Fish tails stir up world's oceans
Creatures large and small may play an unsuspectedly important role in the stirring of ocean waters, according to a study.
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 research is published in today's edition of the journal Nature.
The notion that fish and other sea swimmers might somehow contribute significantly to currents as they move forward was first proposed in the mid-1950s by Charles Darwin, grandson of the legendary evolutionary biologist of the same name.
But this was dismissed by modern scientists as a fishy story.
In the 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.
Old theory re-established
But the new study by Kakani Katija and Joan Dabiri, of the California Institute of Technology, goes a long way toward reviving the 20th century Darwinian view, and uses the quiet pulse of jellyfish to prove the case.
Katija and Dabiri devised a laser-based system for measuring the movement of liquid.
They donned scuba gear and then released dye in the path of a 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.
Influence on climate
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, says the new paper challenges conventional thinking.
"Should the overall idea of significant biogenic mixing survive detailed scrutiny, climate science will have experienced a paradigm shift," he says.