For centuries, sailors have recounted tales of navigating through vast expanses of the ocean that glow with a continuous, milky-white light. These rare events, known as milky seas, transform the water’s surface into a uniform, luminous field stretching as far as the eye can see. Under the darkness of a moonless night, the sea can appear to be a field of snow or a layer of clouds, creating a disorienting and otherworldly experience for those who witness it.
The Science Behind the Glow
The steady luminescence of a milky sea is not a reflection of the moon or stars, but is generated by immense colonies of bioluminescent bacteria. A specific bacterium often identified in these events is Vibrio harveyi.
This bacterial light is controlled by a process called quorum sensing. Individual bacteria do not glow on their own but communicate using chemical signals. Only when the population multiplies to an enormous density, sometimes reaching 100 million cells per milliliter, do they collectively activate their light-producing genes to create the sustained glow.
The conditions for such a massive bacterial bloom are often linked to large patches of microalgae. These algae provide the necessary nutrients for the bacteria to thrive. This symbiotic relationship allows a milky sea to form and persist for days or even weeks.
Distinguishing Milky Seas from Other Bioluminescence
The ocean hosts various forms of light production, but milky seas are distinct from more common bioluminescence. Many people are familiar with the brief, sparkling flashes of light in breaking waves or a boat’s wake. This fleeting light is produced by larger organisms called dinoflagellates, a type of plankton.
These dinoflagellates flash only when they are physically disturbed, using the light as a defense mechanism to startle predators. Their glow is momentary and localized to the point of disturbance, creating a trail of sparks in the water.
In contrast, the light from a milky sea is a steady, continuous emission from bacteria that does not require physical stimulation. This creates a calm, uniform luminescence over a vast area, appearing more like a glowing fog than distinct sparkles.
Observing the Phenomenon
Historically, knowledge of milky seas came from firsthand accounts in ship logs, which were often dismissed as maritime legends. These descriptions appeared in literature as well, notably in Jules Verne’s “Twenty Thousand Leagues Under the Seas.” For centuries, these anecdotal reports were the only evidence of their existence.
The study of milky seas was revolutionized by environmental satellites with low-light sensors. In 1995, scientists matched satellite imagery with a ship’s log, providing the first objective confirmation of a milky sea from space. Instruments like the Day/Night Band (DNB) on NOAA’s satellites now allow researchers to detect and track these events.
Modern observation tools have confirmed that milky seas occur most frequently in specific regions. The northwestern Indian Ocean, particularly off the coast of Somalia, and the waters surrounding Indonesia are known hotspots. One event observed south of Java in 2019 spanned over 100,000 square kilometers and lasted for more than a month.