TURN OFF THE LIGHT: Giant tube worms and mussels thrive near hydrothermal vents. Their tiny embryos can hitch rides on the swift currents of ocean eddies. Image: Monika Bright, University of Vienna, Austria
Giant swirling masses of seawater known as mesoscale eddies roam the world's oceans. Whipped up by surface winds and girded by the Coriolis effect (produced by Earth's rotation), eddies may grow to several hundred kilometers in diameter and are known to transport heat, chemicals and biology throughout the oceans' shallower depths. A new study published April 29 in Science suggests that eddies may have a deeper reach than previously thought, helping to shape some of the most remote ecosystems on Earth—deep-sea hydrothermal vents.
Hydrothermal vents occur at oceanic ridges where brand-new seafloor emerges from the ductile depths of the mantle. Temperatures at these vents can top 400 degrees Celsius and thick plumes of iron and sulfur cloud the permanently pitch-black waters. But life teems even where the sun doesn't shine. In the vicinity of these vents, where temperatures hover at a cozy 30 degrees Celsius, thick mats of chemosynthetic bacteria convert vent chemicals into energy, and in turn support colonies of giant tube worms, huge beds of mussels and a variety of crustaceans.
"It's as close to what I imagine another world would look like," says Diane Adams who worked on this study as a biological oceanographer at the Woods Hole Oceanographic Institution. (She is now at the National Institutes of Health.) It's not Kansas down there, but the new research suggests that powerful deep-sea cyclones—the whirling underbellies of eddies—sweep through these communities on a seasonal basis, transporting vent products to far-off waters. By correlating the flux of chemical sediments and organisms with the passage of eddies, researchers say they have identified a new mechanism by which vent products are transported around the world.
The vast distances that separate hydrothermal vents make them similar to isolated island ecosystems, Adams says. Yet there is evidence of high gene flow among vents. Deep-sea gastropods, like snails and mussels, have been found hundreds of kilometers from their native vents. These creatures are most mobile during their free-floating larval stage, but just how life traverses long stretches of cold dark oceans from one vent oasis to another has not been well understood.
To study the movement of vent products, the researchers set up sediment traps and current meters near the hydrothermal vents along the East Pacific Rise, an ocean ridge located about 800 kilometers off the southern coast of Mexico and a mile and a half below sea level. Over a five-month period from December 2004 to April 2005, the traps collected samples of sediments and larva while the meters recorded deep-sea current velocities. Reviewing the data, researchers noticed a dramatic decrease in larvae and sediments in March and April that corresponded to a period of unusually strong currents. With speeds of 15 centimeters per second, these currents were two to three times the norm, and they were continually changing directions.
Intrigued, the researchers checked satellite observations for large-scale hydrodynamic events during that period that might account for anomalous currents. They found several, including a 375-kilometer-diameter eddy that crossed the study site from February to March 2005, just before the strong deep currents and drop off in sediments and larvae. A computer simulation of the eddies produced similar deep-sea currents, giving support to the correlation between the two observations.
"Based on the data that they have, even though it's circumstantial, it's a nice story," says Richard Thomson, a physical oceanographer at Fisheries and Oceans Canada who was not involved with the study. Thomson contends that atmospheric forces are well known to influence movement in the deep ocean, but that the key new idea here is that surface-generated eddies can be a transport mechanism for vent products. "These eddies tend to have some integrity, they entrap material in them and carry it out into the deep ocean," he says. More data is needed to confirm the connection between eddies and the transport of vent products, Thomson says, but he also acknowledges the extraordinary resources required to gather longitudinal data on deep-sea phenomena.
The eddies that pass over the East Pacific Rise are generated by westward winds that come through mountain passes in Central America. About two to three eddies cross the rise each year, mostly during the winter and spring. Eddies are globally distributed, of different strengths and sizes, and some can last for months. Some form from instability in the Gulf Stream, whereas others seem to increase in strength and frequency during El Niño years. Thus, eddies can lend seasonal and even climate-scale components to the generally weatherless deep ocean. "The deep ocean is not severed from what's happening in the upper ocean; they're linked," Thomson says.
Even if larvae are carried off by these currents, there is no guarantee that they will find their way to another vent in time. The present study has not demonstrated that the larva can survive the journey through cold, nutrient-poor waters, Adams says. The particular chemistry and high pressures of vent habitats are difficult to replicate on terra firma, so the majority of deep-sea species have not been cultured in laboratories and much is unknown about their life cycles. One tube worm species that has been cultured has a larval stage of about 38 days, Adams says, which may be an approximate limit on how long they can last adrift. "The next question is where are they going?" Thomson says, "And how are they doing, how do they survive?"