Red tide in Santa Cruz
The sight of the ocean near Santa Cruz Harbor on Monday afternoon.
(Kevin Painchaud / Lookout Santa Cruz)
Environment

‘Not uncommon’ but little understood: Red tides have returned to Santa Cruz beaches — what to know?

Better known to scientists as harmful algal blooms, red tides can cause irritation of the eyes, ears or nose for those with certain conditions in those areas but are generally not harmful. And they do sometimes come with bioluminescence — waves glowing at night.

Nope, you weren’t imagining that reddish-brownish tint to the water along Santa Cruz County beaches the past week — the red tide is back with a vengeance.

When coastal water temperatures warm up into the higher 60s, typically between these later summer months and early fall, it’s a common occurrence. But also one that’s not widely understood.

So Lookout reached out to a local water quality expert and the greater pool of marine science information available to break it down for you:

What is it? And why is it that color?

It’s a phenomenon better known to scientists as harmful algal blooms — “the result of high concentrations of a phytoplankton species of the Margalefidinium genus,” county water-quality specialist Nilo Alvarado told Lookout via email.

Cells in the phytoplankton contain what UC San Diego’s Scripps Institution of Oceanography calls “sunscreen,” and when they congregate near the surface, that’s what gives the water its reddish-brown color.

Is it harmful? Should I avoid the water?

“While the color of the water looks dramatic, this bloom should not concern anyone without any extreme health conditions,” Alvarado wrote. Red tides “can produce organic compounds that can affect other microorganisms and marine life, but are not known to have direct effects on humans.”

“As a general rule when swimming in any natural body of water, avoid ingestion,” he wrote. “Large concentrations of any microorganisms might promote irritation of the eyes, ears or nose of people that have sensitivities in these areas.”

Harmful algal blooms can also be harmful to humans as toxins work their way up through the food chain, particularly in shellfish; Healthline suggests following local guidance when eating fish and shellfish as the best way to avoid any of those issues.

How long will it last? No one can exactly predict.
(Kevin Painchaud / Lookout Santa Cruz)

How long will it last?

Red tides are common for Monterey Bay, but they can be quite difficult to predict.

“Algal blooms occur for a variety of reasons such as increases in light, nutrients, and water temperature,” Alvarado said, with temps between about 57 and 64 degrees most conducive to the Margalefidinium species. “The bloom we are currently experiencing is not uncommon for Monterey Bay, except for the timing. We might expect the water conditions we are experiencing more typically in the fall, and therefore could potentially see this bloom lasting anywhere from days to weeks.”

What about the cool glowing waves?

Yes, red tides often come with bioluminescence — waves glowing at night — though that, too, is unpredictable and depends on the kind of phytoplankton in the bay.

The Scripps Institution suggests hitting the beach at least two hours after sunset for a possible light show. If and when it becomes a phenomenon like it did last year, social media will be the place for immediate discovery.

While there were a few opportunities to view it here, Southern California got a consistent run of late-night viewing opportunities. This video from the San Diego area last year gives you a taste.