From The Woods Hole Oceanographic Institution: “Five big discoveries from WHOI’s Ocean Twilight Zone Project”

From The Woods Hole Oceanographic Institution

Creatures of the ocean twilight zone © Woods Hole Oceanographic Institution

5.16.24
Véronique LaCapra
Alison Pearce Stevens
Kathryn Baltes

Six years since it began, WHOI’s Ocean Twilight Zone project brings new and exciting insights to bear.

Deep below the ocean surface, where sunlight almost disappears, the twilight zone abounds with life. From the tiniest microbes to the largest fish and everything in between, most of this shadowy world has long remained a mystery—but WHOI’s Ocean Twilight Zone (OTZ) Project is changing that.

Reaching from approximately 200 to 1,000 meters (656 to 3280 feet) below the surface and stretching across the global ocean, the twilight or mesopelagic zone lies out of reach of many traditional methods of data collection. This limits how easily—and how well—scientists can study its inhabitants. We have known about the abundant life in the ocean’s midwater since it was first detected by Navy sonar in World War II, and that many of its residents travel to and from surface waters to feed each night. Theirs is the largest migration on the planet, a continuous wave of motion that tracks the rising and setting of the sun. Yet we know very little about the creatures that undertake this remarkable journey.

WHOI researchers are wrapping up a six-year, multi-million-dollar project that began with an audacious goal: to revolutionize our understanding of the mesopelagic and the animals that live there. The OTZ Project has enabled ocean scientists and engineers to collaborate in new ways; to develop new technologies and approaches; to deploy them at sea in new combinations; and to collect huge amounts of new information—visual, acoustic, genetic, and more—to begin illuminating the mysteries that lurk at the heart of one of the world’s largest ecosystems.

Here are just five of the many discoveries WHOI’s OTZ Project team has made so far.

1. In the darkness, life follows the light

Life in the twilight zone is exquisitely attuned to light. Its inhabitants can detect subtle differences in light levels that the human eye would miss. Although scientists have long known that a wide variety of organisms live in the twilight zone, they assumed the animals were randomly distributed in relatively few layers throughout the water column.

The OTZ Project has revealed a much more nuanced picture. In fact, different organisms occupy many distinct layers, adjusting their position in the water column to maintain their desired level of light. Copepods occupy one layer, shrimp another, lanternfish another, and so on. As light levels change—at dawn, dusk, or when a cloud passes overhead—this highly structured ecosystem moves vertically to compensate.

Diel Migration Wave
This animation shows the daily movement of species from the ocean twilight zone to the surface waters to feed and back—the largest animal migration on earth. (Video by WHOI Creative Services, © Woods Hole Oceanographic Institution)

2. The twilight zone serves up a feast for big fish

Commercial fisheries catch tuna and swordfish near the surface, but these large ocean predators may depend on the twilight zone for as much as two-thirds of their diet. To arrive at this important new finding, OTZ Project researchers put satellite tags on large, commercially important fish to track their movements and behavior. In a separate study, researchers dissected and genetically analyzed the contents of their stomachs to find out what they were eating. The team discovered that these large predators spend far more time in the twilight zone than anyone had previously realized—and eat far more midwater fish. They may also dive into the twilight zone to escape their own predators, or to use the colder waters to adjust their body temperature or remove parasites.

These new data suggest that the expansion of commercial fishing into the twilight zone could deplete a critical food source for tuna, swordfish, and other commercially important fish—among other negative impacts.

A scientist holds out an array of lanternfish collected during the NASA-funded EXPORTS (EXport Processes in the Ocean from Remote Sensing) mission in 2021. (Photo by Marley Parker, © Woods Hole Oceanographic Institution)

3. The twilight zone has fish by the gazillions — but they may not make a good fishery

A 2014 study using shipboard acoustics in the Pacific Ocean suggested the global amount of fish biomass in the twilight zone was at least 10 billion tons—10 or more times previous estimates. To get a more detailed picture of life in the twilight zone, OTZ Project researchers used acoustic imaging at depth to generate fine-scale images of life in the midwaters. By combining these acoustic data with information from animals caught in nets, visualized in dozens-to-hundreds of images per second, and derived from the DNA they leave behind in the water, the researchers have begun to identify what kinds of organisms the acoustic signals are detecting—and better quantify their abundance. Their results from the northwest Atlantic Ocean indicate that twilight zone organisms are almost certainly less numerous than the 2014 study suggested.

In ongoing research, biologists on the OTZ Project team are examining the life-history traits of some of the most common twilight zone fish. By studying the fishes’ size, age, and number of eggs released, they will be able to help determine whether the species reproduce quickly enough to be sustainably harvested by commercial fishing. Although evidence is still under analysis, some of these fish live seven years or more, are slow-growing, and likely slow to reproduce. These characteristics—combined with the new information on their relative abundance—suggest they would make poor candidates for a future fishing industry.

Acoustical oceanographer Andone Lavery (right) analyzes acoustic data to track how life moves in the twilight zone. (Photo by Daniel Hentz, © Woods Hole Oceanographic Institution)

4. For top predators, ocean eddies serve as superhighways to the twilight zone

Eddies—circular currents the size of a city—regularly develop in ocean waters around the globe. The oceanic equivalent of an atmospheric storm, eddies of warm water provide pathways for large ocean predators to reach the twilight zone. Sharks, tuna, and other fish use their environment to regulate body temperature, which limits their ability to tolerate cold water beyond. The relatively warmer temperatures at the center of warm-core eddies provide these “cold-blooded” predators with a cozy throughway to the twilight zone, allowing them to feed on abundant midwater prey at depths that would normally be too cold for them to access.

First of its kind mooring data on midwater biomass collected over the course of a year by the OTZ Project has revealed that these temporary ocean currents—lasting weeks to months—may be hotspots of life in the twilight zone. Data show as much as five times more animal biomass inside warm-core eddies than outside. Eddies, we now know, are essential to understanding the dynamics of ocean systems.

Apex predators like this mako shark use eddies as express elevators to the twilight zone to feed. (Photo by Tom Burns, © Woods Hole Oceanographic Institution)

5. The depth to which carbon sinks depends on its source

The fate of carbon in the ocean depends heavily on biology—both in surface waters and in the twilight zone. At the ocean’s sunlit surface, carbon dioxide from the atmosphere diffuses into the ocean. Tiny, plant-like organisms known as phytoplankton take up this carbon to grow. Phytoplankton are consumed by tiny animals called zooplankton, which in turn are eaten by fish and other larger animals.

Much of the carbon in this food web stays near the surface and ultimately returns to the atmosphere. How much of it moves down into the twilight zone and beyond is highly variable and depends on which organisms are present near the surface.

Assume, for example, that the phytoplankton community is dominated by diatoms. These tiny creatures build glass “houses” from silica dissolved in the water around them—but they can quickly outgrow supplies. Unable to reproduce, they die off and rapidly sink, dragged to the depths by their heavy glass shells.

Similarly, gelatinous animals known as salps can boost the amount of carbon that gets through the twilight zone to the deep ocean. These small, transparent creatures act like “vacuums” in the water, scooping up smaller particles and creating large dense fecal pellets. When released, these pellets of poop can reach the ocean floor in a matter of days, effectively removing carbon from the atmosphere for hundreds or even thousands of years.

A sediment trap is deployed from the stern of R/V Neil Armstrong to measure the flow of carbon in the twilight zone. (Photo by Daniel Hentz, © Woods Hole Oceanographic Institution)

Science with a global impact

OTZ Project researchers are now in the final stages of analyzing the trove of new information it has collected, with more exciting discoveries sure to come. The team’s work has already made an impact on policymakers and the public, helping bring about a sea-change in understanding and awareness of the twilight zone and its crucial importance to ocean ecosystems and Earth’s climate.

One noteworthy example is the United Nations agreement on Marine Biodiversity of Areas Beyond National Jurisdiction (BBNJ), also known as the High Seas Treaty. Two decades in the making, this groundbreaking agreement will help to protect sensitive mid-ocean environments that are beyond the purview of any national authority—such as most of the twilight zone. By cultivating relationships with international policymakers and working to provide broad access to the best available science, the OTZ Project has helped ensure the twilight zone’s inclusion in policy discussions at the highest levels.

WHOI’s work in the twilight zone will doubtless continue to capture the public imagination and inform solutions to sustain this critical region of the ocean and the amazing life it supports.

WHOI Marine Chemist and co-principal investigator on the Ocean Twilight Zone Project, Ken Buesseler speaks at the Plankton and Climate Change panel discussion at COP28 in Dubai. (Photo by Katherine Spencer-Joyce, © Woods Hole Oceanographic Institution)

What is the ocean twilight zone?

The ocean twilight zone is a layer of water that stretches around the globe. It lies 200 to 1,000 meters (about 650 to 3,300 feet) below the ocean surface, just beyond the reach of sunlight. Also known as the midwater or mesopelagic, the twilight zone is cold and its light is dim, but with flashes of bioluminescence—light produced by living organisms. The region teems with life. Recent studies suggest that the biomass of fish in the twilight zone may be ten times greater than previously thought—more than in all the rest of the ocean combined.

Animals in the twilight zone range in size from microscopic to among the largest on the planet. Some organisms spend their lives in its shadowy depths, while others travel to and from the surface every day in the largest animal migration on Earth. Animals in the twilight zone help support the ocean’s food web and transport huge amounts of carbon from surface waters into the deep ocean, helping to regulate global climate.

So far, the twilight zone is largely unexplored and its rich biodiversity has remained mostly beyond the reach of commercial fishing—and the international laws that govern the high seas. But some fishing interests are poised to begin extracting the biological resources of the twilight zone, with unknown consequences for marine ecosystems and Earth’s climate.

What kinds of organisms live in the twilight zone?

Life in the twilight zone includes microscopic bacteria and tiny animals known as zooplankton, along with larger crustaceans, fish, squid, and many kinds of gelatinous animals. With their strange shapes and behaviors, many of the twilight zone’s inhabitants seem like the stuff of fantasy or science fiction, but they are all uniquely adapted to life in a deep, dark, watery world where temperatures stay close to freezing and water pressure can reach 1,500 pounds per square inch.

Most mesopelagic fish are only a few inches long. But their size does not keep twilight-zone animals from being a powerful force in the ocean. The bristlemouth—a small twilight-zone fish with a large jaw full of spiny teeth—is the most abundant vertebrate on Earth, possibly numbering in the quadrillions. And there are countless species still to be discovered.

To survive in such a low-light environment, many twilight-zone species—from microbes to jellies—produce their own light through a biochemical process known as bioluminescence to avoid being eaten and to attract prey.

For example, some fish use an adaptation called counterillumination to keep from being seen by potential predators. Since light in the twilight zone comes from above, many predators look up to search for prey silhouetted against the surface. Consequently, some small prey fish have rows of organs called photophores along their bellies that emit light similar in intensity and color to the light of the surface water above, making them nearly invisible when viewed from below.

Conversely, predatory fish can also use bioluminescence—in their case, to attract prey to catch and eat. Some, for example, have a bioluminescent organ called an esca that dangles from a whisker-like barbel on their chins or backs like a kind of fishing lure to tempt potential prey.

Why do some twilight-zone animals migrate?

During World War II, U.S. Navy sonar operators looking for enemy submarines were puzzled when sonar images seemed to show the seafloor changing its depth, from about a quarter mile down during the day, to near the surface after dark. What appeared to be a shifting seafloor turned out to be plankton, fish, and other twilight-zone animals making a nightly migration to and from the surface to find food.

Not all organisms in the twilight zone migrate, but many do. As darkness falls, a multitude of fish, squid, plankton, and other mid-ocean dwellers swim hundreds or even thousands of feet up to surface waters to feed under cover of darkness, then return to the relative safety of deeper, darker waters at daybreak to avoid becoming food themselves. Theirs is the largest animal migration on the planet, and it happens every 24 hours, sweeping across the world’s oceans in a massive living wave.

Value Beyond View: The Ocean Twilight Zone

Why is the ocean twilight zone important?

The ocean twilight zone provides important ecosystem services, including supporting ocean food webs and commercial fisheries, and transferring carbon dioxide to the deep ocean.

How does the twilight zone support ocean food webs?

The abundance of life in the twilight zone supports a complex food web with connections to both the deep ocean and the surface. Dead animals and marine “snow”— clumps of dead plankton, bacteria, fecal pellets, and other particles rich in organic carbon—sink from surface waters through the twilight zone to the deep ocean, providing food for twilight-zone animals. Some twilight-zone inhabitants migrate to the surface to feed every night, then return to deeper waters during the day. Conversely, satellite tagging has revealed that whales, tuna, swordfish, sharks, and other top predators dive deep down into the twilight zone to feed. Since humans value those predators for their ecological, commercial, and nutritional benefits, we also depend on the twilight zone.

How does the twilight zone help keep carbon dioxide out of the atmosphere?

The ocean absorbs about a quarter of the carbon dioxide that human activities emit into the atmosphere. The twilight zone plays an important role in transferring carbon from surface water to the deep ocean, preventing it from returning into the air as a heat-trapping greenhouse gas. The multistep process is often called the ocean’s “biological pump.” In surface waters where there is plenty of light, tiny plantlike organisms called phytoplankton use energy from the sun to transform carbon dioxide into the energy and matter that allows them to grow. Phytoplankton, in turn, become food for small animals known as zooplankton, which are then eaten by fish and other animals.

Some of the carbon in surface waters becomes part of a kind of underwater blizzard known as marine snow. That “snow,” however, consists of clumps of dead plankton, bacteria, fecal pellets, and other particles rich in organic carbon, which provide food for twilight-zone animals.

Another fast track for carbon into deeper water is through the daily migration of twilight zone animals that feed near the surface at night then bring the carbon in their food back down into the twilight zone during the day.

About 90 percent of the carbon that gets into the twilight zone remains there, but a small percentage of it sinks to down into the deep ocean when animals die or expel carbon-rich fecal matter. Once there, it can remain isolated from the atmosphere for hundreds or even thousands of years.

Why do we need to find out more about the twilight zone as soon as possible?

The twilight zone’s biological richness makes it a potential source of food to support growing human populations—and an attractive future target for intensive commercial fishing operations.

Twilight zone organisms that migrate to surface waters are already being harvested on an industrial scale by the fishing fleets of countries such as Norway and Japan. Every year, factory ships are vacuuming up an ever-increasing quantity of small twilight-zone crustaceans—copepods and krill. Some of the harvest goes into fish paste for direct human consumption, but most is ground into fish meal to support expanding aquaculture or processed for use in pet foods or in “nutraceutical” oils.

Open-water fisheries far from land are currently largely unregulated, and we do not yet know enough to ensure that potential extraction of fisheries from the twilight zone would be sustainable. Nevertheless, countries including Norway and Pakistan have already issued licenses to begin fishing the twilight zone. Work began in 2018 under the United Nations Law of the Sea Convention to promote the preservation and sustainable use of marine biodiversity in areas beyond national jurisdiction, but efforts so far have focused on improving the conservation of surface-water fisheries and the genetic resources of the seabed, not of the twilight zone and its important ecosystem services, which are not well understood.

What do we still need to find out about the twilight zone?

To avoid impacts from overfishing—as has already happened with some coastal fisheries such as Northwest Atlantic cod—we need to know more about twilight zone animals and their interactions. Such information would enable policymakers to design regulations to protect twilight zone ecosystems and the surface-water species that depend on them—and also potentially allow for sustainable harvest of some twilight zone species. Among the questions still to be answered are:

Biomass and biodiversity: What species are there, and in what quantities?
Life histories and behaviors: How long do twilight-zone organisms live? How quickly do they grow? At what age do they reproduce?
Food webs: To what extent do large ocean predators such as whales and tunas depend on twilight-zone organisms as a source of food?
Global carbon cycle: How much carbon do twilight-zone animals transfer to the deep ocean through their daily migration? How much carbon sinks out of the twilight zone into deeper waters on marine snow and in other forms?

Why don’t we know more about the twilight zone?

Researchers have already learned some information about the twilight zone using acoustic imaging, nets, and submersibles. But unlike the surface ocean, which is accessible by ship and can be imaged remotely, the twilight zone isn’t easily studied with ship-based sonar and can’t be imaged with satellite technology. It covers a vast area, stretching around the globe, and changes quickly as water and animals move. Organisms in the twilight zone are unevenly distributed and are often good at avoiding nets from ships or cameras on underwater vehicles. Studying the twilight zone’s numerous gelatinous organisms such as jellyfish, salps, and siphonophores is particularly difficult, because they tend to fall apart in nets and require special lighting to be photographed or filmed.

What’s next for twilight-zone exploration and discovery?

In 2018, WHOI launched an ambitious mission to explore and understand the ocean twilight zone, with initial funding of $35 million from the Audacious Project. The effort is drawing on the expertise of a team of scientists and engineers, combining research, new technologies, and broad public engagement.

WHOI engineers are developing new platforms and vehicles that will enhance scientists’ ability to study this challenging region of the ocean. They will incorporate state-of-the art acoustics to detect twilight-zone animals, high-resolution camera systems to observe their behavior, sensors to measure environmental conditions, and sampling devices to collect small organisms and water for analysis. Researchers also will use genetic techniques and satellite tags to identify biological hot spots and better understand twilight-zone food webs.

By combining these and other new technologies with more traditional methods such as ship-based sonar and net tows, WHOI scientists and engineers hope to rapidly advance our understanding of the twilight zone and the animals living in it.

See the full article here .

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct.

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Mission Statement

The Woods Hole Oceanographic Institution is dedicated to advancing knowledge of the ocean and its connection with the Earth system through a sustained commitment to excellence in science, engineering, and education, and to the application of this knowledge to problems facing society.

Vision & Mission

The ocean is a defining feature of our planet and crucial to life on Earth, yet it remains one of the planet’s last unexplored frontiers. For this reason, WHOI scientists and engineers are committed to understanding all facets of the ocean as well as its complex connections with Earth’s atmosphere, land, ice, seafloor, and life—including humanity. This is essential not only to advance knowledge about our planet, but also to ensure society’s long-term welfare and to help guide human stewardship of the environment. WHOI researchers are also dedicated to training future generations of ocean science leaders, to providing unbiased information that informs public policy and decision-making, and to expanding public awareness about the importance of the global ocean and its resources.

The Institution is organized into six departments, the Cooperative Institute for Climate and Ocean Research, and a marine policy center. Its shore-based facilities are located in the village of Woods Hole, Massachusetts and a mile and a half away on the Quissett Campus. The bulk of the Institution’s funding comes from grants and contracts from the National Science Foundation and other government agencies, augmented by foundations and private donations.

WHOI scientists, engineers, and students collaborate to develop theories, test ideas, build seagoing instruments, and collect data in diverse marine environments. Ships operated by WHOI carry research scientists throughout the world’s oceans. The WHOI fleet includes two large research vessels ( R/V Atlantis and R/V Neil Armstrong); the coastal craft Tioga; small research craft such as the dive-operation work boat Echo; the deep-diving human-occupied submersible Alvin; the tethered, remotely operated vehicle Jason/Medea; and autonomous underwater vehicles such as the REMUS and SeaBED.

On October 1, 2020, Peter B. de Menocal became the institution’s eleventh president and director.

History

In 1927, a National Academy of Sciences committee concluded that it was time to “consider the share of the United States of America in a worldwide program of oceanographic research.” The committee’s recommendation for establishing a permanent independent research laboratory on the East Coast to “prosecute oceanography in all its branches” led to the founding in 1930 of the Woods Hole Oceanographic Institution.

A $2.5 million grant from the Rockefeller Foundation supported the summer work of a dozen scientists, construction of a laboratory building and commissioning of a research vessel, the 142-foot (43 m) ketch R/V Atlantis, whose profile still forms the Institution’s logo.

WHOI grew substantially to support significant defense-related research during World War II, and later began a steady growth in staff, research fleet, and scientific stature. From 1950 to 1956, the director was Dr. Edward “Iceberg” Smith, an Arctic explorer, oceanographer and retired Coast Guard rear admiral.

In 1977 the institution appointed the influential oceanographer John Steele as director, and he served until his retirement in 1989.

On 1 September 1985, a joint French-American expedition led by Jean-Louis Michel of IFREMER and Robert Ballard of the Woods Hole Oceanographic Institution identified the location of the wreck of the RMS Titanic which sank off the coast of Newfoundland 15 April 1912.

On 3 April 2011, within a week of resuming of the search operation for Air France Flight 447, a team led by WHOI, operating full ocean depth autonomous underwater vehicles (AUVs) owned by the Waitt Institute discovered, by means of sidescan sonar, a large portion of debris field from flight AF447.

In March 2017 the institution effected an open-access policy to make its research publicly accessible online.

The Institution has maintained a long and controversial business collaboration with the treasure hunter company Odyssey Marine. Likewise, WHOI has participated in the location of the San José galleon in Colombia for the commercial exploitation of the shipwreck by the Government of President Santos and a private company.

In 2019, iDefense reported that China’s hackers had launched cyberattacks on dozens of academic institutions in an attempt to gain information on technology being developed for the United States Navy. Some of the targets included the Woods Hole Oceanographic Institution. The attacks have been underway since at least April 2017.