In this still from a video, fluorescent dye gets dragged along right behind the jellyfish before eventually trailing off in its wake.

Listen to a Podcast of John Dabiri discussing his work with jellyfish.

Plankton Stirs the Oceans

The winds, the tides, and—say what?—the ocean's tiniest swimmers may have roughly equal effects on the large-scale mixing that distributes heat, nutrients, and gases throughout the world's oceans, according to a new Caltech study. Oceanographers had previously assumed that water's viscosity would damp out any turbulence created by weak-swimming plankton. But John Dabiri (MS '03, PhD '05), associate professor of aeronautics and bioengineering, and grad student Kakani Katija think that so-called Darwinian mixing, named for Charles Darwin—no, not that Darwin; his grandson—might be able to transport huge volumes of water in very small batches.

"Darwin's grandson discovered a mechanism for mixing, similar in principle to the idea of drafting in aerodynamics, whereby an individual organism literally drags the surrounding water with it as it goes," Dabiri explains. Every day, billions of tiny krill and copepods migrate hundreds of meters from the depths of the ocean toward the surface. Darwin's mechanism suggests that they might drag some of the colder, heavier bottom water up with them toward the warmer, lighter water at the top. This would create instability, and eventually the water would flip, mixing itself as it went.

When Dabiri and Katija modeled this mathematically, they found that at very small scales, the water's viscosity actually enhanced Darwin's mechanism, magnifying the effect. "It's like a human swimming through honey," Dabiri explains. "What happens is that even more fluid ends up being carried up with a copepod, relatively speaking, than would be carried up by a whale."

Katija and collaborators Monty Graham (from the Dauphin Island Sea Laboratory), Jack Costello (from Providence College), and Mike Dawson (from the University of California, Merced) then traveled to the South Pacific island of Palau to study this effect among jellyfish, which are the focus of much of Dabiri's work and much easier to see than krill. When fluorescent dye was injected into the water in front of the jellyfish, the dye traveled right along with them, often for long distances.

Dabiri and Katija calculated the impact of this so-called biogenic ocean mixing for a broad range of species. Says Dabiri, "There are enough of these small animals in the ocean that, on the whole, the global power input from this process is as much as a trillion watts of energy—comparable to that of wind forcing and tidal forcing."

And then there's a downbound process that they have yet to analyze, says Dabiri. Fecal pellets and marine "snow" made up of falling organic debris probably pull surface water toward the deeps. "This may have an impact on carbon sequestration on the ocean floor," says Dabiri. "It's something we need to look at in the future." Both effects will need to be incorporated into the computer models of global ocean circulation used to study climate change.

A paper on the work, written by Katija and Dabiri, appeared in the July 30 issue of Nature. The work was supported by grants from the National Science Foundation, the Office of Naval Research, the Department of Defense, and the Charles Lee Powell Foundation. —LO