Red tide at Moonlight Beach in Encinitas, Calif. North County Times/Zuma Press/Newscom

It’s best seen at night.  In the Pacific Ocean off San Diego, masses of algae have bloomed to form what’s known as a red tide.

By and large, it is not considered a good thing. The algae are sometimes toxic. They can poison fish or starve them of oxygen.

But then why are people gathering to look, and why are some biologists impressed by it? Because it’s blue.

Peter Franks, a biologist at the Scripps Institution of Oceanography, said the particular organism causing it, Lingulodinium polyedrum, was his “favorite dinoflagellate.”

Why so? “Because it’s intensely bioluminescent,” he wrote on the blog Deep-Sea News. “When jostled, each organism will give off a flash of blue light created by a chemical reaction within the cell. When billions and billions of cells are jostled — say, by a breaking wave — you get a seriously spectacular flash of light.”

Luckily for those who like dinoflagellates, we’re close to a new moon. The blue glow is usually pretty faint, best seen when it’s very, very dark (though the chemical reaction happens at all hours). During the day, the algal bloom usually makes the water murky, with pigments that can turn the water reddish-brown.

It also can create quite a stench, though biologists say the waters are safe for swimming. They go so far as to suggest that you scoop up some water in a glass jar, shake it and watch it glow (we’ll have to trust them if we don’t live near the Southern California coast).

“Oooo,” wrote a person on Twitter. “Electric-blue glowing phytoplankton! Put it in a bottle and shake to waken the dinoflagellates. SHINY!”

Photographer Kevin Baird posted an eerie image on Flickr with the caption, “This is SpongeBob’s aurora borealis.”