Deep sea taxis

Invertebrate larvae at hot, biologically active deep sea vents can be swept up and transported hundreds of kilometers by surface-generated currents caused by the weather, according to research published this week in linkurl:Science.;http://www.sciencemag.org/

Hydrothermal vents on the sea floor
Copyright Woods Hole Oceanographic Institution

The discovery provides one solution to the long-standing question of how materials -- including heat, chemicals and living organisms -- present at hydrothermal vents are transported throughout the ocean."What's neat about this paper is the idea that right next door to a mid ocean ridge system is a source of energetic eddies that can slowly flush things out into the ocean and populate other areas," said linkurl:Richard Thomson;http://www.meds-sdmm.dfo-mpo.gc.ca/sdb-bds/profile-profil.do?id=1230&lang=eng of the Institute of Ocean Sciences in Canada, who was not involved in the research. "That's new and innovative."Documented gene exchange between distant vent ecosystems and observations of the rapid colonization of new vents have long suggested that larvae are somehow dispersed from one hydrothermal vent to another. Yet decades of research have failed to uncover how the animals are transported such long distances. "This is one of the more fundamental and exciting problems in oceanography," said Thomson.To investigate possible mechanisms for long-distance transport of larvae, linkurl:Diane Adams;http://www.whoi.edu/hpb/Site.do?id=4152 and colleagues at Woods Hole Oceanographic Institution, along with collaborators at numerous other institutions, spent six months collecting specimens and data on currents at the East Pacific Rise, a tectonic plate boundary rich in hydrothermal vents located about 500 miles off the coast of Mexico.

Resarchers haul in one of the moorings equipped with current meters and a time series sediment trap
Copyright B. Govenar, Woods Hole Oceanographic Institution

During their observations, the researchers noticed a pattern: dramatic decreases in larval populations at the vents coincided with increases in local current flow, often doubling or tripling in speed compared with normal, slow deep ocean currents. One team member made an offhand remark that the current changes looked like eddies, currents generated by winds and atmospheric forces that flow in a roughly circular motion. "So we went and looked at the satellite [data] to see whether or not one of these eddies was even in the vicinity at the time when we saw this [decrease in larvae]," Adams told The Scientist. "And sure enough, there was one there." Using modeling simulations, the team confirmed that such an eddy could indeed induce extremely deep currents, about 2,500 meters (1.5 miles) below the surface -- deep enough to sweep away larval populations from hydrothermal vents. "It was surprising to see these eddies, at the surface, have such a large effect on what was going on that far down," said Adams, who recently accepted a position at the National Institutes of Health. The team concluded that the eddies are likely responsible for picking up the larvae and transporting them, via the circular flow of water, to other hydrothermal vents on the ridge, even hundreds of kilometers away, as well as pushing some of the larvae out to sea.

Light micrograph of a larva of the limpet gastropod
Copyright S. Beaulieu, Woods Hole Oceanographic Institution

There could certainly be other mechanisms by which materials are transported to and from hydrothermal vents, "but this is definitely an important one," said Adams. "It has the possibility of transporting large quantities [of vent larvae] episodically."And because eddies are caused seasonally by wind and other weather conditions, the findings suggest that climate phenomena such as El Nino have the potential to cause seasonal variations deep in the ocean. "Tying variability in climate to flushing these organisms out into the ocean is really interesting," said Thomson. "It's a great idea." Adams, D.K., et al., "Surface-generated mesoscale eddies transport deep-sea products from hydrothermal vents," Science, 332:580-3, 2011.
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[14th March 2007]