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As Algal Bloom Returns to the Bay, Is Swimming Safe for Humans (and Pets)?

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Reddish-brown water can be seen in the Berkeley Marina on July 31, 2023, in what could be a potential red-tide algae bloom developing in the Bay. (Beth LaBerge/KQED)

The red tide that killed an immeasurable number of fish in San Francisco Bay last year has again emerged, covering parts of the bay in a light brown sheen.

The state agency in charge of regulating flows into the bay held a press conference in early August alerting the public about the return of the tea-colored water. This alert came after an environmental watchdog group, San Francisco Baykeeper, received a series of calls at the end of July about a reddish-brown film floating on the surface of the water in places like Berkeley, Emeryville, Albany and Tiburon.

Environmental scientists believe the algal bloom, while a natural occurrence and likely not harmful to human health, is fueled by treated sewage put out by wastewater treatment plants across the Bay Area — and could be worsened by climate change.

Scientists aren’t sure how large this bloom will grow or how many fish will die, but they are examining the organism closely. Still, scientists are sure of one thing: Humans are a big part of the problem.

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“It’s almost all about us to blame,” said Peter Roopnarine, curator of invertebrate zoology & geology at the California Academy of Sciences. “The bay is kind of a gigantic laboratory flask in some ways where you can put in ingredients, can mix them and it’s not easy for those to have an outside influence from the ocean.”

Keep reading for what you need to know about this latest algal bloom.

What is red tide?

Ominous sounding, the term red tide is broadly defined. It typically means anytime a bloom of phytoplankton — microscopic plant-like organisms that are the base of the marine food web — discolor a body of water to the point it is visible to the human eye.

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Red tides aren’t always red. They can in fact range from rusty orange to brown to green, but millions (if not billions or trillions) of these little creatures are needed for a red tide to be visible.

Not every red tide, or algal bloom, is toxic — although they can be. The species behind the recurring algal bloom in San Francisco Bay is called Heterosigma akashiwo and isn’t known at this time to be toxic to humans (see more below). The microscopic critter looks like a swimming potato chip with a tail, said Raphael Kudela, a phytoplankton ecologist at UC Santa Cruz.

He said the organism thrives in the bay because the shallow water warms up quickly. Plus, it’s full of tasty treats it likes to fill up on.

“It’s just really happy when it’s in the bay,” he said. “As long as it’s happy, it’s just going to keep going, and going, and going. And that’s basically what a red tide is.”

White sailboats float in reddish brown water.
Reddish-brown water due to an algae bloom can be seen in the Berkeley Marina on July 31, 2023. (Beth LaBerge/KQED)

Is the red tide harmful to humans or pets??

Scientists don’t think the red tide in the bay harms human health or pets, but they are hesitant to say that for certain.

For that reason, experts do not recommend swimming in — or otherwise coming into contact with — murky water.

David Senn, senior scientist at the San Francisco Estuary Institute, (SFEI) said there’s very little information available about the toxicity of the algae on people when they come in contact with it. That’s why the general guidance is: Don’t take a dip into areas of the bay where blooms are present.

“It makes sense to avoid discolored water in general, particularly in this case,” he said.

What is known is that an algae bloom of this nature often kills fish by eliminating the oxygen they need to breathe in the water. (Jump to more information about what to do if you find a dead fish or other organism near a red tide.)

A photo submitted by Damon Tighe to inaturalist.org, showing a dead bat ray found in Emeryville. (Damon Tighe via inaturalist.org)

But there is a caveat for humans, said Kudela at UC Santa Cruz. Dense red tides are often accompanied by organic material and bacteria, which can harm human health.

“The Heterosigma is not going to harm humans or dogs. But just being in that organic soup, you could be exposed to pathogenic bacteria or other things,” he said. “If the water is super thick and red, you probably don’t want to be in it anyway.”

In short: While the risks are probably low, it’s just a good idea to keep yourself — and your pets — out of a body of water that has a red tide.

What do I do if I spot a red tide??

To report a red tide or algae bloom you can reach SF Baykeeper’s pollution hotline by:

Algal blooms can also be reported to the California State Water Resources Control Board’s Freshwater and Estuarine Harmful Algal Bloom (FHAB) Program. You can reach the agency hotline by:

If you’re reporting a red tide, be sure to include the time, date, location and a description of what you saw.

Spotted a dead fish near a red tide?

To report a dead fish in an area where there is red tide present, you can download the app iNaturalist. The California Department of Fish and Wildlife is collecting these observations of dead organisms related to the algae bloom in San Francisco Bay, so make sure you log a photo, location and (if possible) species.

Read more about how to report dead fish using iNaturalist from The Oaklandside.

A map of the Bay Area on inaturalist.org showing user reports of dead organisms around a red tide (inaturalist.org)

Can I continue fishing in the bay with the algae present?

UC Santa Cruz’s Kudela said it’s perfectly fine to continue fishing in the bay, even in areas where the algae bloom is present because the algae do not contaminate the meat of the fish.

‘While it can kill the fish, if you’re eating the meat and cleaning it, there’s no risk,” Kudela said.

“It’s up to you whether you want to fish or eat stressed fish, but there’s nothing wrong with fishing.”

Keep reading for more background on the 2023 algal bloom.

What is causing this year’s algal bloom?

Heterosigma akashiwo got lucky in 2023 — for the second year in a row.

A series of environmental conditions are allowing this minuscule creature to show up en masse. This organism needs four things to flourish: light, warm water, relatively calm water and oodles of food. It got all four this summer.

Scientists think the organism deposited itself in sediment under the bay after it died off last year, acting like a dormant seed until the right conditions allow it to grow.

This year, the San Francisco Regional Water Quality Control Board has confirmed the organism in bay waters near Emeryville, Albany, the Berkeley Marina, Richardson Bay and Belvedere Cove.

Since the bay’s east side is often shallow — water heats up faster there than in other parts of the region — the red tide was first spotted there, likely because it’s an ideal place for the algae to thrive.

“It’s definitely responding to warm temperatures,” said UC Santa Cruz’s Kudela. The organisms swim near the water’s surface during daytime hours to gather more light and swim toward nutrients.

With winter storms long in the rearview mirror, tides in the bay also are at their mellowest point of the year, which decreases ocean water circulation into the bay.

The algae also have a lot to eat for several reasons. They munch on tiny particles, nutrients of nitrogen and phosphorus. While naturally occurring, there is an influx of these nutrients into the bay from 37 wastewater treatment plants that discharge treated sewage into the bay. This rush of nutrients provides a buffet for the algae to feast upon, allowing them to grow voraciously.

“San Francisco Bay has some of the highest levels of nutrient pollution of any estuary in the world,” said Jon Rosenfield, senior scientist with SF Baykeeper.

Dark reddish-orange looking water splashes agains rocks in the foreground with the San Francisco skyline in the distance.
An algae bloom in the San Francisco Bay near the Berkeley Marina on July 31, 2023. (Beth LaBerge/KQED)

When will the red tide go away?

In the near term, scientists say nothing can be done to stop a red tide once it shows up.

“You just have to wait for conditions to change,” said Kudela.

The options are limited. The algae can consume all the nutrients like PAC-MAN eating dots in a video game until there isn’t enough left to support the organism. Or a heavy rain storm or a wind event can churn up the bay, altering the water’s temperature, and reversing the conditions that can support it. Some scientists think that’s what caused the red tide to dissipate last year.

“We had some really big wind events, and it was gone,” said William Cochlan, a marine biologist and an emeritus professor at San Francisco State University. “The winds stir up the whole system, they don’t get enough light and that’s usually the end of the bloom.”

But the algae, in this case, are tricksters. Scientists believe that when their conditions change, they produce a cyst and embed themselves into the sediment at the bottom of the bay, not emerging again until the right conditions persist for them to grow.

SFEI’s Senn said they don’t yet know what triggers the cysts to hatch. His team is testing the bay’s water and monitoring satellite imagery to see if the bloom is expanding.

“What struck the match last year was a mystery. But once the match was struck, it was enough for this organism to get a toehold and then it had fuel to continue to grow,” said Senn.

What are scientists and officials doing to address the red tide?

In the long term, the region’s 37 wastewater treatment plants can limit the nutrient pollution they dump into the San Francisco Bay.

These plants regularly discharge treated sewage water full of tiny particles, which algae love to devour. Cochlan said these plants contribute “the vast majority” of the nutrients (or algae food) into the bay, especially during the summer when the water is warmer.

Next year, the San Francisco Bay Regional Water Quality Control Board will consider changing its rules for these plants, when their permits are up for renewal. This is an opportunity for regulators to clamp down on the nutrient load the plants are allowed to release.

While these plants treat the sewage, most do not filter out all the nutrients before discharging the water into the bay. Fixing the problem could cost at least $12 billion and maybe twice that much, according to Bay Area Clean Water Agencies, which represent local water districts.

Some plants are already redesigning their facilities to reduce the amount of nutrients released into the bay, said Eileen White, the water board executive officer. But not all plants are doing that — and there is currently no formal requirement for plant operators to reduce the discharged nutrient loads.

Climate scientists also say red tides will likely occur more often as the climate warms and raises water temperatures. The even bigger fix is addressing climate change at its root by ending the burning of fossil fuels that cause global warming.

What does climate change have to do with red tides?

Climate change will likely increase the frequency of red tides because the bay and ocean will be warmer. The bay could also absorb more runoff during significant wet years and less water during periods of drought. This off-and-on cycle could accentuate the warm and relatively calm conditions the algae proliferate in.

“We could expect more unpredictable and more frequent algal blooms of all kinds,” said Kudela at UC Santa Cruz. “It’s not surprising that we’re seeing bigger and longer-lasting blooms as the climate shifts.”

The increasing water temperature is critical to the algae’s lifespan, and a warming climate only increases the odds of the species thriving in the bay, said San Francisco State University’s Cochlan.
“We may be exacerbating these plumes, making them more frequent, of greater duration and more intense by increasing temperature through climate change,” he said.

“We have to expect to see these changes in the bay or the coast with climate change,” Cochlan said.

Cleaning up what the treatment plants dump into the bay and warming water is “like a one-two punch — and we really need to tackle both of the problems,” said Emily Jeffers, an attorney at the Center for Biological Diversity.

“But the one that we can address more readily and more quickly is the nutrient loads from the wastewater-treatment facilities.”


Is the Bay Area red tide related to the algal bloom off the Southern California coast?

The red tide in San Francisco Bay is unrelated to the algal bloom in the Pacific Ocean, currently off the coast of Southern California. The organism causing the death of dolphins and sea lions is a different type of algae, Pseudo-nitzschia, that causes the production of a neurotoxin called domoic acid.

That toxin is transmitted via the complex food web. Birds and marine mammals can die when ingesting the toxin in animals and fish that they eat.

“They get a big dose of domoic acid; it causes brain damage,” said Kudela. “Humans can get the same thing, but almost nobody in California gets sick because it’s super well regulated. But it can get into the shellfish and things like spiny lobster.”

The algae bloom in the bay and in the Pacific Ocean are similar because they’re both phytoplankton blooms and flourish in relatively warmer water, although at somewhat different temperatures. The more dangerous algae currently circulating in the Pacific Ocean could spread into the bay if the conditions are right. But Kudela said the likelihood of that happening is slim because the bay is consistently too warm for the species.

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So tell us: What do you need to know more about? Tell us, and you could see your question answered online or on social media. What you submit will make our reporting stronger, and help us decide what to cover here on our site, and on KQED Public Radio, too.


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