Is it safe to swim in San Francisco Bay during algal bloom?


You’ve probably heard by now that an algal bloom is overtaking San Francisco Bay and tens of thousands of dead fish are washing up on beaches and around Lake Merritt. The murky soup that’s the reddish, brownish color of root beer formed because a type of algae called Heterosigma akashiwo is growing out of control. It’s among the types of algae that can cause red tides.

It’s clearly not good for the fish, but what about humans? Is it safe to swim in the bay that may offer a place to cool off during the upcoming heat wave?

State and local health officials are advising that humans avoid swimming in the bay if they can see algal bloom in the water. While the algae is not toxic to humans, it could cause skin or eye irritation, said Eileen White, a spokesperson for the San Francisco Bay Regional Water Quality Control Board, one of several government agencies monitoring the bloom.

“We’re recommending that people stay out of the water out of an abundance of caution,” White said.

Blobs of the bloom are spread throughout the bay, and they’re growing and also moving around with the winds and tides. 

“If it’s there, the water is a reddish-brownish color,” said White. “You can tell by the look of the water. It’s pretty obvious. If the water doesn’t look good, don’t swim in it. If there’s no algal bloom, then it’s OK.”

White also advised pet owners to keep dogs out of the bay in areas where the bloom is present. She said boating on the bay is perfectly safe, and on Wednesday morning, she saw crew boats on Lake Merritt. 

Aquatic Park in San Francisco is a popular spot for bay swimming, and White said she could not confirm whether the bloom was in this area. Several swimmers with the Dolphin Club said they have seen the bloom come and go here. 

“I swim 3 or 4 mornings a week for forty minutes in the cove at Aquatic Park,” Joe Illick, a Dolphin Club member, told SFGATE in an email. “During the past two weeks my eyes have been watering and I bring up more mucus than usual. Nothing deeply disturbing. I thought it could be my age (87) but of course it may be the water.”

The bloom has been observed in waterfronts in Alameda County, including locations such as Lake Merritt, Alameda Grand Marina and Martin Luther King Jr. Regional Shoreline, the county’s health department said. The county said it is posting signs at popular swimming locations saying that it is highly recommended people and their pets avoid the water until the blooms dissipate.

“Contact with the algae blooms can cause skin irritation and burning eyes to humans and can cause more dangerous effects to dogs,” the Alameda County Public Health Department said in a statement.

Alameda, the Oakland Inner Harbor and Lake Merritt are the first locations where researchers identified the bloom, White said. The algae continued to proliferate and by mid-August, it had pushed into waters near Richmond and Belvedere to the north, and San Mateo and Foster City across the bay. It has since consumed the southern part of the bay crept into the central portion.