What lives in the ocean’s twilight zone? New technologies might finally tell us
The dynamic midwater ocean teems with organisms such as this bristle worm.LARRY MADIN, © WOODS HOLE OCEANOGRAPHIC INSTITUTION
By Eli KintischAug. 23, 2018 , 10:45 AM
The sea's murky depths might host more life than we thought. That's the preliminary conclusion of scientists who this week completed the inaugural cruise of the Ocean Twilight Zone (OTZ) initiative, a 6-year, $35 million effort that is using innovative technologies—and an unusual funding model—to document the ocean's mysterious midwater layer.
The weeklong North Atlantic Ocean expedition was aimed primarily at testing the OTZ initiative's new workhorse: a 5-meter-long towed sled, dubbed Deep-See, that bristles with cameras, acoustic sensors, and samplers. But the trial also produced some eye-opening observations. At times when traditional surface instruments traced just a single, relatively dense layer of midwater organisms beneath the ship, for instance, Deep-See revealed a host of creatures distributed throughout the twilight zone, which extends from 200 meters to 1000 meters below the surface. "We kept seeing organisms all the way down," says Andone Lavery, a physicist with the Woods Hole Oceanographic Institution (WHOI) in Massachusetts, which is leading the project. "That was really surprising."
It was a promising start for OTZ scientists. They are targeting a little known ocean layer, between easily studied surface waters and the dark abyss, which submersibles have explored. "The midwater zone has been severely neglected," says Heidi Sosik, a biological oceanographer with WHOI.
Researchers know it teems with life, including fish, crustaceans, jellies, worms, and squids. And they have speculated, based on acoustic and net surveys, that the total biomass of midwater fish might dwarf the current global catch of surface-dwelling fish by 100 times. But they have struggled to document this twilight ecosystem. "The big questions are who are the players [and] who's eating who," says biological oceanographer Mark Benfield of Louisiana State University in Baton Rouge. There are "species just waiting to be discovered," says WHOI physical oceanographer Gordon Zhang.
The project also aims to get a better grip on how twilight zone creatures influence the global carbon cycle. Midwater organisms perform perhaps the biggest daily migration on the planet, rising each night toward the surface to feast on a sunlight-fed bounty of plankton and fish. Then, as the sun rises, they sink back to the depths. That plunge prevents carbon captured at the surface from going "right back into the atmosphere," where it would amplify global warming, says marine ecologist Tracey Sutton of Nova Southeastern University in Dania Beach, Florida.
Traditional tools have proved inadequate for exploring midwater ecosystems. Ship-mounted acoustical sensors—which use sound waves to locate objects—have trouble precisely detecting deep-swimming organisms. Towed nets can crush beyond recognition the gelatinous creatures found in the midwater. Because many are bioluminescent, the catch can resemble a ball of fire within the mesh, scaring away other animals. The pressure wave formed by an oncoming net can also warn off creatures.
Deep-See is engineered to overcome those challenges. "It's the equivalent of having the ship down at 600 meters," Benfield says. That allows the sled's acoustic sensors to deliver higher resolution data, because the sound waves don't have to pass through hundreds of meters of water. And because the sensors track seven different frequency bands, they might allow researchers to discern an animal's size and possibly even species. "It's like color TV versus black and white," Lavery says.
Deep-See's cameras, meanwhile, can image creatures as small as 50 microns in length, seven times per second. Other devices measure light and environmental variables. All told, 2 terabytes of data flow by cable from the rig to the ship each hour.
During Deep-See's recent trial, researchers compared the animals captured by the acoustic sensors and cameras to those caught in nets. The acoustic sensors imaged "big fish just a few meters away," says marine biologist Michael Jech of the National Oceanic and Atmospheric Administration in Woods Hole. But the large lights used by the cameras appeared to scare away creatures. The scientists are now brainstorming solutions, Jech says, which might include slowing the tow and trying different light colors.
WHOI scientists have already scheduled two more OTZ cruises and expect many more, across the globe, by the time the project ends in 2024. As a finale, they hope to establish permanent, tethered monitoring observatories in the midwater. "Who knows what we'll have in 20 years," Benfield says. "We may look at Deep-See as a primitive forerunner."
Funding for the project is coming from an unusual source: The Audacious Project, a new initiative by TED, the ideas-spreading nonprofit based in New York City. Audacious raises money from multiple private donors and vets proposals on their behalf, which cuts paperwork for grantees. Over the past 3 years, it has made seven awards, including the OTZ initiative, a methane-sensing satellite, and health care and hunger programs.
OTZ scientists are now combing through the more than 30 gigabytes of data they amassed. One goal: to see whether the data confirm that first impression of abundant midwater life. They are also looking ahead. "The big thing I want," Lavery says, "is to get Deep-See out in the ocean again soon."
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