Surfers, kayakers and beach-combers in San Diego, California, were greeted by a bizarre and beautiful sight: the ocean waters emitting a bright blue glow as the waves crashed and the tide came in.

The cause of this unusual phenomenon was a tiny organism called Lingulodinium polyedrum. This dinoflagellate (a type of algae) blooms every few years in the waters around San Diego, forming what is known as a red tide.

While the algae gives the water a soupy red color during the day, nighttime is when the show begins. Every time the algae is jostled — either by the movement of the tides or the slice of a kayak moving through the water — it emits a bright blue bioluminescent glow.

The breathtaking effect appeared earlier this week, and scientists aren't sure how long it will last.

Bioluminescence is a fairly common phenomenon among certain dinoflagellate species, says Cynthia Heil, senior research scientist at Bigelow Laboratory for Ocean Sciences in East Boothbay, Maine. "It's the same reaction that occurs in fireflies, which is triggered by turbulent motion."

The last time the blue glow faded in San Diego was in 2013. "If you asked me 10 years ago, I would have said no," Melissa Carter, a programmer analyst at the Scripps Institute of Oceanography in San Diego, said in 2012. "We used to see the blooms ever three to seven years. However, over the last eight years, we've seen a bloom four out of the eight years, which has been the highest frequency of these blooms over the past 30 years."

Although the frequency has increased, it's still hard to predict when it will appear again since scientists still don't understand the variables that cause the algae to bloom. "A very intricate stage has to be set for this plankton to bloom," Carter said. The exact conditions are not known, but variables could include water temperature, wind speeds, the presence of other bacteria or viruses in the water, among other conditions.

Carter and her fellow scientists study the bloom whenever it occurs and learn what they can when they can. "Every time there is a bloom, we are collecting our standard measurements and this helps in testing basic hypothesis about what we think is happening and adding to our understanding and predictability of future blooms," she said.

In 2017, a student-led team at Scripps developed a model that takes ecological data and can "identify patterns in the apparent randomness that can be used to predict red tides off Southern California," the institution said in a statement. "This research shows that the challenge is being overcome using innovative techniques that offer us information such as how to predict red tides. That’s important for knowing when to close fisheries and swimming areas, and for the health of residents who live along affected waters," said National Science Foundation’s (NSF) Division of Environmental Biology Deputy Director Alan Tessier.

Bloom or bust

San Diego algae bloom red tide While the bioluminescence is stunning to see, people should be cautious about getting in the water due to the bacteria feeding on the algae. (Photo: Kevin Baird/Flickr)

In addition to predicting future red tides, scientists are also asking why certain algae species go through boom periods where they form red tides, while others do not. Carter notes that only about five of the 50 or so species they see on a regular basis in San Diego form these massive blooms. "Why does only a small subset of species have the ability to out-compete all the others and dominate the community for weeks to a month at a time?" she asks.

There are other places you can experience similar phenomena. Heil says she has encountered bioluminescent algae in Moreton Bay, Australia, where it is caused by a dinoflagellate called Noctiluca scintillans . In Maine, a species called Alexandium fundyense causes glowing red tides, although it doesn't occur in the same concentrations as the ones in San Diego and Australia. "It just appears almost like stars twinkling in the sea rather than the water itself glowing," she says. One of the most famous places you can see glowing blooms is Bioluminescent Bay in Puerto Rico, where the glow is reportedly bright enough to read by.

If you do happen to encounter glowing waters, take a bit of caution. Although most are harmless, some of them can be slightly toxic if ingested. The algae in Moreton Bay, for instance, contain high levels of ammonia. The red tides in San Diego have been linked to increased levels of ear and sinus infections, although that could be more from the bacteria in the water that are feeding on the algae than from the algae itself.

But no matter where you encounter bioluminescent waters, take the time to stop and enjoy them. "They can be spectacular," Heil says.

Editor's note: This article has been updated since it was originally published in November 2012.

Why San Diego's tides are glowing blue
Bioluminescent algae blooms every few years on the California coast.