Things that Glow in the Water at Night



image credit: University of Maryland

I often get reports from coastal residents that the waters in their canals seem to be "glowing" at night during certain times of the year. Fortunately, my colleague Betty Staugler, the Florida Sea Grant Extension Agent in Charlotte County has produced a fact sheet about just such a phenomenon.

Things that Glow in the Water at Night

Have you ever been out on the water at night and observed the water glowing? The glow may be shades of blues or greens and seem to occur when the water is disturbed by anything from a moving fish to a paddle swept through the water. Many people have observed this phenomena and they often wonder:

Q: What’s causing this glow? 
A:  The glow is caused by bioluminescent producing plant and animal organisms.Many forms of life produce bioluminescense including small single celled bacteria, dinoflagellates, diatoms, copepods and comb jellies just to name a few. Bioluminescense is the term used to describe light generated by living organisms. The glowing light that residents are seeing is most likely the result of bioluminescent dinoflagellates, although zooplankton could also be the cause. In Florida one of the most common biouminescent species in Pyrodinium bahamense, which is common throughout the Caribbean.


Q: What causes these organisms to glow, and how does it benefit them?
 A: Bioluminscence was once thought to be produced by the friction of salts or by the element phosphorus in the water. Today we know that certain animals possess light producing organs called photophores and glands that emit light through a chemical reaction which involves a light producing protein called luciferin. Luciferins store energy. This energy is released in the form of photons, or light by enzymes called luciferases. The reasons for these bioluminscent displays are varied. Some organisms bioluminate to attract a mate as is the case in fireflies. Others bioluminate to attract prey. An example of this would be the anglerfish which dangles its glowing lure to attract potential prey. In the case of dinoflagellates, bioluminescence is used to evade predators and acts as a defense mechanism. It is believed that dinoflagelletes produce light when disturbed and will give a light flash lasting a fraction of a second. The flash is meant to attract a predator to the creature disturbing or trying to consume the dinoflagellate. The light flash also surprises the predator causing it to worry about other predators attacking it, making the predator less likely to prey on the dinoflagellate.


Q: Are there other phenomena that can cause similar effects? For example, what about high levels of phosphorus in the water
A: Phosphorus in the water by itself does not produce a glowing effect; however, high concentrations of nutrients and in particular phosphorus would increase the population of dinoflagellates. Although most glows in the water are the result of bioluminescence, some organisms have the ability to fluoresce. Fluorescence is similar to bioluminescence but the trigger is changed. Instead of luciferin and luciferase, fluorescence is triggered when a pigment absorbs light from an outside source. Fluorescence is able to produce the widest spectrum of colors because the emitted color is determined by the fluorescent pigment which absorbs the incoming light. In the case of fluorescence, the emitted light is only visible while the trigger is present. Phosphorescence is similar to fluorescence except that the excited product is more stable, so the glow will last after the trigger has been removed. Glow in the dark stickers phosphoresce.

Q: Are there any harmful effects to other marine life, or that anglers and boaters should be concerned about? Is it OK to eat fish caught from areas where the water is glowing?
A: Luminescence does not pose a health issue, but some bioluminescent species can produce toxins, including Pyrodinium bahamense. These toxins can be bioaccumulated in the food web. The specific threat to health varies between ecosystems. It’s important to note that dinoflagellates comprise a very large group of approximately 2000 different species and of these only a very small percentage are toxic. Dinoflagellates are important primary producers. They fuel food webs, providing food for zooplankton, which feed small fish and so on. It is said that photoplankton produce most of the earth’s oxygen.


Q: Why does the glow seem to be most prevalent in summer, and why is the glow sometimes very bright and intense, other times very muted?
A: You might see increases in summer months because this could correspond to when these bioluminescent organisms are reproducing (natural life cycles), or because summertime is when we have increased freshwater runoff from rains resulting in more nutrients being flushed into the systems which in turn can lead to more blooms of these organisms. As far as intensity of the glowing, the intensity of the bioluminescence depends on the intensity of the bloom and the health of the algae in the bloom.