Aerial shot of the Berkeley Marina. Streaks of darker, tea colored water are visible in the usually blue water.
The Berkeley Marina as seen from above on July 28, 2023. The dark streaks on the water represent areas where there is an abundance of the toxic Heterosigma akashiwo algae. Credit: SF Baykeeper

A harmful algal bloom known as a red tide is spreading through Berkeley Marina, and scientists believe it’s the same species that caused a gruesome fish die-off in Oakland’s Lake Merritt and mass mussel death at the Berkeley Marina last August. 

No mass fish deaths in the Bay Area have been reported yet, but there is cause for concern.

Heterosigma akashiwo, the toxic species associated with the red tide, is always present in low numbers in the Bay’s water, but rarely in such abundance. Before last year’s bloom, there had not been one of that magnitude in the Bay’s estuarine environment for nearly two decades. 

No one knows exactly what makes it bloom or how long it’ll last, but scientists know that the algae feed on nitrogen and phosphorus, nutrients found in agricultural runoff and sewage. Warm temperatures and tidal movements play a role as well.

Baykeeper asks anyone who encounters water that looks or smells suspicious to report it to its pollution hotline.

Environmental nonprofit San Francisco Baykeeper received a tip on July 26 notifying them of streaks of tea-colored water at the Berkeley Marina, which they later confirmed by flying a drone over the area. Over the next five days, the organization received a string of reports of discolored water in Richmond, Tiburon, Emeryville and Alameda, said Julia Dowell, a field investigator for SF Baykeeper. It’s spreading fast: Dowell, an Alameda resident, said she didn’t notice any discoloration in the Alameda estuary when she checked Saturday. The dark, murky swirls of water had already begun to spread by Sunday.

Piles of dead anchovies at Lake Merritt
Piles of dead anchovies washed up onto the shore at Lake Merritt on August 30, 2022. Credit: Amir Aziz

In April, a kayaker found raw sewage leaking from under a seafood restaurant at the Berkeley Marina and reported it to the SF Baykeeper pollution hotline. That leak has since been fixed, but it’s unclear how long the pipe had been leaking. 

“These issues are compounding,” Dowell said, asked whether the sewage leak could have contributed to the algal bloom. “I don’t think we can point out one particular discharge and say that that is the fuel for this.” SF Baykeeper attributes the high levels of nitrogen and phosphorus in the Bay’s waters to the sewage outflows of the area’s 37 wastewater treatment plants. 

It’s been a challenging few months for local marine life. In February, after a series of heavy atmospheric rivers, dead leopard sharks and bat rays were found in tidal lagoons like Berkeley’s Aquatic Park, perplexing naturalists and marine experts. Suspects include decreased salinity, decreased oxygen, or toxic algae. (Dowell said those deaths are likely unrelated to the current red tide due to the timing.)

Heterosigma akashiwo is not a known public health risk to water contact or shellfish consumption, according to the California Department of Public Health. Still, it’s a good idea for people and their pets not to enter discolored ocean water “due to the risk of irritation from any chemicals that may be produced by the dense algae, or other related biological activity in the water,” a department spokesperson wrote in an email. 

Last year’s mass marine die-off started roughly three weeks after the bloom was initially spotted growing in the Alameda area, Dowell said. Only time will tell if this bloom will impact marine life to the same extent or follow last year’s timeline. 

A second consecutive year of fish die-offs could be “devastating” for the local ecosystem, she warned, but there’s not much that can be done at this point. For example, white sturgeon, the largest freshwater fish in North America and one of myriad species impacted by last year’s die-off have struggled to recover. Other fish species affected include the striped bass, sharks, bat rays, smelt and anchovy, according to the San Francisco Bay Regional Water Quality Control Board. 

Scientists don’t fully understand the link between the algae blooms and the fish deaths yet, but they do know that the fish likely died of suffocation. As algal matter dies off during and after a bloom, the decomposition process uses oxygen in the water, causing fish and other marine life to suffocate.

The die-off in Lake Merritt, a tidal lagoon connected to the San Francisco Bay, in August 2022 is estimated to have impacted 10,000 fish, according to the state’s water board. The San Francisco Bay Regional Water Quality Control Board’s field investigation found extremely low dissolved oxygen levels in the lake water. Since then, the city of Oakland has turned back on at least one of the fountains in the lake, which has helped re-oxygenate some of the water.

See Lake Merritt’s dissolved oxygen levels in real time

The Lake Merritt Institute, a nonprofit that educates residents and helps keep the lake’s waters clean, said the lake remains unprotected from another fish kill because the fountains can only aerate up to a quarter acre of the 140-acre lake.

In response to the die-off, the city of Oakland launched the Lake Merritt Water Quality Management Pilot Project, which includes plans to install two devices to increase oxygen levels and prevent stratification in the lake. The city hasn’t announced when the devices will be installed.

This story was updated on Aug. 10 with new information.

Seagulls pick at dead mussels on the Berkeley Marina dock on August 30, 2022. Credit: Supriya Yelimeli

Iris Kwok covers the environment for Berkeleyside through a partnership with Report for America. A former music journalist, her work has appeared in the San Francisco Chronicle, KQED, San Francisco Examiner...