Even Tiny Organisms Can Stir Up an Ocean

August 4, 2009 at 3:33 am Leave a comment

What gets the sea all riled up? Winds and tides do, certainly, but scientists have long wondered how the movement of fish and other organisms — even tiny ones, like zooplankton — might contribute to ocean mixing.
A study by Kakani Katija, a doctoral student at the California Institute of Technology, and her adviser, John O. Dabiri, has determined that the movement of the ocean’s organisms makes a significant contribution. But the researchers are not talking about the effects of swishing fins or flippers. Rather, they report in the journal Nature, the mixing is caused just by the movement of bodies through the water.

The concept, called fluid drift, was first proposed by Charles Darwin’s grandson, also named Charles. The idea is that a body moving through a fluid will set some of the surrounding fluid into motion. The effect depends on the body’s shape and size and is enhanced if the fluid is viscous. For a small organism, seawater is relatively very viscous.

The researchers made theoretical calculations but also performed experiments using jellyfish in a saltwater lake on the island of Palau in the Pacific Ocean. During the day, they used dyes “to understand visually what the jellyfish wake looks like,” Ms. Katija said. At night, they used a method called laser velocimetry, adapted by Ms. Katija for underwater use, to quantify more precisely the water movement. (A video is at nytimes.com.)

The researchers estimated that globally, the ocean mixing contributed through this drift effect by small marine creatures was roughly equal to that contributed by winds and tides. But their estimates were conservative, they said, because they considered only creatures swimming alone.

What’s more, Ms. Katija said, mixing is not limited to living organisms. The drift effect, she said, “is applicable to any body that moves in water.” Even a small particle, like a fecal pellet, will cause a small amount of mixing as it travels down in the water column.

By HENRY FOUNTAIN

 New York Times

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Entry filed under: Dive In, Natural Waterfront. Tags: , , .

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