Small eddies play a big role in feeding the oceans

image: This video from the North Pacific Ocean shows concentrations of phosphate nutrients 500 meters below the ocean surface. Eddies represent small whirlpools transporting phosphate from the nutrient-rich equator (lighter colors), northward to nutrient-depleted subtropics (darker colors).
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Credit: Courtesy of Jonathan Lauderdale, Oliver Jahn, Christopher Hill, Stephanie Dutkiewicz and Michael Follows, et al.

Subtropical gyres are huge rotating ocean currents that generate sustained circulations in the subtropical regions of the Earth just north and south of the equator. These gyres are slow-moving whirlpools that circulate in massive pools around the world, picking up nutrients, organisms, and sometimes trash, as currents swirl from coast to coast.

For years, oceanographers have been puzzled by conflicting observations in subtropical gyres. On the surface, these massive currents appear to harbor healthy populations of phytoplankton — microbes that feed the rest of the ocean food chain and are responsible for sucking up a significant portion of carbon dioxide from the atmosphere.

But judging by what the scientists know about gyre dynamics, they figured the currents themselves wouldn’t be able to hold enough nutrients to sustain the phytoplankton they were seeing. How, then, were the microbes able to thrive?

Now, MIT researchers have discovered that phytoplankton can receive nutrient deliveries from outside the gyres, and that the delivery vehicle comes in the form of vortices – much smaller currents that swirl around the edges of a gyre. These eddies draw nutrients from nutrient-rich equatorial regions and push them to the center of a gyre, where nutrients are then absorbed by other currents and pumped to the surface to feed phytoplankton.

Ocean eddies, according to the team, appear to be an important source of nutrients in subtropical gyres. Their replenishing effect, which the researchers call a “nutrient relay,” helps maintain populations of phytoplankton, which play a central role in the ocean’s ability to sequester carbon from the atmosphere. While climate models tend to project a decrease in the ocean’s ability to sequester carbon over the next few decades, this “nutrient relay” could help maintain carbon storage in the subtropical oceans.

“There’s a lot of uncertainty about how the ocean’s carbon cycle will evolve as the climate continues to change,” says Mukund Gupta, a Caltech postdoc who led the study as a graduate student. at MIT. “As our paper shows, getting the carbon distribution right is not straightforward and depends on understanding the role of eddies and other small-scale motions in the ocean.”

Gupta and his colleagues report their findings this week in the Proceedings of the National Academy of Sciences. The study’s co-authors are Jonathan Lauderdale, Oliver Jahn, Christopher Hill, Stephanie Dutkiewicz and Michael Follows of MIT, and Richard Williams of the University of Liverpool.

A snowy puzzle

A cross-section of an ocean gyre looks like a stack of nesting bowls stratified by density: warmer, lighter layers sit on the surface, while cooler, denser waters make up deeper layers. Phytoplankton live in the sunlit upper layers of the ocean, where microbes need sunlight, warm temperatures and nutrients to thrive.

When phytoplankton die, they sink through the layers of the ocean as “sea snow”. Some of this snow releases nutrients into the stream, where they are pumped up to feed new microbes. The rest of the snow flows out of the gyre, to the deepest layers of the ocean. The deeper the snow sinks, the more difficult it is to bring it to the surface. The snow is then trapped or sequestered, along with any unreleased carbon and nutrients.

Oceanographers believed that the main source of nutrients in subtropical gyres came from recirculating sea snow. But since some of that snow inevitably sinks to the bottom, there must be another source of nutrients to account for the healthy phytoplankton populations on the surface. The exact nature of this source “has left the oceanographic community a bit perplexed for a while,” says Gupta.

Swirls at the edge

In their new study, the team sought to simulate a subtropical gyre to see what other dynamics might be at work. They focused on the North Pacific Gyre, one of Earth’s five major gyres, which circulates across most of the North Pacific Ocean and spans more than 20 million square kilometers.

The team started with the MITgcm, a general circulation model that simulates physical circulation patterns in the atmosphere and oceans. To reproduce the dynamics of the North Pacific gyre as realistically as possible, the team used an MITgcm algorithm, previously developed at NASA and MIT, which tunes the model to match actual observations of the ocean, such as ocean currents recorded by satellites, as well as temperature and salinity. measures taken by ships and dinghies.

“We use a simulation of the physical ocean that is as realistic as possible, given the model machinery and available observations,” Lauderdale explains.

The realistic model captured finer detail, at less than 20 kilometers per pixel, compared to other models that have more limited resolution. The team combined the simulation of the physical behavior of the ocean with the Darwin model – a simulation of microbial communities such as phytoplankton, and how they grow and change with ocean conditions.

The team ran the combined North Pacific Gyre simulation over a decade and created animations to visualize the pattern of currents, and the nutrients they carry, in and around the gyre. What emerged were small eddies that ran along the edges of the huge gyre and appeared to be rich in nutrients.

“We picked up little eddying motions, basically like weather systems in the ocean,” Lauderdale says. “These vortices transported packets of nutrient-rich waters from the equator north to the center of the gyre and down along the sides of the bowls. We wondered if these eddy transfers were an important delivery mechanism .

Surprisingly, the nutrients first move deeper, away from sunlight, before being sent back up where the phytoplankton live. The team found that ocean eddies could supply up to 50% of the nutrients in subtropical gyres.

“It’s very important,” says Gupta. “The vertical process that recycles nutrients from sea snow is only half the story. The other half is the regenerative effect of these vortices. As subtropical gyres contribute a significant portion of the world’s oceans, we believe this nutrient relay is of global importance.

This research was funded in part by the Simons Foundation and NASA.

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Written by Jennifer Chu, MIT News Office


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