The sand at Mission Bay looked ordinary enough at sunset.
Then the sun dropped, the crowds thinned, and the water did something nobody near San Diego expects to see.
A footstep in the shallows left behind a trail of electric blue light.
A wave broke and glowed like a neon ribbon instead of white foam.
People waded in anyway, phones out, watching their own hands light up the dark water.
A scientist had a far plainer explanation for it.
Warm water pushed into the bay set the glow in motion
The bloom did not start with anything mysterious. A marine biologist at the Scripps Institution of Oceanography in La Jolla pointed to ordinary ocean mechanics once reports started coming in. Dimitri Deheyn said recent large waves may have pushed more ocean water into Mission Bay, helping create the right conditions for the bloom. He offered that as a likely explanation, not a settled one.
Mission Bay is mostly enclosed, a lagoon rather than open coastline, so it trades water with the ocean slowly. Big surf can act like a natural pump, shoving offshore water past the inlet and into the bay’s warmer, calmer interior.
“The water in the bay is very warm, so that combination of both those factors may have led to the bloom,” Deheyn said.
Microscopic plankton called dinoflagellates do well in exactly that mix of warmth and incoming seawater. When enough of them crowd into one basin, the bay stops behaving like plain seawater after dark.
Millions of single cells flash at the same disturbance
Each dinoflagellate is a single cell, invisible on its own, yet it carries a built in chemical trigger. Touch, motion or pressure from a wave, a fin or a footstep sets off a flash of blue light inside the cell almost instantly.
Multiply that flash by the cells packed into one stretch of shallow water, and a single ripple becomes a visible sheet of glowing blue.
Why the cells flash at all is still a hypothesis rather than a demonstrated mechanism. Deheyn calls the leading idea the burglar alarm hypothesis, where grazing shrimp trigger the light of the dinoflagellates and so become visible to the fish waiting beneath them.
Either way, the brightest light traces motion instead of sitting still, following a paddle, a swimming fish or a dog’s paws through the shallows.
A week of sightings spread across several beaches
The bloom did not stay in one place. Bioluminescence was spotted in parts of Mission Bay throughout the week, drawing crowds hoping to catch a glimpse of the glowing water, and it was seen along stretches of Sail Bay and Ski Beach late at night and early in the morning.
A San Diego photographer who has shot bioluminescence for years said he got a tip about a week earlier that a bloom could be developing there, and soon hundreds of people were showing up each night to see it for themselves.
During the day, Deheyn said, the blooms can appear reddish brown, which is why they are often called red tides, even though the organisms responsible glow blue at night. Scientists say the local glow comes from blooms of the alga Lingulodinium polyedra.
Red tides are unpredictable, and not all of them produce bioluminescence. Comparable algae counts turn up in other enclosed coastal waters, including a Panhandle bay where cell counts have topped 100,000 per liter.
No toxin warning accompanied this week’s glow
Unlike some red tides elsewhere in the world, this bloom checked out safe. While some red tides in other parts of the world can produce toxins, Deheyn said this bloom in San Diego is not toxic and is safe to swim in. That distinction matters, because the same family of organisms behaves very differently depending on which species dominates the water.
Even a harmless bloom is not entirely inert. Scripps scientist Michael Latz has noted that some people are sensitive to inhaling air associated with a red tide, so the organisms must be producing other compounds that can affect human health.
A large red tide years ago did lead to local fish deaths, though the algae behind this particular glow is harmless. As a dense bloom dies off, decomposing plankton can draw oxygen out of shallow water, which is a matter of low oxygen stress rather than toxins.
No fixed schedule exists for the next bloom
The next glow depends on swell and temperature lining up again. Deheyn said that if the waves slow down, bioluminescence could begin showing up along the open coast instead, which means the show may simply move rather than end.
Scripps Oceanography notes there is no dedicated red tide monitoring program, though the Southern California Coastal Ocean Observing System runs weekly sampling for potential harmful algal toxins.
Other places live with their own unschedulable water events, including the glacial outburst flooding near Juneau, Alaska. For now, the people who waded into Mission Bay after dark got to watch an ordinary wave turn into light with nothing more complicated than their own two feet.
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