Jellyfish Have Big Mixing Effect on the Oceans

Jeanna Bryner, livescience.com 29 Jul 09;

Pulsating jellyfish and their swim pals stir up the oceans with as much vigor as tides and winds, scientists have found. Their study also found that the shape of the aquatic blobs affects their mixing abilities.

Until now, oceanographers had dismissed the idea that such tiny ocean creatures could play a role in mixing various layers of ocean water on a large scale. The argument was based on evidence that any swishing from fish tails, say, would get dampened by the ocean's viscosity (a measure of a fluid's resistance to flow - honey has a high viscosity compared with water).

But the new study, which is published in the July 30 issue of the journal Nature, reveals a mixing mechanism first described by Charles Darwin's grandson that is actually enhanced by the ocean's viscosity, making these tiny sea critters major players in ocean mixing.

"We've been studying swimming animals for quite some time," said John Dabiri, a Caltech assistant professor of aeronautics and bioengineering. "The perspective we usually take is that of how the ocean - by its currents, temperature, and chemistry - is affecting the animals. But there have been increasing suggestions that the inverse is also important - how the animals themselves, via swimming, might impact the ocean environment."

After all, each day, billions of tiny krill and some jellyfish migrate hundreds of meters from the depths of the ocean toward the surface where they feed. And with swarms of the gelatinous organisms popping up across the world's oceans, if the swimmers do indeed mix the water their impact could be major.

"There are enough of these animals in the ocean," Dabiri said, "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."

Biologic blender

Dabiri and Caltech graduate student Kakani Katija discovered the jellyfish mixing with computer simulations and field measurements of jellyfish swimming in a lake in Palau in the Pacific Ocean.

In their field experiments, the researchers squirt fluorescent dye into the water in front of the Mastigias jellyfish and watched what happened as the animals swam through the dyed water. Rather than being left behind as the jellyfish swam by, the dyed water travelled along with the swimming creatures.

Here's how the researchers think it works: As a jellyfish swims, it pushes water aside and creates a high-pressure area ahead of the animal. The region behind the jellyfish becomes a low-pressure zone. Then, the ocean water rushes in behind the animal to fill in this lower pressure gap. The result: Jellyfish drag water with them as they swim.

"What's really cool about these jellies [is] they have huge variation in their body shapes," Katija told LiveScience.

And they found such differences can impact the amount of water that hitches a ride with the jellies. For instance, moon jellyfish (the kind typically seen at aquariums) have saucer-shaped bodies and can carry a lot of water with them. But other bullet-shaped jellyfish would drag smaller volumes of water.

Global impact

The ocean churning has broader implications.

Without any mixing, the surface of the ocean would lack nutrients, as any food gets gobbled up immediately, while the ocean bottom would remain deplete of oxygen. "With this mechanism, through mixing the animals can pull nutrient-rich fluid up to nutrient-poor areas and pull oxygen-rich fluid down to oxygen-poor regions," Katija said.

And on larger scales, the biologic blender could impact the ocean circulation, which affects the Earth's climate.

Dabiri and Katija say such mixing effects should be incorporated into computer models of the global ocean circulation.

Fauna play key role in circulating seas, says study
Yahoo News 29 Jul 09;

PARIS (AFP) – Creatures large and small may play an unsuspectedly important role in the stirring of ocean waters, according to a study released Wednesday.

So-called ocean mixing entails the transfer of cold and warm waters between the equator and poles, as well as between the icy, nutrient-rich depths and the sun-soaked top layer.

It plays a crucial part in marine biodiversity and, scientists now suspect, in maintaining Earth's climate.

The notion that fish and other sea swimmers might somehow contribute significantly to currents as they moved forward was first proposed in the mid-1950s by Charles Darwin, grandson of the the legendary evolutionary biologist of the same name.

But this was dismissed by modern scientists as a fishy story.

In 1960s, experiments compared the wake turbulence created by sea creatures with overall ocean turbulence. They showed that the whirls kicked up by microscopic plankton or even fish quickly dissipated in dense, viscous water.

On this evidence, sea creatures seemed to contribute nothing to ocean mixing. The clear conclusion was that the only drivers of note were shifting winds and tides, tied to the gravitational tug-of-war within our Solar System.

But the new study, published in the British science journal Nature, goes a long way toward rehabilitating the 20th century Darwin, and uses the quiet pulse of the jellyfish to prove the case.

Authors Kakani Katija and Joan Dabiri of the California Institute of Technology devised a laser-based system for measuring the movement of liquid.

They donned scuba gear and then released dye in the path of swarm of jellyfish in a saltwater lake on the Pacific island of Palau.

The video images they captured showed a remarkable amount of cold water followed the jellyfish as they moved vertically, from deeper chillier waters toward the warmer layers of the surface.

Katija and Dabiri say the 1960s investigators had simply been looking in the wrong place.

They had been on the alert for waves or eddies -- signs that the sea was being stirred up in the creatures' wake -- rather than vertical displacement of water.

What determines the amount of water that is mixed is the size and shape of the animal, its population and migratory patterns.

Churning of the seas is a factor in the carbon cycle.

At the surface, plankton gobble up carbon dioxide (CO2) through photosynthesis. When they die, their carbon-rich remains may fall gently to the ocean floor, effectively storing the CO2 for millennia -- or, alternatively, may be brought back to upper layers by sea currents.

William Dewar of Florida State University in a commentary, also published in Nature, said the new paper challenged conventional thinking.

"Should the overall idea of significant biogenic mixing survive detailed scrutiny, climate science will have experienced a paradigm shift," he said.

Shrimpy Sea Life May Mix Oceans as Much as Tides and Winds Do
Researchers are applying observations made by Charles Darwin's grandson to find that small organisms carry water with them as they go--which means they might play a big role in mixing vast tracts of ocean water
Katherine Harmon, Scientific American 29 Jul 09;

Here's a puzzle: A child pees in the shallow end of a pool and then swims to the deep end. Which end should you avoid? Conventional wisdom holds that the deep end would be safe (until the pool's normal circulation mixed the contaminated water throughout). But according to new research—and old observations by Charles Darwin, the grandson of the more famous Charles Darwin—it would be wise to avoid most of the child's path through the water.

The force at work, called induced drift, happens in the sea, too. In centuries past, people thought that the movement of ocean water was the result of the sun's and moon's tidal forces, Earth's rotation and weather, along with fishes' fluttering tails, notes William Dewar, a professor of physical oceanography at Florida State University in Tallahassee. As it turns out, those earlier thinkers might not have been as off base as many contemporary scientists have assumed.

According to a paper that will be published tomorrow in Nature, the induced drift caused by billions of swimming creatures, especially small crustaceans, could be a force on par with the tides and wind in mixing ocean water. (Scientific American is part of the Nature Publishing Group.)

In a swimming pool the mixing might not be so necessary—or even desirable—but in the open ocean, mixing is an important way to move nutrients among layers and to maintain temperature balances that keep currents flowing.

The idea isn't new: Dewar, who wrote an accompanying views piece, co-authored a 2006 paper in the Journal of Marine Research, which observed increased water turbulence in large schools of krill and proposed that the creatures' tiny flutters could be churning waters on a large scale. "Zooplankton on the whole are pretty small," Dewar concedes. "Because of that, there are some legitimate concerns about how effectively they can mix [layers of] water."

Wouldn't the water's viscosity—its resistance—quickly overcome tiny amounts of turbulence caused by small zooplankton?

Actually, the new study's authors, Kakani Katija and John Dabiri from the California Institute of Technology in Pasadena, have shown that far from hindering the movement, water's viscosity actually enhances it. The principle observed by Darwin was that a solid body, whether it is a car or krill, moving through a fluid (air or water, respectively) will tend to take some of that fluid's particles along with it—hence the concept of induced drift. And as that fluid becomes thicker and more inclined to stick to the object, the amount of drift increases.

To put it simply, Dewar explains: "These animals go swimming, and they take the water along with them. It looks like they're pretty good at this."

For their research, Katija and Dabiri trained their sites not on krill but on small jellyfish, which can also swarm in large schools. They tracked how individual jellyfish carried water as they swam upward in the water column by observing the track of glowing dye injected into the water [see video below] as well as by measuring the kinetic energy the jellies generated in their wakes.

But why settle for such small sea dwellers? Although one might expect massive animals, such as whales, to have more impact on mixing individually, Dabiri, an assistant professor of aeronautics and bioengineering, explains that smaller organisms that travel in large schools—crustaceans and zooplankton for example—would have more of a global impact because they're so widespread and numerous.

Per Darwin's theory, however, it is not just critical mass that matters, but body shape. Dabiri explains that the quickest and most efficient swimmers—those that are smooth and bullet-shaped—are the least effective mixers, whereas slower and more saucer-shaped creatures will drag along proportionately more water.

How much water is moving? For it to have much importance for mixing purposes, water needs to be carried about a meter. From the observations and numerical simulations, Dabiri notes, "We expect that fluid is being carried at least on the magnitude of meters—if not tens of meters."

Extrapolating from their work, Katija and Dabiri suggest that in large schools these organisms likely have an even greater mixing power. In a massive krill migration for example, "it will be much more difficult for water to slip through the cracks" and not be carried along, Dabiri says.

But no one is quite sure how—and whether—the dynamic is actually playing out across the world's oceans. "It's not clear how you will go from that to a global model," Dewar says. Other considerations include how organisms' swimming style would affect water transport and how the combined force of these animals' drift might add up to a worldwide impact on ocean circulation. If it turns out to be as large a component as some are beginning to think, it will need to be incorporated into computer climate models. And that would be no small task because today's models are not nuanced enough to include data at the level of a school, much less an individual animal—to say nothing of complexities involving possible feedback loops down the road.

"Our paper raises more questions than it answers," Dabiri acknowledges. But, he says, it is casting light on what might be an important dynamic of oceans that has been right under our noses—or at least our hulls.

Jellyfish And Other Small Sea Creatures Linked To Large-scale Ocean Mixing
ScienceDaily 29 Jul 09;

Using a combination of theoretical modeling, energy calculations, and field observations, researchers from the California Institute of Technology (Caltech) have for the first time described a mechanism that explains how some of the ocean's tiniest swimming animals can have a huge impact on large-scale ocean mixing.

Their findings are being published in the July 30 issue of the journal Nature.

"We've been studying swimming animals for quite some time," says John Dabiri, a Caltech assistant professor of aeronautics and bioengineering who, along with Caltech graduate student Kakani Katija, discovered the new mechanism. "The perspective we usually take is that of how the ocean—by its currents, temperature, and chemistry—is affecting the animals. But there have been increasing suggestions that the inverse is also important—how the animals themselves, via swimming, might impact the ocean environment."

Specifically, Dabiri says, scientists have increasingly been thinking about how and whether the animals in the ocean might play a role in larger-scale ocean mixing, the process by which various layers of water interact with one another to distribute heat, nutrients, and gasses throughout the oceans.

Dabiri notes that oceanographers have previously dismissed the idea that animals might have a significant effect on ocean mixing, saying that the viscosity of water would damp out any turbulence created, especially by small planktonic animals. "They said that there was no mechanism by which these animals could impact large-scale ocean mixing," he notes.

But Dabiri and Katija thought there might be a mechanism that had been overlooked—a mechanism they call Darwinian mixing, because it was first discovered and described by Charles Darwin. (No, not that Darwin; his grandson.)

"Darwin's grandson discovered a mechanism for mixing similar in principle to the idea of drafting in aerodynamics," Dabiri explains. "In this mechanism, an individual organism literally drags the surrounding water with it as it goes."

Using this idea as their basis, Dabiri and Katija did some mathematical simulations of what might happen if you had many small animals all moving at more or less the same time, in the same direction. After all, each day, billions of tiny krill and copepods migrate hundreds of meters from the depths of the ocean toward the surface. Darwin's mechanism would suggest that they 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.

What the Caltech researchers also found was that the water's viscosity enhances Darwin's mechanism and that the effects are magnified when you're dealing with such minuscule creatures as krill and copepods. "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."

"This research is truly reflective of the type of exciting, without-boundaries research at which Caltech engineering professors excel—in this case a deep analysis of the movement of fluid surrounding tiny ocean creatures leading to completely revelatory insights on possible mechanisms of global ocean mixing," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech.

To verify the findings from their simulations, 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) traveled to the island of Palau, where they studied this animal-led transport of water--otherwise known as induced drift--among jellyfish, which are the focus of much of Dabiri's work.

"From a fluid mechanics perspective, this study had less to do with the fact that they're jellyfish, and more to do with the fact that they're solid objects moving through water," Dabiri explains.

Katija's jellyfish experiments involved putting fluorescent dye in the water in front of the sea creatures, and then watching what happened to that dye—or, to be more specific, to the water that took up the dye—as the jellyfish swam. And, indeed, rather than being left behind the jellyfish—or being dissipated in turbulent eddies—the dye travelled right along with the swimming creatures, following them for long distances.

These findings verified that, yes, swimming animals are capable of carrying bottom water with them as they migrate upward, and that movement indeed creates an inversion that results in ocean mixing. But what the findings didn't address was just how much of an impact this type of ocean mixing—performed by impossibly tiny sea creatures—could have on a large scale.

After a series of calculations, Dabiri and Katija were able to estimate the impact of this so-called biogenic ocean mixing. And, Dabiri says, it's quite a significant impact.

"There are enough of these animals in the ocean," he notes, "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."

In other words, the amount of power that copepods and krill put into ocean mixing is on the same scale as that of winds and tides, and thus their impact is expected to be on a similar scale as well.

And while these numbers are just estimates, Dabiri says, they are likely to be conservative estimates, having been "based on the fluid transport induced by individual animals swimming in isolation." In the ocean, these individual contributions to fluid transport may actually interact with one another, and amplify how far the ocean waters can be pulled upward.

In addition, says Dabiri, they have yet to consider the effects of such things as fecal pellets and marine snow (falling organic debris), which no doubt pull surface water with them as they drift downward. "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."

Dabiri says the next major question to answer is how these effects can be incorporated into computer models of the global ocean circulation. Such models are important for simulations of global climate-change scenarios.

The work was supported by grants from the National Science Foundation's Biological Oceanography, Ocean Technology, Fluid Dynamics, and Energy for Sustainability programs, and by the Office of Naval Research, the Department of Defense's National Science and Engineering Graduate Fellowship, and the Charles Lee Powell Foundation.

Journal reference:

1. Dabiri et al. A viscosity-enhanced mechanism for biogenic ocean mixing. Nature, July 30, 2009

Adapted from materials provided by California Institute of Technology, via EurekAlert!, a service of AAAS.