Toxic algae blooms becoming more common at Lake Temescal and other East Bay swim sites

As global temperatures rise, the frequency and size of dangerous blue-green algae blooms are increasing worldwide. For East Bay residents, that means more warnings not to touch the water at local lakes and reservoirs. 

Warnings and closures have been seen across the East Bay in recent years, from Lake Anza in Berkeley to Lake Merritt in Oakland, where an algal bloom deprived the water of oxygen and killed thousands of fish in 2022. Coming into contact with the toxic algae — a type of cyanobacteria — can cause minor effects like skin irritation but can be more dangerous when ingested, especially by children or pets, causing flu-like symptoms.

North Oakland’s Lake Temescal was closed to swimmers for a total of 670 days due to cyanobacteria blooms between 2014 and 2022, according to records of water testing from the East Bay Regional Parks District. A caution sign was posted for swimmers for 955 days in that period. That sign remains, with a “danger advisory” currently posted at the lake.

Oakland resident Naazgol Koushafar, 24, grew up near Temescal Park and swam in the lake as a kid, but she hasn’t seen anyone in the water in years. She walks her dog, Meeku, at the park, but is careful not to let her near the water.

“I’m scared for her to drink it,” Koushafar said. “In the last five or six years, there’s always caution tape.”

Becky Tuden, an ecological services manager for the East Bay Regional Parks District, said Lake Temescal was treated in 2019 with alum, a compound that reduces some of the bacteria’s available nutrients. Tuden said that while the treatment was temporarily effective a toxic bloom was detected the following summer.

Not all cyanobacteria are harmful, and it’s not fully understood why toxin-producing cyanobacteria are blooming more frequently.

That’s what Andreja Kust, a researcher at UC Berkeley’s Advanced Genomics Institute, is trying to figure out. She studies the role of microbes in ecosystems by sequencing their genome, and cyanobacteria is her specialty. Kust said these blooms coincide with the seasons and typically die off when the weather gets cooler. But the problem is getting worse each year.

“They are becoming more intense and they are lasting longer,” Kust said.

Scientists have already identified the genes in blue-green algae that lead to toxin production, Kust said, but they need to study the whole organism to understand why those toxic variants are on the rise and what measures could curb their growth.

The biggest factor overall, Kust said, is temperature. Rivers and lakes around the world are warming due to climate change, creating an environment in which cyanobacteria thrive.

“They really like the warm temperature,” Kust said. “They really proliferate, they grow really fast.”

Shallow water with little circulation also promotes cyanobacteria growth. That’s a problem in Lake Temescal, where sediment filled with nutrients used by the bacteria is accumulating and making the water shallower. The depth is now around 15 feet, compared to 60 to 80 feet when the reservoir was constructed in the 1860s. 

Tuden’s department has proposed a plan to dredge the bottom of the lake to correct the issue, with an estimated cost of $23 million.

Increasing the depth of the lake could be a longer-term solution than water treatment, Kust explained. It would leave nutrient-rich sediment layers in deeper, colder waters where blue-green algae doesn’t thrive.

Kust collects water samples from around the Bay Area to take back to the lab in Berkeley for testing. On Oct. 23, she collected a sample from an active cyanobacteria bloom in the Sacramento River near Rio Vista. Around this same time last year, she said, blooms in that area were dying off. 

But Kust is confident research will illuminate the causes of these blooms in the near future.

“There are a lot of unknowns,” she said. “But what we are getting better at is … the genomics, so we can see much deeper into what’s going on.”

(Top photo: Lake Temescal has been closed for months at a time since harmful algae blooms were first detected there in 2014, by Julian Wray)

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