Called induced fluid drift, it involves the tendency of liquid to “stick” to a body as it moves through water — and a little bit of drift could add up quickly on a global scale.
“The mere act of swimming implies that some water travels with the swimmer,” said CalTech engineer Kakani Katija, co-author of the study in Nature Wednesday. “Drift applies to all animals, to anything with a body.”
That the mere motion of animals could play a profound role in water-column commingling was once considered absurd. The sea would surely absorb the force of a flapping fin, to say nothing of a phytoplankton’s flagellae. It was a basic principle of friction, applied to water.
But in recent years, this consensus has sprung some leaks. When added up, winds and tides don’t quite provide enough energy to account for the amount of water-mixing observed in the seas. In 2004, a study found that a school of fish could cause as much turbulence as a storm. Other researchers soon suggested that ocean swimmers could account for the gap. Soon after that, ocean physicists measured enormous turbulence generated by a swarm of krill, a crustacean considered too small to have meaningful mixing effects.
Missing from their equation, however, was a physical explanation for how tiny forces could avoid being swallowed by the friction of the sea. One possibility, originally proposed by Charles Darwin’s grandson, also named Charles, was that the act of swimming created pressure differentials that pulled water along with a body, an invisible suitcase to be unpacked along the way by cross-currents.
“As a body moves in a fluid, a high-pressure field is created in front of the body, and a low-pressure field behind. Because fluid moves from high to low pressure, the fluid that’s adjacent to the rear of the body moves along with it,” said Katija. “You get a permanent displacement of the water.”
Katija and CalTech bioengineer John Dabiri have provided the first direct observation of this phenomenon. Using fluorescent dyes and underwater video cameras, they’ve made visible the invisible, producing videos of swimming jellyfish trailed by the water they came from.
If the video seems like an infinitesimal drop in the bucket compared to winds or tides, consider that most of the ocean — excepting the top 300 feet or so — is so placid that a couple hand-held kitchen mixers could stir a cubic mile of it.
According to Katija and Dabiri, induced fluid drift should be caused by any swimming animal. Their next task is to verify that it does, and to put numbers to how much water is moved by each animal, how it mixes, how the figures vary by body shape and size and population density.
Future findings could have a profound influence on climate models, which do not now account for this so-called biogenic mixing.
If swimming generates tide-scale forces, then “it has an impact on global climate. This is a rather novel twist to the whole climate story,” said William Dewar, a Florida State University oceanographer. “How one would extend existing models to include a biosphere mixing input is not clear, largely because no-one has spent much time thinking about it.”
copyright: article: Brandon Keim