Trillions of zooplankton, many smaller than a grain of rice, hover hundreds of feet below the surface of the sea, waiting for their signal. These tiny animals were thought to be drifters because they were suspended in the ocean. The swarms start to rise just before the sun goes down.
A group of zooplankton join in as they climb. As the first beams of morning light begin to cascade across the sea, the multitudes are already turning back down to the deep. As sunset and sunrise slide from east to west across the Pacific Ocean, then the Indian, the Southern and the Atlantic, swarms make the same upward journey.
Humans don't know that diel vertical migration is the largest routine migration of life on the planet. Estimates show 10 billion animals make these excursions each day. They ascend from more than 3000 feet below. It's an amazing achievement. Making a one-way trip of 1,000 feet is equivalent to a human swimming more than 50 miles. The animals go through different areas of the ocean. The water is about 20 degrees colder than the surface and the pressure is about 30 times what it is up top. Why would so many animals make such a long journey?
The long answer is to eat. Zooplankton hide from squid and fish during the day. They rush to the surface at night to feed on the plants that live in the top few hundred feet of water.
This is the wind of vertical migration. Crosscurrents and eddies can be found. Researchers are starting to understand the details of the new technologies. The specifics will help answer questions about the ocean food web, the carbon budget and the nature of life on Earth.
The earliest recordings of diel migration were made during World War II, when ships and subs using sonar to sweep the oceans for enemy subs detected something odd. The layer was constantly shifting by as much as 3000 feet. Martin Johnson embarked on a research ship in 1945 to sample plankton. There seems to be a correlation between planktonic animals and the scattering layer. The proposal that the layer was made of living creatures raised a lot of questions.
It was hard to answer those questions. The deep ocean is difficult to access and the animals involved are small. Tracking swarms of flea-size organisms through the lightless depths is not easy. The diel migration was described by researchers as a cloud of organisms rising and falling together. Animals were picked up by higher-resolution sonar. Today's surveys can't tell which small animals are moving. Sampling the zooplankton can bring up organisms for identification, but it can't tell you where the animals are in their journey.
New research is showing hidden aspects of the mass migration. The process is tied to what's going on in the sky. When the sun is not in the sky, some animals realign their migrations with the moon. They can start swimming after seeing a solar eclipse. Zooplankton living below 1,000 feet, where light intensity is just 0.012 percent of what it is at the surface, may shift their vertical position by as much as 200 feet. Even though the light changes at the surface weren't apparent to her or her colleagues, she realized this. She and her colleagues noted in a paper that the cruise was overcast, gray and rainy. The zooplankton registered subtle changes in light under the water.
Animal's-eye views of migration have begun to be offered by vehicles with cameras and collection devices that allow them to pair images with chemical signatures from the water column. The Monterey Bay Aquarium Research Institute (MBARI) in California and the University of Delaware sent an underwater vehicle 1000 feet down into the Catalina Basin to take pictures of zooplankton. The echoes it returned were stunning, revealing that the zooplankton were organized in well-defined clusters and migrated in carefully timed ascents.
The idea of vertical migration needs to be thought of as an individual and species-by-species thing. The zooplankton are not the only ones in the evening commute. Many animals use this as a tactic. The deep-sea creatures make the nightly trek to find food and avoid their own prey.
The animals might not be the only ones. The Institute of Coastal Systems in Germany has a professor named Kai wirtz. They were looking to describe the distribution of different plants in the ocean. The ocean's vast and essential blanket of plankton wouldn't be fed by the ocean's circulation alone, he realized.
Scientists have known for a long time that many species of phytoplankton can move by a variety of factors. The top of the ocean is filled with sun but not much else. The bottom has an abundance of vitamins and minerals despite not getting enough sunlight. He wondered why these plants wouldn't use their locomotives to commute between the two spaces. There is not an easy explanation according to him.
It's possible that half of the marine phytoplankton species migrate from below to above on a regular basis. The journey might take hours, days or even weeks, with some organisms reproducing along the way to allow their descendants to continue. The idea presents a radical change in the way scientists think of phytoplankton, which they think of as more of a chemical compound than individual living organisms.
The behavior of marine plants is more advanced than we had thought. predatory copepods were introduced to one of the tanks at Washington State University. The hungry copepods made the traditional nighttime ascent and daytime descent when the scientists replicated typical day- night light cycles. The phytoplankton in both tanks swam up during the day and down at night to maximize their sunlight exposure and reduce their risk of being eaten by the zooplankton.
The plants in the tower with the copepods retreated deeper at night, putting more distance between themselves and the enemies above. They don't know how the phytoplankton sense their predator's actions. The researchers said that the behavioral response could have important ecological consequences.
The extent of climate change is one of the consequences of phytoplankton migration. In 1995 Steinberg and other scientists were trying to figure out how much carbon dioxide is released into the atmosphere and how much is pulled from it. More carbon was disappearing from the ocean than could be accounted for. Then he looked into the dark.
She became well versed in the local fauna as a result of her research at the Institute of Ocean Sciences. She had to go on a night dive. It was a completely different community when she plunged off the side of a small boat. She recalls that she was in the water with animals of all kinds. She changed direction and started studying diel migration. She knew it could hold part of the carbon answer.
On the ocean's surface, a lot of carbon dioxide is sucked from the atmosphere, but it is released back into the air quickly. When zooplankton migrate and eat the marine plants, they become a kind of biological conveyor, transporting carbon down into the deep sea, where it can be sequestered for hundreds or thousands of years.
Michael Stukel, a plankton and marine biogeochemistry researcher at Florida State University, spends a lot of time looking through a microscope at zooplankton's fecal pellets. Individual excretions are small, but when they happen on such a large scale, they have global significance.
Fecal pellet from migrators descend through the water column They are joined by other biological particles and they create "marine snow". The global sequester of carbon means the planet is not as hot as it otherwise would be.
Estimates of the amount of carbon sequestered by migrating organisms vary widely. Climate models will be improved by better data and by better understanding of how climate change will affect organisms. Ken Buesseler is a senior scientist at the Woods Hole Oceanographic Institute.
The answers to the big questions about these migrators are likely to come from work. She's adding vision and cameras to the cars to make sure they can see what's happening to migrators. She now has the ability to train a vehicle and find an animal.
The technology is being trained on creatures that look like ghosts. She wants to understand how to make these systems more robust because these animals have semi transparent tissue and are hard for a vehicle to see. To capture usable images and video, the team needs a robot that can swim and produce light. That's a big concern. Most of these creatures can't see red light and a cruising mode that reduces turbulence is one stealthy strategy. Satellites in space are being used by researchers to observe the density of animals that come up to feed at night. They can see into the water as deep as 65 feet with lidar.
Scientists are combing the water column for the genetic traces of organisms that have left their home. The team dropped bottles from the research ship as it was drifting in the Gulf of Mexico. The researchers were looking at the life below. They were able to deduce the location of organisms from the samples. The results were published in 2020 The DNA indicated that copepods and zooplankton had more presence than fish and other large targets.
A global network of ocean monitors that can watch these processes day in and day out is what researchers need most. Large-scale fishing has mostly been done in the ocean's surface layer. Some countries, including Norway and Pakistan, are issuing commercial fishing permits for the middle swath of ocean in order to suck in the diel migrators and process them into food and fish oil.
These animals are being squeezed out of daytime habitats due to dead zones and rising oxygen-minimum zones in ocean water. Climate change is affecting the mixing of water layers in the ocean. Less food for zooplankton is caused by fewer phytoplankton. Scientists studying these animals are under a lot of pressure. It's rare that we have the chance to understand a system before it's used. We're kind of racing against time.
Benoit-Bird and her colleagues will return to the sea this summer to better understand the movements of copepods and other migrators. She wants the expedition to help them learn how these tiny animals self-organize during the day to stay connected with other species.
The sun will keep setting and rising. Many animals will follow the underwater tides of darkness and light, eating, excreting and changing the balance of elements on our planet.
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