Sea Animals Change Climate Via Flutters and Flaps?

Brian Handwerk
for National Geographic News
July 30, 2009
Predicting global warming is far from an exact science, and it may have just gotten even more complicated.

Despite having been largely ignored by climate science, sea creatures' countless tentacle snaps, fin flaps, and tail twitches are responsible for a third or more of all "ocean mixing"—as much as winds or tides—according to a new study of jellyfish.

This mixing of seawater layers— and their salt, nutrients, carbon dioxide, and even heat—helps guide ocean circulation, which, like the atmosphere, moves heat around the planet.

If animal-driven ocean mixing is extensive and unaccounted for, as the new study argues, climate models—including those used to forecast global warming—may be off target.

(Related: "Ocean 'Thermostat' May Be Secret Weapon Against Warming.")

Ocean in Motion

Some scientists argue that an animal such as a jellyfish is simply too small to create turbulence on a scale that could effectively mix layers of ocean water.

"You have to be stirring the fluid with a big-enough spoon to actually mix together waters of really different temperatures," explained Florida State University oceanographer William Dewar, who was not involved in the new study. Dewar comments on the debate in today's issue of the journal Nature.

But the study authors believe that even small swimmers stir the ocean in a big way, via a mechanism that Sir Charles Darwin, grandson of the legendary scientist, described half a century ago.

As an animal moves through the sea, it pulls some of the surrounding water along for the ride, explained Kakani Katija, a Ph.D. candidate in bioengineering at the California Institute of Technology.

Katija and colleagues witnessed this effect while diving with jellyfish in Palau's saltwater Jellyfish Lake, which is relatively protected from other mixing agents like wind and tides.

During dives in the lake, the team used turkey baster-like tools to squirt brightly colored dye along jellyfish's leading edges. As the jellyfish swam on, each took along an "aura" of the dyed water.

The amount of water moved, of course, varies with an animal's size and shape. But the distance an animal travels is also a factor.

"The drifting fluid essentially stretches along the length of the migration, so if an animal migrates 500 meters [1,600 feet], that [water] will move 500 meters," explained Katija, who co-authored the new study, which appears in today's issue of Nature.

And though jellyfish don't cross seas like finned animals, in a single day the spineless beasts often ascend and descend hundreds of yards, migrating through multiple ocean layers.

Stirring It Up

While seas may seem turbulent on the surface, deep below they're surprisingly calm—and can be mixed by even small-scale disturbances.

In the subsurface ocean, for example, a typical handled mixer "would provide enough energy to mix a cubic kilometer [about 264,200 gallons] of ocean," Florida State's Dewar said.

Needless to say, "if a giant squid swims through, you've got a significant bit of mixing right there."

But research scientist André Visser countered that the seas are "quite a thin soup of organisms"—there might not be enough animals to create significant global mixing.

"Organisms can mix the water, that's true," said Visser, of the Technical University of Denmark. "We know that dense assemblages of swimming fish or of bivalves pumping water on the sea bed can have large effects on local conditions.

"But the question is, do they have impact on a global scale that can [influence] climate? That's where the whole story becomes a bit fuzzy."

Missing Pieces

Study co-author Katija said she's confident that the mechanism works on a global scale. But even she has questions about just how swimming animals affect the ocean.

"For example, what happens if you have a huge population, or school, of animals that are moving in concert?" she said. "Do you perhaps treat that whole population as a body itself?"

Also blurry are the effects of different swimming styles on water transport: a shark's vigorous tail thrusts, say, versus a jellyfish's slow-motion undulations. Even the countless things that sink passively to the ocean floor, like fish droppings, could conceivably help to mix the waters.

For now, the concept is highlighting just how much we don't know about ocean mixing and how it affects our climate, both present and future.

"I think it's really quite important work, because at the moment nobody accounts for any biological feedback in climate models," Florida State's Dewar said. "I think this paper will cause people to think about it a lot more."

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