雅思阅读 the deep sea

Deep-sea vents　Ocean-floor migration
　　How surface winds blow deep-sea critters from vent to vent

　　EVER since their discovery in the 1970s, deep-sea vents—chimney-like structures on the ocean floor that belch hot water and dissolved minerals into the surrounding ocean—have been one of the hottest topics in marine biology. The vents support populations of bacteria, giant worms, clams, shrimp and other creatures in the inky darkness, often several kilometres below the surface. Unlike virtually every other ecosystem on the planet, these deep-sea communities do not rely on the sun for their food. Instead of using photosynthesis, the bacteria at the bottom of the food chain harvest energy from chemicals supplied by the vents themselves.
　　The vents are both widely spaced and transient, which means their denizens live a precarious existence. Yet travel between vent systems is apparently possible, even across miles of desolate ocean floor. Creatures confined to islands rapidly head in a different genetic direction to mainland relatives; but researchers have found surprisingly little genetic variation between the populations of even quite widely spaced ocean-bottom vents. Last year one paper described how a vent system that had been wiped clean by a volcanic eruption was quickly recolonised by a variety of larval creatures, some of which seemed to have travelled from another vent more than 300km (190 miles) away. Exactly how has remained a mystery.
　　Now a group of scientists led by Lauren Mullineaux at America’s Woods Hole Oceanographic Institution has described in Science how such transfers could happen—and, in the process, discovered something surprising about how surface weather influences the deep ocean, traditionally thought of as an isolated, closed world.
　　The group was monitoring vents more than 2km beneath the surface in the Pacific Ocean off the coast of Central America when it found that quantities of larva and certain chemicals being emitted both fell sharply during periods of unusually strong deep-sea currents. This is consistent with larva and effluent from the vents being swept away into the open ocean. As a colonisation strategy, dispersal by ocean current would be unreliable—with vent systems so scarce, most of the larva swept out to sea would presumably perish—but plenty of plants on land adopt a similar approach using the wind.
　　The research offers more than just an insight into the life cycle of subsea gribblies. The powerful deep currents were strongly correlated with the passage of wind-generated mesoscale eddies: swirls of water tens or even hundreds of kilometres across on the ocean surface. The idea that surface winds can influence deep-sea currents is surprising, and it suggests that the atmosphere’s influence may extend far deeper into the oceans than previously thought. The researchers found that the surface eddies—and presumably, the deep currents as well—tended to form between autumn and spring, and were more common during El Nio years. Even in the frigid darkness of the deep ocean it seems there may be weather and seasons of a sort.
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