The bluish glow of a
Image credit: By catalano82 (Red Tide Luminescense) [CC BY 2.0], via Wikimedia Commons

In the mid-1990s, when I was living in San Diego for the first time, I worked at an office near the ocean. So barely a day went by when I didn’t walk along the beach or at least drive by it. One day, something looked a bit different—the water, normally greenish, had taken on a bit of a red hue. I heard a few people talking in passing about something called a “red tide,” as though it were a common occurrence that anyone who had lived there for more than a few months should be familiar with. It didn’t sound like anything I should be either worried about or especially interested in—just some random marine phenomenon, the details of which I was content not to know.

Light Waves

But driving home from work late that night I glanced at the waves breaking on the beach again and did a double take. I could have sworn I saw light coming from the water. Maybe it was just a reflection of the moon, I thought, or someone playing with a flashlight. I kept looking, and every few seconds I saw another glow of bluish light that lasted for just a second or two and then faded away. I pulled over and walked down to the water to take a closer look. Sure enough, every time the waves broke, the leading edge of the wave emitted a faint blue (or perhaps green) glow. I stood there and stared at the water for the longest time—I thought it was one of the strangest and most beautiful things I’d ever seen.

Later I asked around about the glowing water and someone said, “Oh yeah, the red tide. It’s caused by a phosphorescent plankton.” Apparently this happens every several years or so in or near San Diego. Glowing organisms were familiar enough; where I grew up—far from the ocean—fireflies were common. But though I never thought twice about seeing a sparkling field or lawn at night, glowing water seemed incredibly foreign and exotic. Years later, though, living in British Columbia, I sometimes saw warnings against collecting bivalve mollusks (such as oysters, clams, and mussels) due to something that was also called a “red tide”—but never a mention of glowing waters. Was the phenomenon that had entranced me years earlier in southern California actually a toxic danger?

The Tide Turns

The answer turns out to be somewhat complicated. First the easy part: the red color of the water in a red tide is indeed caused by a type of plankton, specifically one of quite a few species of dinoflagellates, single-celled organisms that have characteristics of both plants and animals. Dinoflagellates, like other phytoplankton, create their own food through photosynthesis, a plantlike behavior. However, they don’t have roots, stems, leaves, or seeds, and they do have the ability to move (thanks to a pair of hairlike appendages called flagella)—thus making them more like animals. Although the dinoflagellates are often found in ocean water in very small quantities, sometimes the conditions of heat, sunlight, salinity, water currents, and so on are just right, and the organisms multiply extremely rapidly, creating a reddish or brownish algae bloom.

All dinoflagellates are not created equal. The variety that caused the glowing waters I saw in California in the 1990s is Noctiluca scintillans, which by itself is relatively harmless, but it often appears together with the less colorful but more toxic Gonyaulax catenella. The latter, along with other non-bioluminescent species of dinoflagellates such as Gonyaulax tamarenis (which appears in the Atlantic Northeast) and Karenia brevis (which appears in the Gulf of Mexico), are responsible for the dire warnings; they produce toxins that can cause illness or death if consumed. Red tides not only make shellfish unsafe for humans, they kill fish, manatees, dolphins, and other marine life. A red tide in 2018 was caused by a different species—Lingulodinium polyedra—which is also toxic, though apparently less so than some of the others.

Because of the negative impact of red tides (at least in some areas), research is underway to determine the exact conditions that cause them and what can be done to prevent or destroy the blooms—or at least reduce their toxic effects. I’m all for protecting marine life, but I’d be sorry to see the disappearance of Noctiluca scintillans and Lingulodinium polyedra. I’d gladly give up shellfish for a few weeks to experience another evening of glowing waves.

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on July 6, 2003, and again in a slightly revised form on July 18, 2004.