Every night a mysterious layer rises from the murky depths of the ocean, as though the seafloor has suddenly become detached and is floating to the surface. This cloud-like specter is ubiquitous, and so dense that during World War II it confounded the U.S. Navy's sonar, leading to the belief that enemy submarines might be able hide within it.

This enigmatic “deep scattering layer”, so named for the way it deflects sonar pings, turned out to be slightly more innocuous than enemy camouflage. It's zooplankton. The deep scattering layer is a stampede of sea monkeys whose combined biomass renders their nightly trek to feed on phytoplankton near the surface the largest animal migration on the planet. During the war, the Navy funded covert research operatives aimed at discerning sea monkey from submarine. Today, research on the migration of zooplankton is as important as ever because researchers now recognize the critical importance of these organisms in the global carbon cycle. As to whether or not these salty sea critters could still hide an enemy in their ranks? That remains open to interpretation.

For nearly two centuries oceanographers knew that zooplankton rose higher in the water column at night. The discovery goes back to the expeditions of the HMS Challenger  in the late 1800s.

“We knew that something was going on at nighttime,” says Dr. Deborah Steinberg, Professor at the Virginia Institute of Marine Science. What no one appreciated was the vast expanse of this nightly sojourn. “The Navy would have been aware of it, but there were no acoustics in the 1800s,” says Dr. Steinberg. “It wasn’t until the 1940s they realized how widespread it was, when they could detect this layer of migrating organisms with sound.”

World War II era sonar first mapped the extent of the mysterious "deep scattering layer"—which simply turned out to hordes of zooplankton. Credit: Wikimedia Commons

Using sound, or sonar, scientists discovered this migration was a global phenomenon, one that occurs across the world’s oceans, in estuaries and in fresh water lakes.

Animal migrations call to mind the annual trips of birds, monarch butterflies, or old people: massive flocks seeking greener pastures or a warm respite from winter weather. When it comes to sheer biomass and relative distance traveled however, birds, butterflies and grandparents have nothing on the zooplankton that make up the deep scattering layer. In terms of biomass, their migration is the largest on the planet, and they make that trek each night.

“It’s like walking 25 miles to breakfast every morning,” says Dr. Steinberg. At their largest, zooplankton are but a dozen or so millimeters. Scaling their body size to that of a human and considering the viscosity of sea water, their trek is like a nightly marathon through pudding. Dr. Steinberg adds, “It’s a long way to go for a little critter.”

Dr. Steinberg’s research focuses on the ecology of zooplankton. She explains that during the day, zooplankton avoid predation and save energy by slowing down metabolism in the dark, chilly waters of the deep ocean. Once the sun sets, “they come up under the cover of darkness so they can eat in peace,” she says.

Dr. Steinberg’s experiments often take her to the high seas, where she and her team catch the critters in the deep scattering layer to study. They use nets reminiscent of those used to catch butterflies, but enormous and designed to be towed through the water behind the ship.

“I can always tell who the migrators are,” Dr. Steinberg says describing the organisms concentrated in the plankton net. “They just look like they’ve come from the deep sea.” Because the red end of the light spectrum is rapidly lost with increasing depth through the water column, blood red is a common color for the creatures that spend their time in the abyss.

Despite the sinister coloration of zooplankton, their tiny stature and loopy swimming patterns make them seem anything but menacing. Should Dr. Steinberg be worried that an enemy lurks in their midst, trailing stealthily behind her research vessel? Could sea monkeys actually be used to camouflage a submarine?

“There’s always been a fear that that could be done,” concedes Dr. Kelly Benoit-Bird, Professor at Oregon State University. She is an expert in using acoustics to study the ecology of marine organisms and co-author of a perspective piece about this topic that will appear in the upcoming issue of the Annual Review of Marine Science.

In the review, Dr. Benoit-Bird and co-author Dr. Gareth Lawson of the Woods Hole Oceanographic Institution describe how acoustic techniques have been used to study marine life since researchers first used echo sounders to detect fish in the late 1920s. Sonar and other acoustic tools experienced a surge of developments during World War II, when the vastness of the deep scattering layer was discovered and the ability to discern friendly sea creature from foe became a critical challenge.

Despite the new sonar machines developed for war ships, for a time it remained impossible to resolve the mysteries of what the deep scattering layer contained. Historical texts detail how Admiral John S. Thatch, commander of the Atlantic Task Force following World War II, reputedly sent many frantic SOS signals complaining that a 200-foot thick cloud-like layer was confounding the sonar resolution of his ship.

These days acoustic techniques have come a long way. “Certainly there have been advances in technology and computing power, and we can record at higher resolutions, but the physics is the same,” says Dr. Benoit-Bird.

Modern acoustic techniques still can't discern a single sea monkey from amidst an army of them, but what they can now detect is still pretty nifty. In their review article, Drs. Benoit-Bird and Lawson show how with current technology acoustic scatter plots that can discern whales, schools of fish, individual squid, and even sea birds diving to catch fish—all using only sound beams and modern computing power.

The dominant feature in the plots rendered by acoustic queries into the benthos is still that dense, impenetrable cloud of the deep scattering layer.

Today, the Navy is still interested in teasing apart what they call the “bioclutter” in the ocean. “A lot of our work is funded by the Office of Naval research,” explains Dr. Benoit-Bird. “They don’t tell you what to do but they’re aware of what you're doing.”

While defense tactics are undisputedly important, it’s the immense biogeochemical impact of the organisms in the deep scattering layer that motivates the research of oceanographers using these acoustic techniques today.

The daily migration of zooplankton helps remove carbon from the atmosphere and surface waters and then sequesters it deep in the ocean where it can remain for centuries, preventing it from warming our planet as a greenhouse gas in the atmosphere.

In today's world, that looming threat of climate change is perhaps a more worthy adversary than skulking submarines. This is what provides the impetus for today’s zooplankton migration research. According to Dr. Benoit-Bird, the research goal of the acoustics field is figuring out one thing, “Where is the carbon going, not where is the enemy going.”

Kyle Frischkorn is a graduate gtudent in the Dyhrman Lab at Columbia University's Lamont-Doherty Earth Observatory