Aug 3, 2009
Jellyfish mix up the oceans
The amount of ocean mixing caused by ocean-dwelling organisms has to date proved controversial. While large clusters of planktonic animals can cause increased turbulent dissipation, this disturbance may be lost as heat before it mixes the water.
Further investigation has revealed that a mechanism discovered by Charles Darwin's grandson fifty years ago is the major cause of ocean mixing by organisms. In this induced drift phenomenon, as a creature such as a jellyfish travels through the ocean it causes water to move along behind it.
"Through this mechanism, an animal can mix a fluid efficiently when it swims," Kakani Katija Young of the California Institute of Technology, US, told environmentalresearchweb. "The resultant mixing in the ocean from biological sources is on the same order as winds and tides. Therefore biological inputs to current ocean and global climate models need to be taken into account."
Young and colleague John Dabiri used a sophisticated underwater laser-video camera system (SCUVA) and fluorescent dye to measure the fluid flow caused by jellyfish in a saltwater lake on the Pacific Island of Palau. Later they calculated the fluid mixing that would be produced by smaller organisms, such as single-celled plankton.
"Our observations are the first of their kind that identify drifting fluid that follows an animal when it swims," said Young. "Unlike mixing by fluid structures generated in an animal's wake, the drift mixing mechanism is in fact enhanced when an animal becomes smaller and is active along the length of animal migration."
The researchers found that the amount of water that drifts after a moving animal depends on its volume and length-to-diameter ratio. For jellyfish of the Mastigias genus, which have a body diameter of 1–10 cm, induced drift dominated the mixing process, causing 90% of the potential energy increase.
On a worldwide scale, the team calculated that induced fluid drift provides ocean mixing power of the order of 1012 W of total kinetic energy input per unit time – comparable to that of winds and tides. The researchers reckon this figure is conservative because it does not account for animals swimming in shoals, which may increase the effect, and does not include passively sinking particles such as marine snow or faecal pellets.
"Should the overall idea of significant biogenic mixing survive detailed scrutiny, climate science will have experienced a paradigm shift," wrote William Dewar of Florida State University in an accompanying News and Views article in Nature. "To quote Carl Wunsch, modellers will 'need to start thinking about the fluid dynamics of biology', to which he added, 'that's a tough one' – as, indeed, it is."
Now Young plans to investigate the effects of swimming distance, animal body surface motion, propulsion methods and population density on the drift volume. "I hope to conduct additional measurements and simulations to address the full impact of biogenic mixing on the ocean and global climate," she said.
The researchers reported their work in Nature.
About the author
Liz Kalaugher is editor of environmentalresearchweb.