Subtropical gyres are massive rotating ocean currents that form continuous circulations in the Earth’s subtropical regions just north and south of the equator. These slow-moving whirlpools, called gyres, cycle within enormous basins all over the world, collecting nutrients, organisms, and sometimes garbage as the currents rotate from coast to coast.

Oceanographers have been perplexed by contradicting observations within subtropical gyres for years. The phytoplankton, which feeds the rest of the ocean food chain and is in charge of absorbing a sizeable percentage of the carbon dioxide in the atmosphere, appears to be abundant near the surface of these powerful currents.

However, based on what is known about the dynamics of gyres, scientists concluded that the currents alone wouldn’t be able to sustain the phytoplankton they were observing. So how did the microbes manage to survive and thrive?

MIT scientists have now discovered that phytoplankton may receive nutrient supplies from outside the gyres, and the delivery vehicle is in the form of eddies, much smaller currents that whirl around the borders of a gyre. The nutrients are pushed towards the center of a gyre by these eddies, which then pick them up and pump them to the surface to nourish phytoplankton. These eddies suck nutrients in from high-nutrient equatorial regions.

The research team discovered that ocean eddies seem to be a significant source of nutrients in subtropical gyres. Their ability to replenish nutrients in the water plays a key role in keeping phytoplankton populations stable, which in turn contributes to the ocean’s capacity to remove atmospheric carbon. Climate models predict a loss in the ocean’s ability to retain carbon over the next decades; nevertheless, this “nutrient relay” may assist maintain carbon storage across the subtropical oceans.

“There’s a lot of uncertainty about how the carbon cycle of the ocean will evolve as climate continues to change, ” says Mukund Gupta, a postdoc at Caltech 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 fine-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 at MIT, and Richard Williams at the University of Liverpool.

A snowy puzzle

A cross-section of an ocean gyre resembles a stack of nesting bowls that is stratified by density: Warmer, lighter layers lie at the surface, while colder, denser waters make up deeper layers. Phytoplankton live within the ocean’s top sunlit layers, where the microbes require sunlight, warm temperatures, and nutrients to grow.

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

Oceanographers thought that the main source of nutrients in subtropical gyres came from recirculating marine snow. But as a portion of this snow inevitably sinks to the bottom, there must be another source of nutrients to explain the healthy populations of phytoplankton at the surface. Exactly what that source is “has left the oceanography community a little puzzled for some time,” Gupta says.

Swirls at the edge

In their new study, the team sought to simulate a subtropical gyre to see what other dynamics may be at work. They focused on the North Pacific gyre, one of the Earth’s five major gyres, which circulates over 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 the physical circulation patterns in the atmosphere and oceans. To reproduce the North Pacific gyre’s dynamics 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, and temperature and salinity measurements taken by ships and drifters.  

“We use a simulation of the physical ocean that is as realistic as we can get, given the machinery of the model and the available observations,” Lauderdale says.

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

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

“We were picking up on little eddy motions, basically like weather systems in the ocean,” Lauderdale says. “These eddies were carrying packets of high-nutrient waters, from the equator, north into the center of the gyre and downwards along the sides of the bowls. We wondered if these eddy transfers made an important delivery mechanism.”

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

“That is very significant,” Gupta says. “The vertical process that recycles nutrients from marine snow is only half the story. The other half is the replenishing effect of these eddies. As subtropical gyres contribute a significant part of the world’s oceans, we think this nutrient relay is of global importance.”

Source: 10.1073/pnas.2206504119

Image Credit: Getty

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