AILSA CHANG, HOST:
Thousands of dead fish are covering beaches in the Bay Area from San Francisco to San Jose. Lesley McClurg from member station KQED reports that a red tide has hit the region.
LESLEY MCCLURG, BYLINE: About a month ago, Mary Spicer says the color of the water started to turn red and then brown.
MARY SPICER: And then the brown color got like a denser, thicker, chocolaty brown.
MCCLURG: For more than a decade, Spicer has kayaked several times a week on the San Francisco Bay. It breaks your heart to see harbor seals peeking their heads out of dense, murky water.
SPICER: It's really worrisome.
(SOUNDBITE OF BIRDS CALLING)
MCCLURG: An estuary connects the bay to Lake Merritt, a few miles inland in Oakland. A group of people stare wide eyed from the shore there. Sabrina Wicker plugs her nose and shakes her head.
SABRINA WICKER: I thought it was, like, leaves or something, but it's, like, all those are all fish of all different sizes, floating, dead.
MCCLURG: The sandy beach below her feet is a web of twisted, tiny corpses. Crabs lay on their backs. Open mussel shells are strewn all along the shore. Kobi Hutchinson points to the mess. He has worked at the boathouse for the past five years.
KOBI HUTCHINSON: There are a few fish that are left. You can see them sometimes. They're close to the shore, but they're kind of gasping for air. You can tell they're really on the surface because the dissolved oxygen in the lake is so low.
MCCLURG: The algal bloom known as red tide causes fish to suffocate, and toxins in the algae can destroy fish gills. Laurie Bagley, a local volunteer, has watched the situation worsen by the day.
LAURIE BAGLEY: I just started sobbing when I realized the enormity of it, that they're literally dying, like, at our feet. I feel like an emotional wreck.
MCCLURG: Officials estimate as many as 10,000 fish died in Lake Merritt in recent weeks. Signs around the lake warn visitors to avoid playing or swimming in the water.
EILEEN WHITE: Generally, this algae bloom is not toxic to humans, but it could cause skin irritation, eye irritation for people.
MCCLURG: Eileen White is the executive officer for the San Francisco Bay Regional Water Board. She says algal blooms are not uncommon in the bay, especially during the summer around marinas and along shorelines. But this one has lasted longer and is more widespread than ever before.
WHITE: We're continuing to take samples. We're continuing to monitor. We're trying to understand the cause of it.
MCCLURG: White says the unusual conditions are likely a combination of warmer water temperatures driven by climate change and high nutrient levels in the water, specifically nitrogen and phosphorus, which algae love to eat. Scientists have long warned the bay is primed for a disastrous red tide.
SEJAL CHOKSI-CHUGH: It's kind of a wake-up call to try to control the level of nutrients in the bay.
MCCLURG: Sejal Choksi-Chugh is the executive director for the environmental group San Francisco Baykeeper.
CHOKSI-CHUGH: And there really is only one main lover. It's wastewater treatment plants. We have 40 wastewater treatment plants around the bay.
MCCLURG: These plants remove solids and bacteria from sewage, but utilities do not clean nutrients out of wastewater before discharging it.
CHOKSI-CHUGH: What we really need to be doing is investing in our infrastructure so that we can upgrade those plants. San Francisco has some of the worst water recycling programs of anywhere in California.
MCCLURG: Choksi-Chugh says human waste from 8 million Bay Area residents feeding the algae is why sharks, bat rays and huge sturgeon are washing up on beaches across the region.
CHOKSI-CHUGH: And that was just sort of a tip of the iceberg - the level of fish, because most of the fish that died sunk to the bottom of the bay. And so we don't even know the magnitude of the problem of the fish kills, how many fish died.
MCCLURG: In recent days, the chocolaty water has begun returning to its shimmery blue. But Choksi-Chugh fears the ecological effects from this summer's red tide may haunt the region for years to come. For NPR News, I'm Lesley McClurg in San Francisco. Transcript provided by NPR, Copyright NPR.