Tiny Krill Key to Ocean Turbulence, Study Says
for National Geographic News
|September 21, 2006|
Mass movements of marine crustaceans called krill generate turbulent currents that may help pump nutrients from the ocean depths to surface waters, researchers say.
In British Columbia, Canada, billions of the tiny swimmers churn the seas as they migrate each night between their safe daytime havens deep underwater to food-rich surface waters (related video: "Krill Swarm Seas by the Billions").
A new study, which appears in tomorrow's issue of the journal Science, shows that the creatures' nightly migration greatly increase the daily mixing of deep and shallow waters.
The finding, by Eric Kunze of the University of Victoria and colleagues, is the strongest evidence to date that marine organisms contribute significantly to turbulent mixing in the oceans, particularly in coastal areas.
Turbulent mixing caused by wind and other physical forces circulates nutrients that are vital to the productivity of marine ecosystems.
(Related news: "Ocean 'Conveyor Belt' Sustains Sea Life, Study Says" [June 2004].)
Ocean mixing also strongly affects the exchange of heat and gases, such as carbon dioxide, between the atmosphere and the ocean.
Stir It Up
To determine the crustaceans' mixing effect, researchers looked at the daily migration of the krill species Euphausia pacifica in Saanich Inlet, a fjord-like ocean arm in southern British Columbia (map of British Columbia).
E. pacifica is one of the most common krill species in the northeast Pacific Ocean, reaching densities of up to 10,000 individuals per 1.2 square yards (1 square meter).
The 0.4-inch-long (1-centimeter-long) creatures spend the daylight hours about 328 feet (100 meters) below the ocean surface to avoid detection by visually oriented predators such as herring.
At night the animals rise en masse to feed on algae that predominantly live in surface waters.
The researchers found that the rapid nightly ascent of E. pacifica generated turbulence up to 10,000 times higher than background levels.
While the peak rate of upward swimming lasted only 15 minutes, the burst of turbulent energy the krill generated greatly increased ocean mixing.
It's the fact that the animals move in synch that creates a significant effect, Kunze says.
Individual krill generate a jet of water that propels them forward at a top speed of about 2 inches (5 centimeters) a second (related wallpaper: solitary shrimp krill).
Turbulence generated by E. pacifica would be quickly dampened if the animals were more widely spaced.
But during the vertical ascent, Kunze says, "the krill are swimming together like a school of fish.
"This allows them to draft each other like bicyclists, making the school, rather than the individual, the defining area of the turbulence."
Kunze says the energy generated by the animals' collective ascent is comparable to that found in strong tidal channels.
The researchers have not yet conducted studies to determine the exact effects of the biologically generated turbulence in the Saanich Inlet ecosystem.
It is well known, however, that turbulent mixing of ocean layers is an important source of nutrients for algae and other plantlike organisms that form the basis of the marine food chain.
Cold waters that rise from ocean depths carry nutrients to depleted surface waters in a process called upwelling.
But plant growth in the surface layer often exceeds what would be expected based on the amount of nutrients delivered by known physical mechanisms such as winds and tides.
That mystery might be solved if migrating krill are effectively stirring the pot, raising nutrients that would help enhance their own food supply.
"I don't think it's going too far to say the animals are doing a little farming," said biologist Mark Huntley at the University of Hawaii at Manoa.
"The [algae eaten by the krill] are going to respond fairly quickly to an influx of nutrients."
Huntley was one of the first scientists to suggest that marine life may be an important contributor to turbulent mixing in the world's oceans.
In 2004 he and a co-author predicted the amount of turbulent energy generated by various types of schooling marine speciesfrom Antarctic krill to Atlantic bluefin tuna.
The turbulence measured by Kunze's group was very close to Huntley's prediction for krill in the Antarctic.
"We were surprised by the closeness of the agreement," Kunze said.
The researchers caution that it remains to be seen how widespread and significant the phenomenon of biologically generated turbulence really is.
But the study raises the possibility that an important source of mixing in the upper ocean has been largely overlooked.
"One of our reasons for publishing our preliminary results from Saanich Inlet," Kunze said, "was to alert the larger oceanographic community to this phenomenon, so they can look for it in their data."
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