Cloud shadows lead to mini-migrations | William & Mary

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by David Malmquist, VIMS

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September 27, 2021

Zooplankton repeatedly swims up and down due to subtle changes in daylight intensity

Spurred on by the setting sun, flocks of zooplankton and small fish migrate from the depths of the ocean to the surface to feed, while under cover of darkness they avoid predators. At dawn they swim hundreds of meters back into the darker, safer waters of the ocean’s “twilight zone”. A new study shows that some zooplankton repeatedly swim up and down within this daytime sanctuary, responding to cloud shadows so subtle that they escape the oceanographers on board.

The lead author of the study, Melissa Oman of the University of Rhode Island’s Graduate School of Oceanography, says, “Our results ask some really good questions about whether this diurnal behavior has any evolutionary or environmental benefit.” The newly discovered high-frequency “mini-migrations” also seem to significantly increase the metabolic requirements of zooplankton and their ability to reduce the build-up of the greenhouse gas carbon dioxide in the earth’s atmosphere.

Joined Omand in the field of study Deborah Steinberg and Karen Stamieszkin of the Virginia Institute of Marine Science by William & Mary. Your discovery, the cover story of the month Proceedings of the National Academy of Sciences, comes from data collected during the northeast Pacific Ocean. were collected NASA’s EXPORTS field campaign in 2018. EXPORTS, for EXport Processes in the Ocean by RemoTe Sensing is a multi-institutional, 5-year project that involves more than 40 leading scientists from 17 organizations in 11 countries.

Measurements of light intensity (white lines) and zooplankton abundance (black line) with sea depth (z, in meters) show that zooplankton floats up to stay in the water at its preferred brightness when cloud shadows prevent sunlight from penetrating so deeply into the water Ocean arrives.  When clouds get thinner or pass by, zooplankton swims back down.  Model results (purple line) show that the zooplankton reacts to changes in brightness of only 10% or 20% - a difference that is imperceptible to the ship's crew.Steinberg, CSX professor and professor of life sciences at VIMS, is one of the leading scientists in the EXPORTS project. She has been conducting field studies on vertical zooplankton migration for three decades, most recently during EXPORTS ‘second and final field campaign, a cruise to the North Atlantic in May 2021.

The daily migration between the depths of the ocean and the surface was named the greatest migration on earth, both because of the large number of immigrants and the distance these tiny creatures travel back and forth.

“For creatures this small – many the size of a grain of rice – walking 900 feet a day is like walking 25 miles to and from breakfast every day,” Steinberg says.

“We’ve known daily vertical migration – an adjustment to avoid visual predators – for over a hundred years,” she adds, “but we had no idea that this high-frequency migration was also taking place. It just shows how little we still know about the ecology and behavior of organisms in the deep sea. “

The team collected their data using a radiometer that measures surface sunlight and a sonar-like device that can detect zooplankton in the water. Comparing these two data streams showed that zooplankton floats to the surface to stay in the water at its preferred brightness when the thickening of the cloud cover prevents sunlight from reaching that deep into the ocean. When the clouds thinned, they swam back down. According to a model created by Omand, the zooplankton reacted to changes in brightness of only 10 or 20% – a difference that was imperceptible to the scientists on board.

The EXPORTS project combined data from satellites, ships and underwater instruments to better quantify the Earth's carbon cycle.  (Courtesy NASA)“It’s amazing how sensitive these small animals are to light,” says Steinberg. “It was overcast for most of our six-week cruise, but we’ve found that some zooplankton can only detect and respond to very subtle changes in light intensity because of changes in cloud thickness. Environments with scattered clouds moving through otherwise clear skies are likely to trigger even stronger mini-migrations. “

“It’s so cool to have a glimpse into the daily life of these little animals,” says Omand. “Hopefully our research will shed some light on the clues these animals use and why they do what they do.”

Effects on the Earth’s carbon cycle

Daily hikers play a key role in the Earth’s carbon cycle by eating surface-dwelling phytoplankton and then moving down the carbon that these microscopic plants photosynthesized from the water (this distance then allows the surface ocean to produce more CO. to record2 from the air). The CO2 removed from the atmosphere and transferred as carbon. exported to the deep sea this “biological pump” contributes nothing to current global warming.

VIMS professor Deborah Steinberg is preparing to use a MOCNESS sampling net during the 2018 EXPORTS cruise to the Northeast Pacific.  (Photo by Jason Graff / OSU)The newly discovered mini-migrations have an unknown but possibly significant influence on the global carbon transport via the biological pump. The average distance for each stage of the mini hikes is only around 15 meters, but over the day the repeated excursions add up to more than 200 meters, more than 30% of the average nightly walking distance. Steinberg says the effects of this additional energy use are clear.

“The amount of carbon that migrating zooplankton needs to meet their energy needs, and thus the amount that they can absorb and transport into the depths, could be higher than previously predicted,” says Steinberg.

Quantifying the role of mini-migrations in the earth’s carbon balance requires further research. More information is needed to fully understand why zooplankton floats up and down energy throughout the day in response to small changes in light, and whether this behavior is common in various species and in the oceans around the world.

Steinberg attributes the team’s discovery to the interdisciplinary nature of the EXPORTS program.

“Programs like EXPORTS are important,” she says, “because they enable scientists from a wide variety of disciplines – in our case a physical oceanographer and zooplankton ecologist – to combine and interpret their field observations. Melissa brought the expertise to spot high-frequency migration, while Karen and I helped put it in an ecological context and understand its effects. “


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