The currents caused by large, swirling eddies at the ocean's surface may reach all the way to the sea floor, a new study suggests. The unexpected finding may help explain how the larvae of organisms living at isolated hydrothermal vents can be transported hundreds of kilometers to colonize new vents. And as climate change affects surface eddies, it may also reach the ocean's depths.
As any sailor knows, conditions at the ocean's surface can change in a moment. But most scientists presume that the environment on the sea floor is fairly stable: Temperatures hover near freezing, darkness reigns, and currents are languid and steady. Well, scratch that last one. Measurements taken at a hydrothermal vent system in the eastern Pacific indicate that currents can be highly variable, in some cases tripling in speed for an extended period, and analyses strongly suggest that eddies at the ocean's surface are to blame.
In a field study, Diane Adams, a marine biologist at Woods Hole Oceanographic Institution in Massachusetts, and her colleagues measured the currents near the seafloor along the East Pacific Rise, a submarine ridge south-southwest of Acapulco, Mexico, that sports many hydrothermal vent systems. They suspended sensors 170 meters above the 2350-meter-deep ridge, high enough to remain unaffected by ridge-related turbulence. They also suspended traps just 4 meters above the sea floor to measure the amount of minerals spewed by the vent systems and settling back to the ocean floor, as well as to count the number of larvae produced by creatures living in the warm oasis. Because the vents usually maintain a steady flow and the creatures that live there reproduce continually, minerals and larvae fall into the traps from the cloudy waters above at fairly steady rates.
From November 2004 through April 2005, typical currents at the site flowed from the north at an average speed of about 5.5 centimeters per second, the researchers report online today in Science. Moreover, says Adams, the currents rarely rose above 10 centimeters per second. But in March 2005, currents shifted suddenly and flowed from the south at speeds that sometimes exceeded 15 centimeters per second. During the same interval, the amounts of sediment and vent-creature larvae that fell into the team's traps dropped dramatically—indicating the cloud of minerals and larvae had been carried away, at least temporarily, as if a strong storm system had swept the stale air from a polluted valley.
Later, while searching for a possible reason for the anomalous currents, Adams and her colleagues found that a large, clockwise-spinning eddy at the ocean's surface—one measuring about 375 kilometers across—had crossed the area at about the same time. Then the team's computer simulations showed that eddies could trigger changes in sea floor currents matching the patterns measured by the instruments, with the best correlation occurring when the effects on deep-sea currents happened 8 days after the eddy passed overhead.
"This is a huge response at the bottom, and passage of the surface eddy probably isn't coincidental," says Dudley Chelton, a physical oceanographer at Oregon State University, Corvallis. "This will change the way we think about the ocean," largely because the effects of such eddies weren't suspected to extend so deeply, adds Cindy Van Dover, a biological oceanographer at the Duke University Marine Laboratory in Beaufort, North Carolina.
The unusual changes in currents may help explain how the larvae of the heat-loving creatures living around hydrothermal vents are dispersed through long stretches of near-freezing waters to reach other warm havens, says Adams. Lab tests show that such larvae drop into a sort of suspended animation when immersed in cold water but can survive in that state for only 30 days or so. Although the occasional eddy-induced currents would likely sweep many larvae to a frigid doom, some of them would luck out and drift to distant oases that couldn't be reached if currents had remained slow and steady.
Many of the surface eddies in this region of the Pacific are generated by winds spilling westward off the coast of Central America, and those winds vary with the seasons and may become stronger or more frequent as climate changes in the future. Therefore, Van Dover says, the eddy-induced currents may offer a way for climate change to affect the deep sea sooner than expected and in a way scientists hadn't been thinking about. Organisms that have evolved in environments that have little if any change in environmental conditions, for example, may not be able to adapt well if currents increasingly mix warm surface waters down to the seafloor.