Paris (AFP) July 29, 2009
Creatures large and small may play an important role in the stirring of ocean waters, according to a study released Wednesday that confirms a theory advanced by Charles Darwin.
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.
In the mid-19th century, Darwin -- best known as the father of evolutionary theory -- proposed that fish and other sea swimmers might somehow contribute significantly to currents as they moved forward.
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 Darwin and his fellow Victorians, 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.
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