Not every algal bloom is toxic to humans and animals, but the bright green bloom currently floating on the surface of Discovery Bay definitely could be, said Raphael Kudela, a phytoplankton ecologist at UC Santa Cruz. This bloom is caused by cyanobacteria, which Kudela describes as “harmful algae that produces really nasty toxins.”
The state water boards warn that these toxins can harm your skin upon contact, causing itching and rashes — and if they’re swallowed, they can have effects like gastrointestinal issues and headaches.
What’s more, the particular compound this bacteria produces — microcystin — also does specific damage to the liver when ingested. “Like many toxins, it’s dependent on how big the person is,” said Kudela, so children are particularly at risk because of their smaller size.
The specific type of bloom in Discovery Bay poses a special risk to dogs — not just because of their size, Kudela said, but because “they get it on their fur, and they are cleaning themselves and licking it off, and then they happen to ingest it.”
As for why these blooms have set up shop in Discovery Bay right now, Kudela noted that it’s actually a fairly regular annual occurrence for this human-created waterway on account of the hospitable environment it provides. “There’s not a lot of water circulation, and these blooms love those conditions,” he said. “They really like lots of nutrients, which comes from humans and agriculture, really warm water and not much turbulence.”
All the same, this year’s bloom is “a pretty intense one,” he remarked.
Are all algal blooms dangerous for swimming?
Despite their sometimes-alarming appearance, algal blooms are “pretty common,” Kudela said — which is why the state’s water boards will test them once they’ve appeared to find out whether they’re producing toxins that could be harmful to humans or animals.
The “red tide” that appeared in San Francisco Bay in 2022 and again in 2023 was a bloom caused by a very different organism to the one currently producing toxins in Discovery Bay: A species called Heterosigma akashiwo. This microscopic organism looks like a swimming potato chip with a tail, Kudela said, and it thrived in the Bay’s shallow, quick-to-warm waters.
That red tide proved particularly dangerous for fish, killing a huge number of them — although scientists still aren’t exactly sure whether that was down to the decomposing cells sucking the oxygen out of the water or if the organism itself produced a toxin. It’s still “a little bit of a mystery,” Kudela said.
Regardless, even though the red tide wasn’t proven to pose health hazards to humans or animals, the general expert advice was still to stay away from the discolored waters. Even though contact with that particular bloom “is probably not going to do much to you,” Kudela said, “you don’t want to be swimming around in really mucky sort of red water.” Read more about the science behind the Bay Area’s red tide in 2022 and 2023.
So if I’m unsure what kind of algal bloom is in the water, is the general advice just ‘stay away regardless’?
Absolutely — the safest thing when encountering discolored water of any kind is just to stay the heck away.
The reason for that, Kudela said, is that even if a particular algae bloom isn’t actively harmful to humans or pets because it’s producing toxins, it’s still “this rich organic matrix that’s loaded with bacteria and other things,” he said — and even if it’s not producing toxins, “you’re still getting a big dose of organic material” by swimming in it or swallowing it.
This can, Kudela said, cause allergic reactions, the skin irritation known as “swimmer’s itch” and also nasty sinus infections if you inhale this matter into your nose. “If the water looks kind of nasty, you don’t want to be touching it,” he said. “You don’t want to be playing in it.”
David Senn, senior scientist at the San Francisco Estuary Institute, echoed this advice to KQED in 2023: Even if an algal bloom in your area hasn’t been proven harmful to humans or pets, “it makes sense to avoid discolored water in general,” he said.
How do I know if a body of water is safe to swim in generally?
If you’re headed to your local lake, river or stream this summer, the California Water Quality Monitoring Council recommends you do the following:
- Look for posted advisories in the form of signs by the water and read them carefully
- Watch for all algae and scum in the water and on the shore, and avoid it
- Keep an eye on children and pets, who won’t necessarily understand the dangers, and make sure they don’t go into the water or put any algae in their mouths
- Don’t drink the water or use it for cooking
- Rinse yourself, your family and your pets well with clean water after playing in any body of water generally.
For fishing, the water boards recommend throwing away the guts of any fish you catch and cleaning the fillets with tap or bottled water before cooking them. You should also avoid eating shellfish if you think a harmful algal bloom is present.
Read more safety tips for protecting yourself against potentially harmful algal blooms.
Why do heat waves mean I’m more likely to encounter an algal bloom?
Algae just love warm water, Kudela said, and “they tend to grow faster when the temperature is warmer.”
In addition, higher temperatures don’t just heat the water — they calm it and prevent it from mixing. And since the algal blooms “don’t like a lot of mixing,” warm and stagnant water provides “perfect conditions to get one of these blooms going,” Kudela said — and “once they get going, it’ll continue until something disrupts it, or they run out of nutrients.”
The impact of climate change and increased warming is something that makes Kudela and his colleagues “very concerned,” he said. “We’re seeing this trend even over the last decade. We’re seeing more blooms, more intense blooms.”
“It’s hard to say that’s directly related to climate change,” Kudela acknowledged. “But climate is certainly one of the factors that’s influencing it.”
What should I do if I spot an algal bloom?
First off, stay away and keep yourself, your children and your pets safe.
To report an algae bloom, you can reach SF Baykeeper’s pollution hotline by:
Algal blooms can also be reported to the California State Water Resources Control Board’s Freshwater and Estuarine Harmful Algal Bloom Program. You can reach the agency hotline by:
Be sure to include the time, date, location and a description of what you saw.
Spotted a dead fish near an algal bloom?
