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Deep sea

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Deep sea

Deep sea zones

The deep sea or deep layer[1] is the lowest layer in the photic zone. For this reason scientists once assumed that life would be sparse in the deep ocean but virtually every probe has revealed that, on the contrary, life is abundant in the deep ocean.

From the time of Pliny until the late nineteenth century...humans believed there was no life in the deep. It took a historic expedition in the ship Challenger between 1872 and 1876 to prove Pliny wrong; its deep-sea dredges and trawls brought up living things from all depths that could be reached. Yet even in the twentieth century scientists continued to imagine that life at great depth was insubstantial, or somehow inconsequential. The eternal dark, the almost inconceivable pressure, and the extreme cold that exist below one thousand meters were, they thought, so forbidding as to have all but extinguished life. The reverse is in fact true....(Below 200 meters) lies the largest habitat on earth.[2]

In 1960 the Bathyscaphe Trieste descended to the bottom of the Mariana Trench near Guam, at 35,798 feet or 6.77 miles (10,911 meters), the deepest known spot in any ocean. If Mount Everest (8,848 metres) were submerged there, its peak would be more than a mile beneath the surface. The Trieste was retired and for a while the Japanese remote-operated vehicle (ROV) Kaikō was the only vessel capable of reaching this depth. It was lost at sea in 2003. In May and June 2009, the hybrid-ROV (HROV) Nereus returned to the Challenger Deep for a series of three dives to depths exceeding 10900 meters.

It has been suggested that more is known about the Moon than the deepest parts of the ocean.[2] Little was known about the extent of life on the deep ocean floor until the discovery of thriving colonies of shrimps and other organisms around hydrogen sulfide, which is highly toxic to almost all terrestrial life. The revolutionary discovery that life can exist under these extreme conditions changed opinions about the chances of there being life elsewhere in the universe. Scientists now speculate that Europa, one of Jupiter's moons, may be able to support life beneath its icy surface, where there is evidence[3] of a global ocean of liquid water.

Contents

  • Environmental characteristics 1
    • Light 1.1
    • Pressure 1.2
    • Salinity 1.3
    • Temperature 1.4
  • Biology 2
    • Chemosynthesis 2.1
  • Exploration 3
  • See also 4
  • Notes 5
  • External links 6

Environmental characteristics

Light

Natural light does not penetrate the deep ocean, with the exception of the upper parts of the photic zone.

Pressure

Because metazoan animals have been retrieved from the deep sea in good condition.

Salinity

Salinity is remarkably constant throughout the deep sea, at about 35 parts per thousand.[4] There are some minor differences in salinity, but none that is ecologically significant, except in the Mediterranean and Red Seas.

Temperature

The two areas of greatest and most rapid temperature change in the oceans are the transition zone between the surface waters and the deep waters, the thermocline, and the transition between the deep-sea floor and the hot water flows at the hydrothermal vents. Thermoclines vary in thickness from a few hundred meters to nearly a thousand meters. Below the thermocline, the water mass of the deep ocean is cold and far more homogeneous. Thermoclines are strongest in the tropics, where the temperature of the epipelagic zone is usually above 20 °C. From the base of the epipelagic, the temperature drops over several hundred meters to 5 or 6 °C at 1,000 meters. It continues to decrease to the bottom, but the rate is much slower. Below 3,000 to 4,000 m, the water is isothermal between 0 to 3 °C. The cold water stems from sinking heavy surface water in the polar regions.[4]

At any given depth, the temperature is practically unvarying over long periods of time. There are no seasonal temperature changes, nor are there any annual changes. No other habitat on earth has such a constant temperature.

Hydrothermal vents are the direct contrast with constant temperature. In these systems, the temperature of the water as it emerges from the "black smoker" chimneys may be as high as 400 °C (it is kept from boiling by the high hydrostatic pressure) while within a few meters it may be back down to 2 - 4 °C.[5]

Biology

Regions below the marine snow' and carcasses derived from the productive zone above, and is scarce both in terms of spatial and temporal distribution.

Instead of relying on gas for their buoyancy, many species have jelly-like flesh consisting mostly of glycosaminoglycans, which has very low density.[6] It is also common among deep water squid to combine the gelatinous tissue with a flotation chamber filled with a coelomic fluid made up of the metabolic waste product ammonium chloride, which is lighter than the surrounding water.

The midwater fish have special adaptations to cope with these conditions—they are small, usually being under 25 centimetres (10 in); they have slow hermaphroditic.

Because light is so scarce, fish often have larger than normal, tubular eyes with only rod cells. Their upward field of vision allows them to seek out the silhouette of possible prey. Prey fish however also have adaptations to cope with predation. These adaptations are mainly concerned with reduction of silhouettes, a form of camouflage. The two main methods by which this is achieved are reduction in the area of their shadow by lateral compression of the body, and counter illumination via bioluminescence. This is achieved by production of light from ventral photophores, which tend to produce such light intensity to render the underside of the fish of similar appearance to the background light. For more sensitive vision in low light, some fish have a retroreflector behind the retina. Flashlight fish have this plus photophores, which combination they use to detect eyeshine in other fish (see Tapetum lucidum).

Organisms in the deep sea are almost entirely reliant upon sinking living and dead organic matter which falls at approximately 100 meters per day.[7] In addition, only about 1-3% of the production from the surface reaches the sea bed mostly in the form of marine snow. Larger food falls, such as Freyella elegans.[9]

Marine bacteriophages play an important role in cycling nutrients in deep sea sediments. They are extremely abundant (between 5x1012 and 1x1013 phages per square meter) in sediments around the world.[10]

Chemosynthesis

There are a number of species that do not primarily rely upon dissolved organic matter for their food and these are found at hydrothermal vents. One example is the symbiotic relationship between the tube worm Riftia and chemosynthetic bacteria. It is this chemosynthesis that supports the complex communities that can be found around hydrothermal vents. These complex communities are one of the few ecosystems on the planet that do not rely upon sunlight for their supply of energy.[11]

Exploration

Describing the operation and use of an autonomous lander (RV Kaharoa) in deep-sea research; the fish seen are the abyssal grenadier (Coryphaenoides armatus).

The deep sea is an environment completely unfriendly to humankind; it represents one of the least explored areas on Earth. Pressures even in the mesopelagic become too great for traditional exploration methods, demanding alternative approaches for deep sea research. Baited camera stations, small manned submersibles and ROVs (remotely operated vehicles) are three methods utilized to explore the ocean's depths. Because of the difficulty and cost of exploring this zone, current knowledge is limited. Pressure increases at approximately one atmosphere for every 10 meters meaning that some areas of the deep sea can reach pressures of above 1,000 atmospheres. This not only makes great depths very difficult to reach without mechanical aids, but also provides a significant difficulty when attempting to study any organisms that may live in these areas as their cell chemistry will be adapted to such vast pressures.

See also

Notes

  1. ^ Navy Supplement to the DOD Dictionary of Military and Associated Terms (PDF).  
  2. ^ a b Tim Flannery, Where Wonders Await Us, New York Review of Books, December 2007
  3. ^ Magnetic Fields and Water on Europa.  
  4. ^ a b Claus Ditlefsen. "About the Marianas" (in Danish) Ingeniøren / Geological Survey of Denmark and Greenland, 2 November 2013. Accessed: 2 November 2013.
  5. ^ Nybakken, James W. Marine Biology: An Ecological Approach. Fifth Edition. Benjamin Cummings, 2001. p. 136 - 141.
  6. ^ http://www.astrobio.net/news/print.php?sid=617
  7. ^ http://www.whoi.edu/oceanus/viewArticle.do?id=2387&archives=true
  8. ^ R. N. Gibson, Harold (CON) Barnes, R. J. A. Atkinson, Oceanography and Marine Biology, An Annual Review. 2007. Volume 41. Published by CRC Press, 2004 ISBN 0-415-25463-9, ISBN 978-0-415-25463-2
  9. ^ http://www.nhm.ac.uk/nature-online/earth/oceans/deep-ocean/session3/index.html
  10. ^ Danovaro, Roberto; Antonio Dell'Anno; Cinzia Corinaldesi; Mirko Magagnini; Rachel Noble; Christian Tamburini; Markus Weinbauer (2008-08-28). "Major viral impact on the functioning of benthic deep-sea ecosystems". Nature 454 (7208): 1084–1087.  
  11. ^ HW Jannasch. 1985. The Chemosynthetic Support of Life and the Microbial Diversity at Deep-Sea Hydrothermal Vents. Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 225, No. 1240 (Sep. 23, 1985), pp. 277-297

External links

  • Deep Sea Foraminifera – Deep Sea Foraminifera from 4400m depth, Antarctica - an image gallery and description of hundreds of specimens
  • Deep Ocean Exploration on the Smithsonian Ocean Portal
  • Deep Sea Creatures Facts and images from the deepest parts of the ocean
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