A likely algae bloom turned Lake Merritt's waters rust red on Aug. 8, 2022. Credit: Darwin BondGraham

There are lots of reasons not to go swimming in Lake Merritt, but officials say a toxic algae bloom may be making the water more hazardous.

The city of Oakland has advised the public to avoid touching, swimming in, or drinking and fishing from the lake, and to keep young children and pets away.

A red-brown discoloration in the water first appeared last week, especially on the southeastern shore of Lake Merritt, around where it connects to the Bay.

“​​The reddish color of the material in the water indicates it could be a Cyanobacteria bloom … which can produce toxins that are harmful to humans and animals,” the city wrote in an advisory Friday. 

While cyanobacteria is also called “blue-green algae,” it can appear in many different colors. Often it looks like globs or streaks in the water, or scum on the surface, but it is impossible to tell just by looking at the water whether toxins are present. 

James Robinson, executive director of the Lake Merritt Institute, told The Oaklandside what’s happening in the lake is likely actually a dinoflagellates bloom, also called microalgae or red tide. 

The city’s public works staff inspected the lake following complaints from residents, and said it planned to submit a report to the San Francisco Bay Regional Water Quality Control Board, the state agency tasked with keeping local water clean. The Oaklandside has asked the city about the results from that inspection and will update this story if we receive a response.

Initial testing in May detected low levels of contamination in Lake Merritt, prompting the city to post warning signs around the southern end of Lakeshore Avenue.

Algae blooms tend to appear in warm, calm water, and researchers believe that changing weather patterns associated with climate change have created conditions for blooms to increase in frequency. Other potential causes include nutrients from fertilizer or animal and human waste entering waterways. 

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Contact with the bacteria can cause rashes, vomiting, and flu-like symptoms for people and pets. 

The city is advising residents to avoid over-watering lawns, to prevent nutrients from entering the water, and to clean up litter at the lake.

Many other bodies of water in the Bay Area and throughout California have been contaminated by similar algae blooms recently. The state updates a list weekly of all algae bloom reports. Currently, there are two places in Alameda County on the list: Big Break Regional Shoreline and Lake Del Valle West Beach. 

Oakland has so far placed a “caution” advisory on Lake Merritt, which is less restrictive than a “warning” or “danger” advisory. 

“The City anticipates ongoing testing and will continue coordinating to provide appropriate notification and warnings to the public,” Oakland’s advisory release said.

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.