Pelagic zone

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"Pelagic" and "Open sea" redirect here. For the writer, see Vasa Pelagić. For the marketplace, see OpenSea. For ocean not controlled by any sovereign territory, also called the "high seas", see International waters.

Layers of the pelagic zone (scaled)
Marine habitats

The pelagic zone consists of the

Ancient Greek πέλαγος (pélagos) 'open sea'.[1] The pelagic zone can be thought of as an imaginary cylinder or water column between the surface of the sea and the bottom. Conditions in the water column change with depth: pressure increases; temperature and light decrease; salinity, oxygen, micronutrients (such as iron, magnesium and calcium) all change. Somewhat analogous to stratification in the Earth's atmosphere
, but depending on how deep the water is, the water column can be divided vertically into up to five different layers (illustrated in the diagram).

topographical
constraints.

The

hadopelagic. Coastal waters are generally the relatively shallow epipelagic. Altogether, the pelagic zone occupies 1,330 million km3 (320 million mi3) with a mean depth of 3.68 km (2.29 mi) and maximum depth of 11 km (6.8 mi).[2][3][4]
Pelagic life decreases as depth increases.

The pelagic zone contrasts with the

groundfish
.

Depth and layers

The pelagic zone is subdivided into five vertical regions. From the top down, these are:

Epipelagic (sunlight)

See also:
Epipelagic fish
From the surface (MSL) down to around 200 m (660 ft)

The illuminated zone at the surface of the sea with sufficient light for photosynthesis. Nearly all primary production in the ocean occurs here, and marine life is concentrated in this zone, including plankton, floating seaweed, jellyfish, tuna, many sharks and dolphins.[citation needed]

Mesopelagic (twilight)

See also: Mesopelagic zone and Pelagic fish § Mesopelagic fish
From 200 m (660 ft) down to around 1,000 m (3,300 ft)

The most abundant organisms thriving into the mesopelagic zone are heterotrophic bacteria.[5] Animals living in this zone include swordfish, squid, wolffish and some species of cuttlefish. Many organisms living here are bioluminescent.[6] Some mesopelagic creatures rise to the epipelagic zone at night to feed.[6]

Bathypelagic (midnight)

See also:
Bathyal zone and Pelagic fish § Bathypelagic fish
From 1,000 m (3,300 ft) down to around 4,000 m (13,000 ft)

The name stems from

Ancient Greek βαθύς 'deep'. The ocean is pitch black at this depth apart from occasional bioluminescent organisms, such as anglerfish. No plants live here. Most animals survive on detritus known as "marine snow" falling from the zones above or, like the marine hatchetfish, by preying on other inhabitants of this zone. Other examples of this zone's inhabitants are giant squid, smaller squid and the grimpoteuthis or "dumbo octopus". The giant squid is hunted here by deep-diving sperm whales.[citation needed
]

Abyssopelagic (abyssal zone)

Main article: Abyssal zone
From around 4,000 m (13,000 ft) down to above the
ocean floor

The name is derived from

Ancient Greek ἄβυσσος 'bottomless' - a holdover from times when the deep ocean was believed to indeed be bottomless. Among the very few creatures living in the cold temperatures, high pressures and complete darkness here are several species of squid; echinoderms including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the sea spider. Many species at these depths are transparent and eyeless.[6]

Hadopelagic (hadal zone)

Main article: Hadal zone

The name is derived from the realm of

trenches.[citation needed
]

Pelagic ecosystem

The pelagic ecosystem is based on phytoplankton. Phytoplankton manufacture their own food using a process of photosynthesis. Because they need sunlight, they inhabit the upper, sunlit epipelagic zone, which includes the coastal or neritic zone. Biodiversity diminishes markedly in the deeper zones below the epipelagic zone as dissolved oxygen diminishes, water pressure increases, temperatures become colder, food sources become scarce, and light diminishes and finally disappears.[7]

Pelagic invertebrates

Some examples of pelagic invertebrates include

cladocerans
.

lecithotrophic (yolk-feeding) eggs and larger offspring.[8][9]

Pelagic fish

Main article: Pelagic fish

Pelagic fish live in the water column of coastal, ocean, and lake waters, but not on or near the bottom of the sea or the lake. They can be contrasted with demersal fish, which do live on or near the bottom, and coral reef fish.[10]

Pelagic fish are often migratory forage fish, which feed on plankton, and the larger predatory fish that follow and feed on the forage fish. Examples of migratory forage fish are herring, anchovies, capelin, and menhaden. Examples of larger pelagic fish which prey on the forage fish are billfish, tuna, and oceanic sharks.[citation needed]

Pelagic reptiles

The pelagic wandering albatross
(Diomedea exulans) ranges over huge areas of ocean and can circle the globe.

Hydrophis platurus, the yellow-bellied sea snake, is the only one of the 65 species of marine snakes to spend its entire life in the pelagic zone. It bears live young at sea and is helpless on land. The species sometimes forms aggregations of thousands along slicks in surface waters. The yellow-bellied sea snake is the world's most widely distributed snake species.[citation needed]

Many species of sea turtles spend the first years of their lives in the pelagic zone, moving closer to shore as they reach maturity.[citation needed]

Pelagic birds

See also: Seabird

Pelagic birds, also called oceanic birds or seabirds, live on open seas and oceans rather than inland or around more restricted waters such as rivers and lakes. Pelagic birds feed on planktonic crustaceans, squid and forage fish. Examples are the Atlantic puffin, macaroni penguins, sooty terns, shearwaters, and Procellariiformes such as the albatross, Procellariidae and petrels.[citation needed]

Aquatic layers
   Pelagic
   Photic
   Epipelagic
   Aphotic
   Mesopelagic
   Bathypelagic
   Abyssopelagic
   Hadopelagic
   Demersal
   Benthic
Stratification
   Pycnocline
   Isopycnal
   Chemocline
  
Nutricline
   Halocline
   Thermocline
   Thermohaline
See also
benthic animals.[11]

References

  1. ^ "pelagic (adj.)". Online Etymology Dictionary. Retrieved 17 February 2020.
  2. PMID 21033734
    .
  3. hdl:1912/3862
    .
  4. ^ Ocean's Depth and Volume Revealed Archived 23 August 2011 at the Wayback Machine OurAmazingPlanet, 19 May 2010.
  5. doi:10.3354/ame01583
    .
  6. ^ a b c The Open Ocean - MarineBio.org
  7. ISBN 978-0-8160-5705-4
    .
  8. ^ Thorson, G (1957). "Bottom communities (sublittoral or shallow shelf)". In Hedgpeth, J.W. (ed.). Treatise on Marine Ecology and Palaeoecology. Geological Society of America. pp. 461–534.
  9. S2CID 84623588
    .
  10. ISBN 978-0-8248-2265-1
    .
  11. doi:10.3389/fmars.2017.00222.
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    .

Further reading
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