Category: Deep Blue Sea

Sharks Take 'Tunnels' into the Depths

Spiraling eddies offer conduits to food in the ocean twilight zone

As the Gulf Stream current curves away from North America and heads east across the Atlantic, it swirls at its edges. If one of these swirls is large enough, it will pinch off, sending a whirling pocket of water—more than 60 miles in diameter—spinning through the ocean like an underwater hurricane.

These swirling pockets, called eddies, may also be full of sharks.

No, this isn’t the plot of the next Sharknado movie. It’s a discovery by researchers from the Woods Hole Oceanographic Institution and the University of Washington, who tracked the movements of several tagged great white sharks. They found that white sharks in the open ocean seem to seek out eddies for a surprising reason: The eddies offer a beeline to a banquet of food.

The Gulf Stream can spin off both warm- and cold-water eddies, and surprisingly, the sharks seem to have a preference. The warmer eddies are spawned when the Gulf Stream snakes farther northward, drawing warm water up from the Sargasso Sea. When the eddy spins away from the current, it traps that warm water in its center. But because that water is typically low in nutrients, these eddies aren’t thought to contain much life.

“The paradigm is that they’re like these ocean deserts,” said Camrin Braun, a graduate student in the MIT-WHOI Joint Program in Oceanography and co-author of the new study published in May 2018 in Scientific Reports

Cold-water eddies are just the opposite. “They trap cold, nutrient-rich water from north of the Gulf Stream,” Braun said. “They’re anomalously cold and anomalously productive.” The extra nutrients can fuel enough phytoplankton growth to make these eddies visible from space.

So it might seem logical that when adult white sharks leave the seal-filled waters of coastal New England and head for the open ocean, they might seek out these whirling blobs of cold water, where phytoplankton at the base of the food chain are blooming. But apparently not. To the scientists’ surprise, the sharks spent most of their time in the warmer eddies.

Playing tag with sharks

The research team followed two mature white sharks. Mary Lee, 16 feet long and 3,460 pounds, was tagged off the coast of Cape Cod in September 2012. Lydia, slightly smaller at 14.5 feet and just under one ton, was tagged off Jacksonville, Florida, in March 2013. (Their trackers have likely reached the end of their five-year lifespan, but both sharks still maintain active twitter accounts: @RockStarLydia and @MaryLeeShark.)

Lydia gained some notoriety as the first white shark to be tracked crossing the Atlantic, while Mary Lee tended to stay a little closer to the coast. Both demonstrated the unexpected preference for warm-water eddies, but it was Lydia, equipped with a second, specialized tag recording her depth, who hinted at a possible explanation.

Both sharks had SPOT tags, an acronym for Smart Position or Temperature. When the sharks came

Forecasting Where Ocean Life Thrives

Scientists focus on seams in the ocean called ‘fronts’

Imagine for a moment that the promises of science fiction have come true and flying cars now enable human society to live in the air. You could move not only horizontally, but up and down too, taking advantage of the full three-dimensional space. There is just one catch: You cannot move faster than the wind. Instead, you, and those around you, tumble around, with your altitude and position entirely dependent on the vagaries of the winds.

This is the locomotion strategy—or lack thereof—of plankton, the microscopic organisms that make up the vast majority of the living organisms in the ocean. They are at the mercy not of the wind, but of ocean currents. In fact, this class of organisms is defined by its inability to move faster than ocean currents. The name “plankton” is derived from a Greek word meaning “wanderer” or “drifter.”

Though they seem so small and powerless, plankton are hardly insignificant. They are the foundation of the food web on which all life in the ocean depends, including the fish that end up on our dinner tables. They produce much of the oxygen we breathe in Earth’s atmosphere. They collectively draw huge amounts of heat-trapping carbon dioxide from the atmosphere into the ocean, regulating the climate of our entire planet.

If we lived in a world where the winds determined our pathways and destinations, we’d crave more understanding of the physics that underlie our weather forecasts. The same is true for the oceans. If want to predict where and when productive plankton-rich areas will form, we need to learn more about the turbulent three-dimensional environment in which plankton live—the forces that govern where different species move and aggregate.

When you hear a TV meteorologist talk about a cold front or a warm front coming through, you know to expect a rapid temperature change and perhaps a storm. It turns out that the ocean has its own underwater ever-shifting frontal systems, and these fronts can help us figure out where the plankton are.

Deep water rising

In the air, fronts form in the seams between those high-pressure and low-pressure systems you see all the time on weather maps. High-pressure air is denser, so it sinks; low-pressure air rises. When an air parcel rises, it cools, and the water it contains condenses and then rains out, causing storms.

Something similar happens in the ocean. Some parcels of seawater may be colder or saltier than others, making them denser than surrounding parcels. Where there is a large density gradient, there is a fast flowing current, and the denser water slips below the less-dense water, which rises toward the surface. This vertical motion is critical for plankton.

Tiny marine phytoplankton function like plants. They harvest energy from sunlight to convert carbon dioxide and other inorganic carbon into food and biomass and produce oxygen as a byproduct. But the two main resources they