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Bay Area Red Tide Crisis Ends, Watchdog Group Declares Algae Bloom Over

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A person wearing glasses smiles while riding in a boat near a large bridge.
Staff Scientist Ian Wren on the deck of the San Francisco Baykeeper in Oakland on August 15, 2023. Wren found that the recent harmful algal blooms in the water have now diminished. (Juliana Yamada/KQED)

The red tide that gave East Bay waters a light brown sheen earlier this month is likely over, declared the environmental watchdog group San Francisco Baykeeper Monday.

“I would say this bloom is done for now,” said the group’s staff scientist Ian Wren on a boat under the eastern half of the Bay Bridge, where the water was olive green instead of a murky tea color brought on by the bloom.

“Almost overnight the bloom died and the water was crystal clear,” he added.

Even though the red tide has dissipated, Eileen White, executive officer of the San Francisco Bay Regional Water Quality Control Board, is hesitant “to declare victory.”

“It is still summertime and at this time I am cautiously optimistic,” she said. “We don’t have enough knowledge of the species. Warmer weather could bring it back and we will continue to monitor the situation.”

Last year the red tide — literally billions of tiny algae called Heterosigma akashiwo — killed an immeasurable amount of fish. This year, the algae killed fewer than 100, according to a state-run citizen science project. Sitings of important Bay Area species, including sturgeons, bat rays and crabs, were among the dead.

The wake of a boat on the water.
Waves from the San Francisco Baykeeper splash in Oakland, on Tuesday, Aug. 15, 2023. (Juliana Yamada/KQED)

“I am so happy it’s only 85 fish and I am glad it didn’t spread to the South Bay,” White said.

Over the last week, Wren said citizen scientists monitoring the bloom looked at bay water “under the microscope and couldn’t find any of the problem algae.”


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.

“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.”

Wren, with SF Baykeeper, isn’t exactly sure what cut the bloom short this year. Still, he said there are a few theories as to why the algae didn’t return in force: it could have been too cloudy decreasing light, it wasn’t warm enough, the bay waters mixed causing the algae to die off or there weren’t enough concentrations of tiny particles in the water that the algae like to dine on.

Wren said output from the region’s 37 regional wastewater treatments is a big part of why the algae blooms can get so bad. The wastewater includes nutrients like phosphorus and nitrogen, which the algae go to town on.

“There could have been a lower level of nutrients to start with and this bloom could have just fizzled out naturally, eating what it could and never having the chance to take off,” he said.

Scientists like Wren and Peter Roopnarine, curator of invertebrate zoology and geology at the California Academy of Sciences, said humans are the main reason why this algae bloom got so bad. He blames wastewater agencies almost continually pumping nutrient-filled water into the bay and a warming world because of the burning of fossil fuels globally.

Small boats and buildings along a waterfront.
The San Francisco Baykeeper is seen in the Oakland Marina in Oakland on Tuesday, Aug. 15, 2023. (Juliana Yamada/KQED)

“It’s almost all about us to blame,” said Roopnarine. “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.”

Wren said the tide is unlikely to return this year, but just because the red tide has disappeared doesn’t mean it won’t come back next year.

When under stress, the algae can create little cysts, like seeds, and plant themselves at the bottom of the bay, lying dormant until the right conditions for the organisms to proliferate return. Conditions include light, warmth and calm water.

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“With the bloom getting so big and widespread last year, it is highly likely a lot more cysts are present throughout the bay, ready to spark again,” Wren said.

It’s sort of like a long-term easter egg hunt with potentially deadly consequences for fish, Wren said. The idea is that when the algae are present, the water holds less oxygen, killing the fish. The algae are not known to have any direct harmful effects on humans or mammals.

“Given that we have had two back-to-back years of blooms, this likely could be the new normal,” he said. “We might see small, medium and large blooms on an annual basis.”

A new normal because Wren said climate change likely means more algae blooms — and not just the lesser toxic bloom this year.

“There are other harmful algae in the bay that could just as easily have taken off with more lethal consequences to wildfire and humans,” he said. “These algae are just waiting to go nuts.”

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