These events are called milky seas. They are a rare nocturnal phenomenon in which the ocean’s surface emits a steady bright glow. They can cover thousands of square miles and, thanks to the colorful accounts of 19th-century mariners like Capt. Kingman, milky seas are a well-known part of maritime folklore. But because of their remote and elusive nature, they are extremely difficult to study and so remain more a part of that folklore than of science.
I’m a professor of atmospheric science specializing in satellites used to study Earth. Via a stat-of-the-art generation of satellites, my colleagues and I have developed a new way to detect milky seas. Using this technique, we aim to learn about these luminous waters remotely and guide research vessels to them so that we can begin to reconcile the surreal tales with scientific understanding.
The bioluminescence in milky seas is caused by a type of bacteria. (Steve. H. D. Haddock/MBARI, CC BY-ND)
Luminous bacteria cause the particles they colonize to glow. Researchers think the purpose of this glow could be to attract fish that eat them. These bacteria thrive in the guts of fishes, so when their populations get too big for their main food supply, a fish’s stomach makes a great second option. In fact, if you go into a refrigerated fish locker and turn off the light, you may notice that some fish emit a greenish-blue glow – this is bacterial light.
Unlike bioluminescence that happens close to shore, where small organisms called dinoflagellates flash brilliantly when disturbed, luminous bacteria work in an entirely different way. Once their population gets large enough – about 100 million individual cells per milliliter of water – a sort of internal biological switch is flipped and they all start glowing steadily.
Now imagine if a gargantuan number of bacteria, spread across a huge area of open ocean, all started glowing simultaneously. That makes a milky sea.
To date, only one research vessel has ever encountered a milky sea.
That crew collected samples and found a strain of luminous bacteria called
Vibrio harveyi colonizing algae at the water’s surface.
While biologists know a lot about these bacteria, what causes these massive displays remains a mystery. If bacteria growing on algae were the main cause of milky seas, they’d be happening all over the place, all the time. Yet, per surface reports, only about two or three milky seas occur per year worldwide, mostly in the waters of the northwest Indian Ocean and off the coast of Indonesia.
We had to wait for a better instrument – the Day/Night Band – planned for the National Oceanic and Atmospheric Administration’s new constellation of satellites. The new sensor went live in late 2011, but our hopes were initially dashed when we realized the Day/Night Band’s high sensitivity also detected light emitted by air molecules. It took years of studying Day/Night Band imagery to be able to interpret what we were seeing.
If scientists want to learn more about milky seas, they need to get to one while it’s happening. Trouble is, milky seas are so elusive that it has been almost impossible to sample them. This is where my research comes into play.
Satellites offer a practical way to monitor the vast oceans, but it takes a special instrument able to detect light around 100 million times fainter than daylight. My colleagues and I first explored the potential of satellites in 2004 when we used U.S. defense satellite imagery to confirm a milky sea that a British merchant vessel, the SS Lima, reported in 1995. But the images from these satellites were very noisy, and there was no way we could use them as a search tool.
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