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Shrimpy Sea Life May Mix Oceans as Much as Tides and Winds Do

Researchers are applying observations made by Charles Darwin's grandson to find that small organisms carry water with them as they go--which means they might play a big role in mixing vast tracts of ocean water
animals move oceans



K.KATIJA/J.DABIRI

Here's a puzzle: A child pees in the shallow end of a pool and then swims to the deep end. Which end should you avoid? Conventional wisdom holds that the deep end would be safe (until the pool's normal circulation mixed the contaminated water throughout). But according to new research—and old observations by Charles Darwin, the grandson of the more famous Charles Darwin—it would be wise to avoid most of the child's path through the water.

The force at work, called induced drift, happens in the sea, too. In centuries past, people thought that the movement of ocean water was the result of the sun's and moon's tidal forces, Earth's rotation and weather, along with fishes' fluttering tails, notes William Dewar, a professor of physical oceanography at Florida State University in Tallahassee. As it turns out, those earlier thinkers might not have been as off base as many contemporary scientists have assumed.

According to a paper that will be published tomorrow in Nature, the induced drift caused by billions of swimming creatures, especially small crustaceans, could be a force on par with the tides and wind in mixing ocean water. (Scientific American is part of the Nature Publishing Group.)

In a swimming pool the mixing might not be so necessary—or even desirable—but in the open ocean, mixing is an important way to move nutrients among layers and to maintain temperature balances that keep currents flowing.

The idea isn't new: Dewar, who wrote an accompanying views piece, co-authored a 2006 paper in the Journal of Marine Research, which observed increased water turbulence in large schools of krill and proposed that the creatures' tiny flutters could be churning waters on a large scale. "Zooplankton on the whole are pretty small," Dewar concedes. "Because of that, there are some legitimate concerns about how effectively they can mix [layers of] water."

Wouldn't the water's viscosity—its resistance—quickly overcome tiny amounts of turbulence caused by small zooplankton?

Actually, the new study's authors, Kakani Katija and John Dabiri from the California Institute of Technology in Pasadena, have shown that far from hindering the movement, water's viscosity actually enhances it. The principle observed by Darwin was that a solid body, whether it is a car or krill, moving through a fluid (air or water, respectively) will tend to take some of that fluid's particles along with it—hence the concept of induced drift. And as that fluid becomes thicker and more inclined to stick to the object, the amount of drift increases.

To put it simply, Dewar explains: "These animals go swimming, and they take the water along with them. It looks like they're pretty good at this."

For their research, Katija and Dabiri trained their sites not on krill but on small jellyfish, which can also swarm in large schools. They tracked how individual jellyfish carried water as they swam upward in the water column by observing the track of glowing dye injected into the water [see video below] as well as by measuring the kinetic energy the jellies generated in their wakes.

But why settle for such small sea dwellers? Although one might expect massive animals, such as whales, to have more impact on mixing individually, Dabiri, an assistant professor of aeronautics and bioengineering, explains that smaller organisms that travel in large schools—crustaceans and zooplankton for example—would have more of a global impact because they're so widespread and numerous.

Per Darwin's theory, however, it is not just critical mass that matters, but body shape. Dabiri explains that the quickest and most efficient swimmers—those that are smooth and bullet-shaped—are the least effective mixers, whereas slower and more saucer-shaped creatures will drag along proportionately more water.

How much water is moving? For it to have much importance for mixing purposes, water needs to be carried about a meter. From the observations and numerical simulations, Dabiri notes, "We expect that fluid is being carried at least on the magnitude of meters—if not tens of meters."

Extrapolating from their work, Katija and Dabiri suggest that in large schools these organisms likely have an even greater mixing power. In a massive krill migration for example, "it will be much more difficult for water to slip through the cracks" and not be carried along, Dabiri says.

But no one is quite sure how—and whether—the dynamic is actually playing out across the world's oceans. "It's not clear how you will go from that to a global model," Dewar says. Other considerations include how organisms' swimming style would affect water transport and how the combined force of these animals' drift might add up to a worldwide impact on ocean circulation. If it turns out to be as large a component as some are beginning to think, it will need to be incorporated into computer climate models. And that would be no small task because today's models are not nuanced enough to include data at the level of a school, much less an individual animal—to say nothing of complexities involving possible feedback loops down the road.

"Our paper raises more questions than it answers," Dabiri acknowledges. But, he says, it is casting light on what might be an important dynamic of oceans that has been right under our noses—or at least our hulls.

 
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