In the world's oceans, water circulates in currents that stretch between the continents and glide along coastlines. Water from the deep mixes with shallower water through vertical movements called upwellings and downwellings. This complex ebb, flow, rise, and fall of seawater—also known as ocean mixing—transports energy, churns nutrients, and stirs dissolved gasses. To understand the driving forces behind ocean mixing is to understand a key element of marine environments.
That's why a pair of scientists from Caltech attracted a lot of attention when they suggested an unconventional way in which ocean water may be mixed. Researchers Kakani Katija and John Dabiri proposed that marine animals drag ocean water along with them as they swim through it. They went on to suggest that the cummulative movement of animals, especially small ones such as jellyfish and krill, could mix water on a scale comparable to that produced by winds and tides. The oceans—previously believed to be an impenetrable force that shapes the lives of the marine animals that inhabit it—may in fact be influenced by the multitude of lifeforms that paddle about is waters.
To test their theory, Katija and Dabiri set out to describe exactly what happens to the water around swimming marine animals. Do ocean creatures drag water along with them as they swim? Or do they slip, streamlined, right through it, leaving a churned up column of water in their wake?
To answer these questions, Katija and Dabiri first looked to an unconventional theory in fluid mechanics to describe how organisms might contribute to ocean mixing. The theory they turned to was developed by the grandson of Charles Darwin (it so happens that Darwin's grandson's name is also Charles Darwin). Darwin's theory, called Darwinian mixing, suggests that when an object moves through a liquid, it pulls some of the fluid with it.
Katija and Dabiri used a computer model to simulate Darwinian mixing and found that as viscosities increased, so did the amount of water an object pulled with it as it moved through the water. They also found that small objets dragged along with them proportionally larger amounts of water than larger objects. But computer models weren't enough to convince them that marine animals were trailing drops of seawater around with them. So Katija and Dabiri went to the western Pacific island of Palau to gather data on the swimming intricacies of jellyfish.
Katija and Dabiri used fluorescent dye, which when released into the paths of swimming jellyfish clouded the water and helped illuminate the flow of water around the animal as it swam. They found that as Darwinian mixing predicted, a layer of water formed around the jellyfish as it swam. The layer moved with the jellyfish, so as the animal ascended toward the surface, it brought with it a sampling of the deeper, cooler waters from which it came.
Despite the afterglow of fluorescent dyes, the convincing computer models of Darwinian mixing, and the claims of large scale ocean mixing by jellyfish, Katija and Dabiri have not enjoyed immediate, widespread acceptance for their work. Instead many scientists still feel that animal movement cannot possibly be a significant contributor to ocean mixing. Some opponents to Darwinian mixing say that when an animals swims, instead of pulling a layer of water with it, it glides right through it. The water is mixed up in the wake of the creature, but the turbulent water does not follow the swimming animal. Regardless of which hypothesis of fluid mechanics is correct, further research will be required to understand what role marine animals have on ocean mixing.
References
Katija K, & Dabiri JO (2009). A viscosity-enhanced mechanism for biogenic ocean mixing. Nature, 460 (7255), 624-6 PMID: 19641595
Photo (top) © Michael Dawson / University of California at Merced. Photo (bottom) © Phdpsx / iStockphoto.
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