What causes Swaths of Ocean to glow a magnificent milky green?

The sky was cloudy and dark, so there were no stars to guide us.

What causes Swaths of Ocean to glow a magnificent milky green?

The sky was cloudy and dark, so there were no stars to guide us. As we moved through the three-meter-deep Arabian Sea, alone at the helm, somewhere between Oman, India, and the dimly lit compass on our ship's gimbal mount, I couldn't see anything in the dark night except for the dimly lit compass. Half an hour into my shift, I noticed that the sails above me started to glow as if there was a rising moon. There was neither a moon nor stars, and there were no other ships or ships. It seemed that the light was coming from below, and it was intensifying. Soon, the entire ocean became glow-stick green. However, the light was muted as if it were shining through a sea filled with milk.

It was August 2010. I had been sailing for more than two months and was volunteering with the NGO, the Biosphere Foundation, to transport the Mir, a 35-meter ketch that they had recently purchased in Malta, to their home port of Singapore. I had become accustomed to the "sea sparkle", caused by dinoflagellates, which ignite when water is stirred, causing ribbons light to twist off Mir's bow. This was not the case. The entire ocean was glowing an opaque, uniform green, as far as my eyes could see. The compass was still in its mount and the light in water created an optical illusion that made the sea seem calm. It looked more like we were floating through phosphorescent skies than turbulent seas.

I woke up the rest of my crew and we spent over four hours engulfed by this green light. We were astonished, but had no idea what we were seeing. The line that separated the blackness from the lambent sea was finally visible. We crossed it and left behind the numinous phantom realm. However, we re-entered a familiar world, although we could still see the gauzy glow to our stern an hour later. We would find out the name of the strange phenomenon that had surrounded our ship 10 days later when we arrived at port. It was a milky ocean.

Over the centuries sailors have described milky seas as rare events where vast expanses of ocean glow uniformly at night. These can sometimes stretch for thousands of kilometers or more. Captain of the Clipper Shooting Star, W. E. Kingman, said this after witnessing one in 1854. "The scene had an awful grandeur; it was like the sea turned to phosphorus and the heavens were hung in blackness and the stars going out. It seemed that all nature was preparing to annihilate the material world."

Moby-Dick even featured a milky sea. Melville described a mariner who was sailing through "a shrouded ghost of the whitened water" that was as "horrible" to him as a ghost.

Our small crew and Melville and Kingman did not know what caused the seas glow. Our crew was able to live in a world that was far more scientifically mapped than the 1800s. This may have explained why Kingman's and Melvilles mariners responded with terror. We, on the other hand, marvelled at the fact that this phenomenon, no matter how strange, was indeed of this world.

Bioluminescence, which is the emission of light by living organisms, is a common phenomenon on Earth. It's especially prevalent in the oceans. Tunicates, crustaceans and dinoflagellates as well as bacteria, glow in the night sky from bioluminescent fish, tunicates and jellyfish. Milky seas are not common, even though they are so large. They are believed to be caused in part by one of the most tiny organisms in the ocean.

Each observation of a milky ocean throughout history was a chance encounter. Mine was the result of that. Only once did a vessel with scientific research capabilities come across one: the USS Wilkes, which steamed through a milky ocean for three consecutive nights off Socotra, Yemen in 1985. David Lapota (a marine biologist who was at that time working for the navy to study bioluminescence) was aboard the Wilkes. Lapota and his group of researchers sampled water to find a profusion Vibrio Harveyi, a well-known, abundant species that luminesces. They also found bits of algae. This led them to believe that legions of this bacteria and possibly other bioluminescent species are responsible for milky seas. This was almost 40 years ago and remains the only field study of a milky ocean.

If scientists believe that milky seas are the result of bacteria, then there is still the question: Why? Bioluminescent bacteria, unlike many other organisms that developed bioluminescence to escape predation is eager to be eaten. The inside of a fish's stomach provides a safer home than floating in open ocean. A single bacterium will not be able to attract the attention of fish, so they need to have a large number to produce microscopic bioluminescence on a macroscale. Each bacterium emits a chemical signal to detect other bacteria nearby. Once they have detected enough bacteria, scientists believe they will begin to glow. This process is called quorum sensing and may be the reason why milky seas are formed.

An abundance of nutrients-rich, decaying organic matter, such as bits of crabs and even long-dead whales, is pushed to the surface by upwelling. This allows bacteria to flourish. These rich waters can become isolated by currents or when different masses of water have different temperatures or salinities. This can lead to a concentrated stew, which can then be prevented from mixing. Scientists call this "the natural flask hypothesis." The bacteria can set off a chemical glow, which can produce the most spectacular display of bioluminescence anywhere on the planet.

The idea of a natural flask might help explain why the light from our boat's first sail into a milky ocean was almost imperceptible when it left, but that we could see when we returned to it hours later. On one side of that particular event, the glowing and non-glowing waters were mixing, while on the other side, due to some sort of oceanic front, a wall-like barrier was being maintained between the specialized--and little understood--conditions that allow milky seas to form and those conditions that do not. Scientists hope to understand more about milky seas by studying them in depth. This may be possible thanks to new satellite technology.

Steve Miller, director at the Cooperative Institute for Research in the Atmosphere has been part of the small group of scientists who have led the effort to demystify milky oceans for almost 20 years. They searched for them in the most unlikely of places, Fort Collins, Colorado. He was the first to find a milky ocean from his office chair.

Miller reached out to me shortly after my blog post about sailing through a milky ocean. He excitedly informed me that the crew of our ship are some of the few people who have seen one. I felt like a minor celebrity after our brief correspondence.

Miller became interested in milky waters in 2004 when he attended an American Meteorological Society conference. Miller and his colleagues looked into whether it was possible to observe marine bioluminescence in space. Any small-scale bioluminescence such as sea sparkle produces too weak a light signal for it to be visible from so far away. Miller was intrigued by the possibility of studying the ocean from space and did some research upon his return home. He discovered dozens of consistent accounts of the so-called milky waters given by mariners over the centuries. Miller, a trained atmospheric scientist, wondered if historical satellite data could be used to locate such events. It wasn't long before he found what he was searching for: a detailed account about a milky ocean that the crew of the SS Lima saw off the Somalia coast on January 25, 1995. It gave the precise coordinates and the time at which the boat entered the luminous phenomenon. Miller used the heading and speed information from the ship's log to calculate the Lima's position at the time that the crew claimed it had left the glowing waters. Miller plotted the dates, points and times and zoomed in to the grainy black and white photo. He tells me, "It was all in black."

Miller didn't give up and scaled the image further, looking through noise from a photograph taken over 800 km away. Miller noticed a small structure in the middle of his computer screen. He initially thought it was a fingerprint smudge. However, when he moved the image around, it moved with him. Zooming in further, a comma-shaped shape appeared in the waters near the Horn of Africa. He re-examined the coordinates of the ship and they aligned perfectly with the comma's borders. He says, "That was when we realized that we had something." The glowing bacteria covered more than 15,000 kilometers of the shape that was larger than Connecticut.

Miller says, "I've been hooked about that ever since because I realized I had just seen a ghost." Although milky seas were more common in fiction and folklore than science, Miller explains that this was the first spaceborne confirmation of a Milky Sea.

A new generation of satellite technology has significantly improved Miller's search for milky oceans since his initial discovery. The National Oceanic and Atmospheric Administration launched two satellites in 2011, and 2017. They are the Suomi National Polar Orbiting Partnership and Joint Polar Satellite System. Although they are not designed to search for milky oceans, these modern satellites have specialized day/night bands instruments that can detect bioluminescence from space at the extreme low end. Miller and his team have been analyzing the satellite imagery since then. They have had the greatest success in two areas where historic sightings of milky oceans have been reported: the northwest Indian Ocean where 70% of all reports have been made, and the waters around Java where 17% of sightings have been reported. Miller and his team have identified 12 milky seas using satellite imagery over the past decade. The most notable was the 2019 event off Java that covered more than 100,000 km, roughly the same area as Iceland. It glowed continuously for 45 nights.

Miller and his team now have the confidence to deploy a research vessel in order to study milky seas while they are still glowing. They will only then be able answer many of the questions scientists have about milky waters, including Miller's favorite: How deep does bioluminescence sink in the water column. Scientists have questioned whether it is a surface slick or a thicker layer of bacteria. Scientists believe that it takes up to 100 million bacteria per cubic cmimeter of water for the milky sea to glow. This could alter the number of bacteria in the ocean by billions or trillions.

Part of me was protective of Miller's groundbreaking research when I first heard about it. We humans must insist on explaining everything. As I discovered more about the causes of milky seas, including upwelling and natural flasks, quorum sensing, and the intentional communal light produced by trillions upon trillions of bacteria, I realized that not finding answers is the same as reducing the wonder of such an event. It actually makes it even more amazing.

We are all Captain Kingman without understanding the world around our eyes. We're terrified of the sight of something that we don’t recognize. We can instead be amazed at the world around us and know that every question we answer is just one of many.

This article comes from Hakai Magazine. It is an online publication that covers science and society in coastal ecosystems. You can find more stories like these at hakaimagazine.com.

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