Deep scattering layer

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The deep scattering layer is a dynamically shifting layer deep in the ocean that sonar waves react to. It is caused largely by the sonar bouncing off the swim bladders (pictured) of mesopelagic fishes

The deep scattering layer, sometimes referred to as the sound scattering layer, is a name given to a layer in the ocean consisting of a variety of marine animals. It was discovered through the use of sonar, as ships found a layer that scattered the sound and was thus sometimes mistaken for the seabed. For this reason it is sometimes called the false bottom or phantom bottom. It can be seen to rise and fall each day in keeping with diel vertical migration.

Sonar operators, using the newly developed sonar technology during World War II, were puzzled by what appeared to be a false sea floor 300–500 metres deep at day, and less deep at night. Initially this mysterious phenomenon was called the ECR layer using the initials of its discoverers.[1] It turned out to be due to millions of marine organisms, most particularly small mesopelagic fish, with swimbladders that reflected the sonar. These organisms migrate up into shallower water at dusk to feed on plankton. The layer is deeper when the moon is out, and can become shallower when clouds pass over the moon.[2] Lanternfish account for much of the biomass responsible for the deep scattering layer of the world's oceans. Sonar reflects off the millions of lanternfish swim bladders, giving the appearance of a false bottom.[3]

Description

File:California headlightfish.png
Most mesopelagic fishes are small filter feeders which ascend at night to feed in the nutrient rich waters of the epipelagic zone. During the day, they return to the dark, cold, oxygen deficient waters of the mesopelagic where they are relatively safe from predators. Lanternfish account for as much as 65 percent of all deep sea fish biomass and are largely responsible for the deep scattering layer of the world's oceans.

The phantom bottom is caused by the sonar misinterpreting as the ocean floor a layer of small seagoing creatures that congregate between 1,000 and 1,500 feet (300 and 460 m) below the surface.[4][5] The name is derived from the fact that the first people to see these measurements erroneously reported that they had discovered sunken islands.[5] At night, the layer disappears as the small creatures composing the phantom bottom move to the surface.[5]

Most mesopelagic organisms, including mesopelagic fish, squid and siphonophores, make daily vertical migrations. They ascend at night into the shallow epipelagic zone, often following similar migrations of zooplankton, and return to the mesopelagic depths for safety when there is daylight.[6][7][8] These vertical migrations often occur over large vertical distances. Fish undertake these migrations with the assistance of a swimbladder. The swimbladder is inflated when the fish wants to move up, and, given the high pressures in the messoplegic zone, this requires significant energy. As the fish ascends, the pressure in the swimbladder must adjust to prevent it from bursting. When the fish wants to return to the depths, the swimbladder is deflated.[9] Some mesopelagic fishes make daily migrations through the thermocline, where the temperature changes between 10 and 20 °C, thus displaying considerable tolerances for temperature change.

Sampling via deep trawling indicates that lanternfish account for as much as 65% of all deep sea fish biomass.[10] Indeed, lanternfish are among the most widely distributed, populous, and diverse of all vertebrates, playing an important ecological role as prey for larger organisms. The estimated global biomass of lanternfish is 550–660 million metric tonnes, about six times the annual tonnage captured worldwide by fisheries.[3]

Sonar scan of the water column
Time lapse video of a 3-D mapping of water column sonar data by the NOAA research ship Okeanos Explorer in the North Atlantic Ocean [11]
Static image of the sonar scan in the video at the left. The backscattered signal (green) above the bottom is likely the deep scattering layer.[11]
External video
video icon The Deep Scattering Layer - Ocean Phenomenon – YouTube

See also

References

  1. ↑ Hersey JB and Backus RH (2005) "Sound Scattering by Marine Organisms" In: M. N. Hill and A. R. Robinson (Eds)Physical Oceanography, Page 499, Harvard University Press. ISBN 9780674017276.
  2. ↑ Ryan P "Deep-sea creatures: The mesopelagic zone" Te Ara - the Encyclopedia of New Zealand. Updated 21 September 2007.
  3. ↑ 3.0 3.1 Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ 5.0 5.1 5.2 Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ Moyle and Cech, 2004, p. 585
  7. ↑ Bone & Moore 2008, p. 38.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Douglas EL, Friedl WA and Pickwell GV (1976) "Fishes in oxygen-minimum zones: blood oxygenation characteristics" Science, 191 (4230) 957–959.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ 11.0 11.1 Water Column Sonar Data National Geophysical Data Center, NOAA.

Further references

External links

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