MOVING OCEANS?: New research is suggesting that jellyfish and other small ocean dwellers might play a big role in mixing the ocean's waters. Pictured are Mastigias jellyfish that the authors studied in Jellyfish Lake, Palau. Image: 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."