What are seamounts？

　　(2006-2007学年第23期)

<p><center><img border=1 src=http://image2.sina.com.cn/edu/en/2007-04-03/U1593P42T31D38176F915DT20070403002148.jpg><br>What are seamounts？</center></p>

　　On the bottom of the oceans stand mountains that rival or exceed the highest on the surface. These are seamounts, or undersea volcanoes. Large-scale mapping of the ocean bottoms, beginning in the International Geophysical Year 1957-58, showed the extent of these geological features.

　　The eight islands of Hawaii, for example, are actually seamounts that have risen above the ocean surface. Riding on the Pacific Ocean Plate, the islands are really the easternmost end of a chain of mountains extending more than 1,000 miles to the west. The Hawaiian islands are moving westward at the rate of four inches a year.

　　Volcanic seamounts form when tectonic plates pass over thin areas “hot spots” in the Earth’s crust.

　　These are far from being lifeless structures. Scientists have found that even seamounts that remain well below the surface can become oases for marine life, providing unique environments for hundreds of species, as well as prime areas for commercial fishing. Seamounts also are often steep and sometimes suffer massive landslides, causing tidal waves or tsunamis.

　　Ecology in seamounts

　　Seamounts often project upwards into shallower zones more hospitable to sea life, providing habitats for marine species that are not found on or around the surrounding deeper ocean bottom. Because seamounts are isolated from each other they form “undersea islands” creating the same biogeographical interest. As they are formed from volcanic rock, the substrate is much harder than the surrounding sedimentary deep sea floor. This causes a different type of fauna to exist than on the seafloor, and leads to a higher degree of endemism.

　　In addition to simply providing physical presence in this zone, the seamount itself may deflect deep currents and create upwelling. This process can bring nutrients into the photosynthetic zone, producing an area of activity in an otherwise desert-like open ocean. Seamounts may thus be vital stopping points for some migratory animals such as whales. Some recent research indicates whales may use such features as navigational aids throughout their migration. Due to the larger populations of fish in these areas overexpoitation by the fishing industry has caused some seamount fauna populations to decrease considerably.

　　The primary productivity of the epipelagic waters above the submerged peak can often be enhanced by the hydrographic conditions of the seamount. This increases the densities of the zooplankton and leads to the high concentrations of fish in these areas. Another theory for this is that the fish are sustained on the diurnal migration of zooplankton being interrupted by the presence of the seamount, and causing the zooplankton to stay in the area. It is also possible that the high density of fishes has more to do with the fish life histories and interaction with the benthic fauna of the seamount. The benthic fauna of the seamounts is dominated by suspension feeders, including sponges and true corals. For some seamounts that peaks at 200-300 meters below the surface benthic macroalgae is common. The sedimentary infauna is dominated by polychaete worms.