An algae bloom turned Lake Merritt's waters rust red Aug. 8. Credit: Darwin BondGraham

For two weeks, Lake Merritt has been turning various hues of red and brown, and authorities are still advising visitors to avoid direct contact with the water until they better understand what’s happening.

The San Francisco Bay Regional Water Quality Control Board is scheduled to conduct a test Aug. 22, in response to a report filed by the city earlier this month after residents complained of discoloration.

“This will include testing for cyanobacteria,” also known as blue-green algae, which can be toxic to humans and animals, said city spokesperson Sean Maher.

“It is highly recommended that people and their pets stay out of the water until the blooms dissipate,” said the Alameda County Environmental Health Department in a statement. “Contact with the algae blooms can cause skin irritation and burning eyes to humans and can cause more dangerous effects to dogs.”

However, the California Department of Public Health examined a sample from the lake and determined that the organism causing the so-called red tide is likely not cyanobacteria but rather Heterosigma akashiwo, a type of microscopic marine algae.

This algae “is not known to cause human illness (but it has been documented to contribute to fish kills),” said department spokesperson Ronald Owens in an email. But he added that people and pets should avoid entering the water during any blooms or periods of discoloration, as “a general precaution.” 

The department conducted the analysis in response to a request by the regional water board for consultation.

“We’re asking people to let us know if they see dead fish,” said Katie Noonan, co-chair of the Rotary Nature Center Friends, which supports science education and stewardship of the lake. So far, one lifeless striped bass has turned up, she said.

“We’re really happy that our monitoring agencies have their fingers on this,” Noonan said.

Last week, a resident contacted The Oaklandside after noticing red water, similar in color to the lake, flowing out of a pipe at the Kaiser Convention Center, which is currently being renovated, and into a storm drain. Maher said the city was investigating the report. There is no indication that the situation is related to the red tide in Lake Merritt.

According to Noonan, the algae is arriving in the lake from the bay, but it’s unclear what exactly is causing the bloom. Blooms of Heterosigma akashiwo can be linked to increases in water temperature, but this isn’t always the case. 

Noonan hopes the concerns around the red tide won’t scare people away from the lake. 

“I love Lake Merritt and think it’s a treasure,” she said. “It’s an educational window for people to view all kinds of current climate and environmental issues, and I hate for people to shun it.”

Natalie Orenstein covers housing and homelessness for The Oaklandside. She was previously on staff at Berkeleyside, where her extensive reporting on the legacy of school desegregation received recognition from the Society of Professional Journalists NorCal and the Education Writers Association. Natalie’s reporting has also appeared in The J Weekly, The San Francisco Chronicle and elsewhere, and she’s written about public policy for a number of research institutes and think tanks. Natalie lives in Oakland, grew up in Berkeley, and has only left her beloved East Bay once, to attend Pomona College.