The following article first appeared in the Spring 1997 issue of Currents. Currents is a quarterly publication of the Bermuda Biological Station for Research (BBSR) . Reprinted by courtesy of the Editor.
The Great Ocean Migration
by David Malmquist
Each night, a migration takes place in the waters around Bermuda that rivals any in the animal kingdom. By studying this migration, BBSR scientist Deborah Steinberg is advancing the work of early deep-sea biologists such as William Beebe. Beebe's dives off Nonsuch Island in the 1930s first put Bermuda at the forefront of deep-sea exploration.
Perhaps unknown to Beebe, however, was that some of the deep-sea creatures he found often leave the deep. Each evening as the sun sets over the Sargasso Sea, millions of these sea creatures migrate up from the depths to feast on microscopic plants growing in sunlit surface waters. At dawn, these animals reverse course, sinking or swimming down to spend another day in the darkness.
By almost any measure, this daily migration rivals the great seasonal movements of caribou on land or arctic terns in the air. Yet it's magnitude was virtually unknown to science until the 1940s, and many of its mysteries remain.
Dr. Deborah Steinberg is studying these mysteries. She is particularly interested in how the migrating animals' consumption of vast quantities of plant material affects the transfer of carbon dioxide from the atmosphere to the ocean. Carbon dioxide is the main greenhouse gas released by burning of fossil fuels, and knowing how long it stays in the atmosphere will help scientists determine how human activities are affecting Earth's climate.
Marine biologists have known since the late 1800s that a daily migration took place in the sea, because their sampling nets came back much fuller at night than during the day. But the extent of the migration wasn't realized until World War II when, during anti-submarine warfare exercises, sailors aboard navy ships detected a puzzling layer on their sonar screens. This "deep scattering layer," or "false bottom" as it was called, rose toward the surface each evening, and sank again the next morning. In fact, submarines hid within this layer, using the sonar echo it produced to disguise their own.
Biologists later confirmed that this false bottom was formed by the bodies of countless marine animals migrating up and down as a layer between the surface and about 1,500 feet, or even deeper. This is known as the "Deep Scattering Layer." It is produced when sound waves reflect off gas-filled swim bladders or fat droplets within the migrating organisms.
Dr. Steinberg is attempting to unravel the mysteries of this layer by studying its migrating animals, what they eat, and the role they play in carbon and nutrient cycling in the open ocean.
Many of the animals that Dr. Steinberg studies are invisible to the naked eye. A glance under a microscope, however, reveals a startling and beautiful world filled with animals stranger than any imagined in science fiction. Included among these tiny migrators are the larvae of larger, more familiar animals such as crustaceans, fish, and eels.
"For animals of this size," says Dr. Steinberg, "swimming upward as far as 1,500 feet each evening, then returning in the morning, takes a lot of energy and requires traveling tens of thousands of body lengths every day. It would be like a person walking 25 miles each way to get to and from breakfast!"
Some of the largest migrators include euphausiids or "krill," shrimp-like animals that reach a few centimeters in length and are best known for their importance as food for whales. Other large migrators include floating, shell-less snails; creatures that resemble jellyfish; and some of the strange deep-sea fish first discovered by Beebe. Other important migrators include the smaller copepods. These "insects of the sea" says Dr. Steinberg, are the most common and abundant animals on Earth. On average, one copepod inhabits about every single liter of sea water.
If it takes so much energy to migrate, why then do these animals make the trip? The most common explanation, says Dr. Steinberg, "is that they feed in surface waters at night to avoid being seen and eaten by their predators." They then return during the day to depths below that reached by sunlight. But the lack of light at those depths prevents plant growth, so they must return again the next night to feed in the sunlit waters where plant life is abundant.
The plants that these migrators eat are rich in carbon in the form of carbohydrates. The plants make these carbohydrates by using energy from sunlight to combine water and carbon dioxide. Carbon dioxide gas enters seawater via exchange with the atmosphere. The plants remove carbon dioxide gas from seawater and incorporate the carbon into their own cells.
By eating plant material in the surface waters at night and swimming downward each day, the migrating animals potentially move a tremendous amount of carbon from the surface to the ocean depths. The animals recycle the carbon into different "forms" at depth by egesting it as feces, respiring it as carbon dioxide, or excreting it as dissolved organic carbon.
According to Dr. Steinberg, this migration occurs not only in Bermuda's waters, but throughout the world's oceans. So although most of the individual migrators are themselves tiny, their numbers are truly staggering. The amount of plant material they eat each night is thus enormous, and makes up an important part in the global carbon cycle.
Yet no one yet knows exactly how much carbon the migrators remove, how deeply they move it, or how much they recycle. If most of the carbon is recycled before they complete their downward migration, mixing of the upper ocean by wind-driven waves will keep a lot of the carbon near the surface. There, marine bacteria and zooplankton can recycle some of the forms of carbon back into carbon dioxide that plants may once more use.
But if the migrators travel below this upper mixed layer before most carbon passes through their guts, the carbon can remain in dissolved forms in the deep water of the ocean, or the solid forms (feces) can sink into sediments on the ocean bottom. Knowing the role that migrators play in transferring this material from the upper ocean to the deep reveals another piece of the carbon cycle puzzle, and ultimately helps resolve the ocean's potential role in taking up excess carbon dioxide from the atmosphere.
Dr. Steinberg is attempting to answer these questions with her colleagues at BBSR by collecting migrators in special nets towed behind BBSR's research vessel R/V Weatherbird II. The research team does much of their work at night, when the migrating animals are nearer the surface and easier to catch. Once collected, she identifies the animals, measures how fast they respire and pass food through their bodies, and dissects them to see what they eat.
Because many migrators swim at a tortoise-like pace of just a few feet per minute, it takes them many hours to swim to and from their feeding grounds. Thus much of the plant carbon they eat at night passes through their guts as solid waste before they swim into the deep layers of the ocean. However, Dr. Steinberg finds that the migrators she has studied so far excrete dissolved forms of carbon during most of their journey, thus they likely bring dissolved forms of carbon back down to depth with them.
But Dr. Steinberg has to date studied only a few of the many animals that migrate, and remains excited about exploring the role other migrators might play.
She also ties her work into the long-term research taking place at the BioStation's off-shore study site, the Bermuda Atlantic Time-series Study or "BATS" station. Once each month for the last 9 years, BioStation scientists have monitored temperature, saltiness, nutrients, plant growth rates, mixed layer depth, and many other properties of the ocean at a site 50 miles southeast of Bermuda. These properties of the ocean vary seasonally, and Steinberg will determine how this animal migration may be affecting some of these properties, such as nutrient and carbon cycles.
In the future, Dr. Steinberg would like to observe and collect the migrating organisms using a remotely operated submarine, as some of the creatures, such as the gelatinous zooplankton, are delicate and can be injured when collected in a net. She has used a remotely operated submarine during previous work in the Pacific Ocean off California, and hopes to use one in the waters around Bermuda.
"After all'" she says, "with the work of William Beebe and his bathysphere, Bermuda is really where using submersible vehicles to study the deep sea got its start."