Alvara Migotto/The Cnidarian Tree of Life project

Dynamo. The humble Mastigias jellyfish and its cousins help keep seawater churning and nutritious.

Little Creatures Can Stir Big Oceans

When it comes to churning up the world's oceans, Mastigias jellyfish are quite the little blenders. New research suggests that large groups of the small, placid creatures--along with all of the sea's other motile beings--can mix as much heat, gases, and nutrients through the water column as the winds and tides do.

On the surface, the sea is a roiling mass. But dip 100 meters below and the water is calm. How, then, do the world's oceans distribute heat and food throughout their depths? Currents driven by salinity and temperature differences can transport a lot. But another part of the answer comes from an idea conceived by the grandson of Charles Darwin. About a half-century ago, the famed naturalist's descendant--also named Charles--proposed that a body moving through a fluid would tend to drag some of that fluid with it. Applied to the oceans, the hypothesis means that the churning action created when aquatic creatures swim--even the smallest and slowest--might stir a significant amount of water.

Most scientists have remained skeptical, however, arguing that small marine creatures in particular could not overcome water's viscosity enough to circulate much of anything. Now, it turns out, the idea first posed by Darwin's grandson may be right.

Bioengineers Kakani Katija and John Dabiri, both of the California Institute of Technology in Pasadena, and their colleagues tested the idea by performing a series of remarkably simple experiments last September in a lagoon on the South Pacific island of Palau. There, they studied how the Mastigias jellyfish mixed water as the softball-size creatures swam peacefully by the scuba-diving researchers. "Their relatively simple swimming motions make them amenable to a careful study of fluid dynamics," Dabiri writes in an e-mail. "We consider them a model system."

The researchers recorded the water-mixing effects of the jellyfish by placing a harmless dye near the creatures and tracking how the dye moved with video cameras. Then the team analyzed the results using computer simulations and calculated the amount of energy produced. The observations showed that despite water viscosity, Mastigias jellyfish mixed a surprising volume of water. So much, based on estimates of the number of living creatures in the ocean and their collective efforts, that the mixing is comparable to that of the winds and the tides, the researchers will report tomorrow in Nature.

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Churn, baby, churn. Watch a Mastigias jellyfish stir up some water.
K. Katija/J. Dabiri

The results "open up fascinating possibilities" for how the marine biosphere can create more favorable conditions for itself, says oceanographer Gary Shaffer of the University of Copenhagen. They imply, for example, that the daily vertical migrations of zooplankton found off the coasts of Peru and Chile help transport nutrients to the ocean surface where those nutrients can feed the phytoplankton that the zooplankton eat, he explains.

Oceanographer William Dewar of Florida State University in Tallahassee agrees. "The study suggests that even relatively small marine animals can play a major role in ocean mixing," he says. "Since many of them move up and down hundreds of meters each day, their effects could be important."

Posted in Earth