Greatest Migration on Earth Happens under Darkness Every Day

The zooplankton menagerie comprises a wild array of tiny animals. Clockwise from top left: A crab larva known as a megalopa from the Atlantic Ocean; a blue and orange plankton from the Canary Islands; a sea butterfly also from the Canary Islands; an arrow worm, found in all oceans, from the surface to the deep; a blue bioluminescent plankton from the Arctic Ocean; and a buglike copepod from the Atlantic deep sea, common in tropical regions. Credit: Solvin Zankl/Minden Pictures; Sergio Hanquet/Minden Pictures; Sergio Hanquet/Minden Pictures (top row, left to right); Solvin Zankl/Minden Pictures; Flip Nicklin/Minden Pictures; Sinclair Stammers/Minden Pictures (bottom row, left to right)

Altering the Carbon Budget

One consequence of phytoplankton migration is the extent of climate change. In 1995 Steinberg and other scientists were trying to piece together the global carbon budget—the amount of carbon dioxide emitted into the atmosphere and the amount pulled from it, in part by marine ecosystems. The numbers weren’t adding up; more carbon was disappearing from the ocean surface than they could account for. Then Steinberg got a look into the darkness.

As part of her research, done at the Bermuda Institute of Ocean Sciences, Steinberg would often dive during the daytime, and she became well versed in the local fauna. But then she got to take a night dive. She plunged off the side of a small boat above 13,000 feet of dark water and soon found “it was a totally different community. I was in the water with animals of every single kind,” she recalls, her voice still ringing with excitement more than a quarter of a century later. That night was her cue to change direction and start studying diel migration. And she had an inkling that it might hold part of the carbon answer.

On the ocean’s surface, phytoplankton suck an enormous amount of carbon dioxide from the atmosphere, but they release much of it right back into the air, often within days. As migrating zooplankton swim up at night and eat these marine plants, they become a kind of biological conveyor, transporting carbon down into the deep sea, where it can get sequestered for hundreds or thousands of years.

To study this crucial movement of carbon, Michael Stukel, a plankton and marine biogeochemistry researcher at Florida State University, spends a lot of time peering through a microscope at zooplankton’s fecal pellets. Individual excretions are small, but when they happen on such an enormous scale, they take on global biogeochemical significance.

Fecal pellets from vertical migrators, rich in carbon, descend through the water column. They are joined by other sinking biological particles, creating “marine snow” that slowly drops to the deep seafloor. Together with the swimming zooplankton carrying their carbon-loaded dinners back down with them, this global sequestration of carbon means the planet is “not as hot as it otherwise would be,” Stukel says.

Estimates of the amount of carbon sequestered by migrating organisms vary widely because so much about the diel migration remains a mystery. Better data will improve climate models, which in turn will improve understanding of how climate change will alter these organisms’ behaviors—and, subsequently, the climate again. “You run into these big questions for humanity, for climate, that we can’t answer, and a fair number of them relate to these migrators,” says Ken Buesseler, a senior scientist at the Woods Hole Oceanographic Institution.

Balancing Act

Answers to the remaining big questions about these migrators are likely to come from work such as that happening in Kakani Katija’s lab at MBARI. There she’s adding stereoscopic cameras and vision algorithms to autonomous vehicles so they can carefully track the movements of specific migrators. She can now train a vehicle and turn it loose to locate an animal and trail it for hours.

Katija’s team is training the technology on gelatinous creatures such as siphonophores, which look like ghostly worms. Because these animals have semitransparent tissue and move quickly and unpredictably, siphonophores are hard for an autonomous vehicle to keep sight of, but that’s what Katija wants: “We’re trying to understand how to make these systems more robust,” she says. To capture usable images and video, the team needs a robot that can swim and produce light—both of which could easily interfere with their subjects’ behavior. “That’s a huge concern,” Katija acknowledges. One stealthy strategy is to use red light, which most of these creatures can’t see, and a cruising mode that minimizes turbulence. Researchers are also turning to satellites in space that can observe the density of animals that come up to feed at night without the risk of disturbing them. Equipped with lidar—laser-based remote sensing technology—they can peer into the water as far down as 65 feet.

To pinpoint which species are moving when and where, scientists are also combing the water column for the genetic traces of transitory organisms. One team dropped large seawater-sampling bottles at various depths from its research ship as it drifted in the Gulf of Mexico. At the same time, the researchers were taking sonar readings of the life below. From the samples, they sequenced strands of DNA to deduce what organisms had been where—and when. The results, published in 2020, revealed poorly resolved spots in the concurrent sonar readings. Although sonar data suggested fish and other relatively large targets accounted for much of the moving biomass, the DNA indicated that copepods and gelatinous zooplankton had a much greater presence.

What researchers need most, they agree, is a global network of ocean monitors that can watch these processes day in and day out to more fully understand the ocean’s systems before humans further disrupt them. For example, large-scale fishing has been done almost exclusively in the ocean’s surface layer, augmented more recently by bottom trawling. But now some countries, including Norway and Pakistan, are issuing commercial fishing permits for the middle swath of ocean, in part to suck in the diel migrators and process them into food for farmed fish and for fish oil.

Expanding dead zones and rising oxygen-minimum zones in ocean water are also squeezing these animals out of livable daytime habitats. And climate change is decreasing the mixing of water layers in the open ocean, bringing fewer nutrients to phytoplankton. Fewer phytoplankton means less food for migrating zooplankton. All of which means the scientists studying these animals are under growing pressure. “It’s not often that we have the chance in history to understand a system before it’s exploited,” Benoit-Bird says. “I feel like we’re kind of racing against the clock.”

To better understand the movements of trillions of copepods, krill and other elusive migrators, this summer Benoit-Bird and her colleagues will return to sea. She hopes their expedition with underwater robots, sonar, imaging and environmental DNA can help them learn how these tiny animals self-organize during the day—rising and falling, tightening and loosening in swarms to stay connected with networks of other species.

In the meantime, the sun will continue to rise and set. As it does, an untold number of animals will follow the underwater tides of darkness and light, eating, excreting and modulating the very balance of elements on our planet.