A rock is tossed into the waves and a small firework explodes in the water. A stick traced in the muddy sand leaves a trail of light. A child jumps in the water and is suddenly bathed in glowing, blue-green crystals.

Some describe summer in the Northwest as magical. But during these long, hot stretches of late summer, nighttime can seem otherworldly as Puget Sound shorelines — including some on Vashon — glow from a natural phenomenon called bioluminescence.

“It’s just enchanting,” said Leslie Enzian, who once saw bioluminescence at KVI Beach. When she and her son saw the light, they jumped in the cold water to play in it. Enzian said she kept her young son up far past his bedtime, but the experience was worth it.

“If you’re in the water and swish your arms ,there are sparkles everywhere … and if you stand up you’re covered in stars,” she said.

Despite several more late-night trips to the beach, Enzian said she hasn’t seen the bioluminescence in a couple of years.

“I’ve been on a search ever since. I tell people to call me if they see it,” she said. “It’s such a rare and magical thing.”

Indeed bioluminescence, appearing mysteriously at any movement in the water, is also somewhat unpredictable. Experiencing the phenomenon, often called sea sparkle, depends largely on being in the right place at the right time.

The term bioluminescence actually refers to any living organism that has the ability to create a fleeting light. Here in the Northwest, bioluminescence — sometimes appearing as small sparks of light and other times like a glow in the water — is created by dinoflagellates, single-celled animal plankton that only live in certain waters during certain times of the year.

“It’s like looking at the stars, but in the water,” said Kathyrn True, a local naturalist who has also seen bioluminescence. She said the phenomenon was especially impressive when she saw it during a late-night kayaking trip.

“The paddle would stir up what looked like melted, glowing droplets, and you saw the fish darting around underneath you,” she said. “It was super pretty.”

Bob Fuerstenberg, a retired aquatic ecologist who lives on Vashon, said conditions are best for bioluminescence during warm spells in the Puget Sound, when smaller plant plankton that the dinoflagellates feed on grow in large numbers. On hot days plankton can often be found in abundance in protected bays and inlets such as Quartermaster Harbor and Tramp Harbor.

“You get some pretty big blooms out there when there’s lots of light and the temperature is high,” he said.

Karlista Rickerson, a local scuba diver who also collects water samples for Public Health — Seattle & King County, noted that the most common bioluminescent plankton in Puget Sound is noctiluca, a dinoflagellate that also turns seawater a murky red. In Latin, she said, noctiluca literally means “night light.”

Noctiluca and other plankton are annoying to divers in the summer, Rickerson said, destroying visibility in the water. But once during a nighttime dive with her son, she said, she discovered that the noctiluca, which many confuse for toxic red tide, took on a whole new life at night.

“I watched my son fall backwards into the water, and his whole body was outlined in diamonds,” she said. “That’s one of the hooks of scuba diving.”

Some beach-goers know to look for bioluminescence when there’s little or no moonlight. This is not only because the sea sparkle is easier to spot in the dark, Fuerstenberg said, but because the tiny creatures are actually capable of moving in the water and will rise higher when it’s darker out.

“Under no moon, they sit closer to the surface and you can get a better look at them,” he said.

Bioluminescent plankton live in oceans around the world. However the reason for their glow is somewhat mysterious even to scientists. Several University of Washington marine biologists reached by The Beachcomber admitted they knew very little about bioluminescence, and even Jeff Adams, a marine water quality specialist with Washington Sea Grant and a local beach naturalist, had to delve into his research library for a refresher on sea sparkle.

It turns out that scientists believe bioluminescence in plankton serves as a defense mechanism, startling or confusing predators that feed on it, Adams said. One paper, he added, called the light a “burglar alarm” that is set off by any physical contact or a rush of water.

“If the contact or flow were created by something that wanted to eat them, then the light may cause enough distraction to prevent becoming dinner,” he said.

Fuerstenberg agreed, comparing it to a smoke screen to distract or blind small fish that eat plankton. “It’s like shining a flashlight in someone’s eyes at night,” he said.

Fuerstenberg, who grew up in Minnesota, compared the glowing plankton to the fireflies that live in other parts of the country. When he moved to the Northwest decades ago, he said, he was disappointed to leave behind the glowing bugs of his youth. But he was truly amazed when, as a student studying marine biology, he came across light green bioluminescence for the first time in the waves off the Oregon coast.

“Seeing it for the first time was just astonishing. I had read about it, … but there’s nothing like seeing it for the first time,” he said.

As it turns out, plankton and fireflies — both considered bioluminescent creatures — emit light through a very similar chemical reaction, Fuerstenberg said. And just as some gather fireflies in jars, creating ad hoc lanterns, Fuerstenberg, as a student, gathered plankton in a bottle at night and shook it to engage the glow.

“It was almost like having a fluorescent lightbulb,” he said with a chuckle. “It was enough light to read notes by.”

Since then, Fuerstenberg said, he has learned there are actually many animals capable of bioluminescence — not to be confused with phosphorescence or fluorescence, he noted, which occur when minerals glow in the dark after being exposed to light.

Though plankton are the most common ocean glowers in the deep sea, and even in the depths of Puget Sound, jellyfish, squid, worms and many other marine invertebrates bioluminesce.

“There’s a lot of light down there in what’s supposed to be a dark place,” Fuerstenberg said.

But while the depths of the ocean may be teeming with light, bioluminescence on Puget Sound’s shores have a special place of wonder and even amazement for those on the water or shoreline at night.

Fuerstenberg said some scientists and scuba divers venture hundreds of feet below the surface to see bioluminescence, while on Vashon, some need only venture outside their back door.

“It’s a big deal to them,” he said, “and here it is right off KVI Beach.”