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Last Summer's Fish-Killing Algae Bloom Is Back in the Bay

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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)

A red tide that has left a light-brown sheen on the water along parts of the East Bay shoreline is the same type of toxic algae bloom that killed thousands of fish in the San Francisco Bay last summer, a local environmental group warned on Monday.

“We have confirmed with our partners that it’s the same species as last year,” Eileen White, executive officer of the San Francisco Regional Water Quality Control Board, said at a Monday afternoon press conference. She said the organism has so far been found in the bay waters near Emeryville, Albany, the Berkeley Marina, Richardson Bay and Belvedere Cove, as well as off the Marin County coast, near Muir Beach.

“The good news is we have not seen any marine animal deaths as a result of this algae bloom,” she said.

San Francisco Baykeeper’s pollution hotline lit up late last week with calls about the tea-colored water seen stretching from Emeryville to Albany, said Jon Rosenfield, the group’s senior scientist.

“Two years in a row is quite alarming,” he said, noting that it remained unclear how bad the bloom will get. “There’s really nothing that people can do to stop a bloom like this once it’s started. It just has to burn itself out.”

Red tides are fueled by elevated levels of nitrogen and phosphorus expelled into the water as a byproduct of treated sewage from wastewater treatment plants — leaving nutrients that algae love to nibble on. Such conditions can cause the algae to grow out of control and sometimes form a rust-colored hue.

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)

Not every algal bloom is harmful, some can even be beneficial to marine habitats. But others produce deadly blooms — as did the one that emerged last August.  That algae species, identified as Heterosigma akashiwo, and also believed to be the cause of the current bloom, killed an untold number fish over a matter of weeks, their rotting, fetid carcasses littered across Bay Area shorelines, including the banks of Lake Merritt.

The algae species emits a toxin that’s especially harmful to fish. It can also spur a biological reaction that depletes oxygen levels in the water, acerbating the marine death toll.


David Senn, a senior scientist at the San Francisco Estuary Institute, said that while it’s too soon to tell, the current bloom may be the result of algae from last year that lay dormant in sediment over the winter.

“It’s not all that surprising that we’re seeing a reemergence of this again this summer,” said Senn, whose group is using satellite imagery to track the bloom in real time.

Rosenfield, with SF Baykeeper, blames this and other recent harmful algal blooms on the region’s 37 wastewater plants that regularly discharge treated sewage into the bay. He said the regional water board has an opportunity next year to change permitting rules — when permits go up for renewal — to clamp down on the nutrient load the plants are allowed to release.

“San Francisco Bay has some of the highest levels of nutrient pollution of any estuary in the world,” he said. “The solution is to remove the fuel load, which means keeping those nutrients out of the bay waters.

Although local wastewater treatment plants remove sewage, most do not filter out all the nutrients before discharging water back 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.

More on red tides

White, with the regional water board, said some local wastewater plants are already redesigning their facilities to reduce the amount of nutrients released into the bay. But she said the push is not uniform and, although it could ramp up next year, there is currently no formal requirement for plant operators to reduce the nutrient loads that are discharged.

“Some of the wastewater agencies have already started planning, but knowing that it’s not like switching on a light switch, it’s going to take time to plan, design and construct,” said White, who formerly served as East Bay Municipal Utility District’s wastewater director.

Scientists also say red tides are likely to occur more often as the climate warms and raises water temperatures.

High nutrient loads 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.”

Jeffers said she learned about the current algal bloom from her 8-year-old daughter, who attended summer camp at the Berkeley Marina last week.

“She said some kids might go in the water and that she didn’t think that was a good idea,” Jeffers said of her daughter. “I told her that was smart.”

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