Pajaro River to Monterey Bay, microcystin also accumulated in clams, mussels and oysters—favorite foods of sea otters—at up to 107 times the concentrations of the surrounding seawater, according to a 2010 study. At least 21 sea otters, the study found, died after ingesting microcystin from Pinto Lake and other water bodies connected to the bay.

Since then, however, Pinto Lake has improved significantly, thanks to a multi-pronged effort by local and state agencies.

In 2013, Watsonville paid residents 25 cents per pound to fish the lake for carp, a bottom-dweller that kicks up nutrient-rich sediment for algae to feed on. Four years later, the city treated the lake with alum, sealing away over 60% of its phosphorus almost immediately. Watsonville has also worked with the county to reduce fertilizer use on residential lawns in the watershed.

The Central Coast Regional Water Quality Control Board joined the effort in 2020, setting the limit for additional phosphorus allowed into the lake annually at 90% below then-average levels.

To help meet that limit, the county Resource Conservation District continues to work with consenting landowners in watersheds across the county to boost fertilizer and irrigation efficiency, plant cover crops and add vegetation to drainage ditches—all methods to reduce nutrient-rich runoff. Working with the county parks department, the district also built a sediment basin to catch pesticides and nutrients upstream of the lake.

The efforts paid off. For the rest of 2017 after the alum treatment, the lake closed due to toxicity only once, for three weeks—compared to three-month-long closures in 2015 and 2016. Although signage still warns of toxic blooms, “it’s been pretty good since they treated it,” Kudela says. Most weeks, his lab detects no microcystin at all.

Algae’s Slow Return

Inevitably, however, nutrient runoff still occurs. Jackie McCloud, environmental sustainability manager at Watsonville Public Works, says blooms have become more frequent in part because eight centimeters of phosphorus-rich sediment have built up on the lakebed since the alum treatment. 

The alum can lock away some of this, she says, but not all of it.

As fresh sediment and the climate crisis give blooms a boost, monitoring toxin levels remains crucial.

So, every week, Kendra Hayashi returns to Pinto Lake. She pulls a sampling bottle from her bag, along with what look like two miniature embroidery hoops and a lightsaber.

The hoops bind a lump of resin between two layers of mesh, forming resin “tea bags.” These will soak in the lake, grabbing whatever toxins they encounter, until Hayashi collects them next week. In addition to microcystins, the resin can snatch up anatoxin, nodularin and cylindrospermopsin—spelling bee nightmare fuel that provides a broader picture of the lake’s health. 

The lightsaber, meanwhile, is actually a fluorometer: a device that can detect the colored pigments associated with certain types of algae. Hayashi slips it just below the surface. “It’s taking a measurement every second,” she says, “so I just try and leave it in the water for a minute so that I can get a good reading of what’s here.”

The water sample in the bottle marks the final step. Hayashi brings all three items back to Raphael Kudela’s lab, where the results have begun to confirm Ryan’s and McCloud’s observations.

“We’re starting to see toxin coming back up,” Kudela says. “Not nearly as much as we used to, but it’s still there.” His lab has begun noticing more frequent pulses of microcystin up to a few parts per billion—low enough for recreation, but not for drinking.

Uncertain Solutions

McCloud hopes for a second alum treatment before the first stops working entirely.

Getting one, however, will require help, probably from the EPA or the California Water Resources Control Board. “As an underserved community, the environmental programs rely on grant funding,” McCloud said in an email. “We don’t have a supplemental budget to do these treatments without state or federal aid.”

Grant funding covered 100% of the original alum treatment. Securing that support again presents a challenge of timing: grants must be applied for and spent under certain deadlines, so the city has to wait for a window when it has the staffing capacity to prioritize a new alum project.

The price of alum has risen, too, McCloud says, so a grant of the same size won’t afford as much treatment.

And as the climate crisis spawns more frequent blooms, Pinto Lake may need more treatments down the road. “I’m unsure if the funding will keep up with the need,” McCloud says.

Despite these challenges, Pinto Lake remains one of our region’s best success stories. It received treatment, and routine testing continues.

Many water bodies aren’t so lucky. Some, like nearby Kelly Lake, experience blooms but receive no toxin testing because they’re on private property, Kudela says.

Others get tested, but only occasionally. County Water Resources lacks the staffing capacity to regularly monitor every water body in its jurisdiction, Sierra Ryan says. Instead, testing occurs reactively, whenever they get a call about a bloom.

But not all algae produce toxins, and the ones that do must be present at high enough levels in order to cause a toxic event. As a result, blooms can shift between toxic and non-toxic as the types and quantities of algae in the water change. “It might not be toxic one day when we go out and test, but it might become toxic the next day,” Ryan says, meaning they might miss a toxic event due to unlucky timing.

This makes tracking the scope of the problem difficult—as does the fact that different water bodies in the region sit within different agencies’ jurisdictions, so no centralized data source exists.

The California Harmful Algal Blooms Portal may help address this. Residents can use the website, operated by the California Water Quality Monitoring Council, to report observations and view a statewide map of active blooms. “Ideally, that’s where we’re going to start having a repository of this data, so it’s not, ‘Well, the city monitors these sites, the county monitors someplace, Watsonville is monitoring other sites,’” Ryan says.

Amidst all these challenges, keeping the public informed remains crucial as blooms continue. Press releases and social media posts advise residents to exercise caution. Signage dots the shores of Pinto Lake. 

After years of blooms, most folks have gotten the message.

On a mid-October morning, Rafael Santillán winds his way down a secret path at Pinto Lake County Park, fishing rod over one shoulder. His dog Dino scampers ahead. The two emerge at the lip of a short cliff over the water, tall enough to prevent Dino from taking a gulp.

In a decade of fishing here, Santillán has heard plenty about the blooms—including one rumor of a scientist developing hand tremors after taking too many samples. He releases his catch instead of eating it, and he never lets Dino near the water.

Even so, Santillán says, “I put my hands in the water all the time.” The lake has never hurt him before. And though its future remains uncertain, odds are, it won’t hurt him this morning either.

A coast live oak casts shade into glassy water as Santillán casts his line and American coots cast their calls. For today, the lake shines clear.


  1. They may wish to consider floating gardens, simple rafts equipped with a large kratky-styled hydroponics system that allows the roots to dangle into it’s “nutrient solution” and choose a crop best suited to hyperaccumulate the nutrients of concern feeding the algae.
    Maybe grow livestock fodder or something useful like biofuels feedstock


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