.png)
What the pelagic ocean?
Any water in a sea or lake
that is neither close to the bottom nor near the shore can be said to be in the
pelagic zone. The word "pelagic" is derived from Greek πέλαγος (pélagos), meaning "open sea".
The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes from the surface of the sea almost to
the bottom. Conditions differ deeper in the water column; the pressure
increases, the temperature drops and less light penetrates. Depending on the
depth, the water column, rather like the Earth's atmosphere, can be
divided into different layers.
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). Fish that live in the pelagic
zone are called pelagic fish. Pelagic life decreases with
increasing depth. It is affected by light intensity, pressure, temperature,
salinity, the supply of dissolved oxygen and nutrients, and the submarine
topography, which is called bathymetry. In deep water, the pelagic zone is
sometimes called the open-ocean zone and can be contrasted with water
that is near the coast or on the continental shelf. In other contexts, coastal water not near
the bottom is still said to be in the pelagic zone.
The pelagic zone can be contrasted with the benthic and demersal zones at the bottom of the sea. The benthic zone is
the ecological region at the very bottom of the sea. It includes the sediment
surface and some subsurface layers. Marine organisms living in this zone, such
as clams and crabs, are called benthos. The demersal zone is just above the benthic zone. It
can be significantly affected by the seabed and the life that lives there. Fish
that live in the demersal zone are called demersal fish, which can be divided into benthic fish, which are denser than water so they can rest on
the bottom, and benthopelagic fish, which
swim in the water column just above the bottom. Demersal fish are also known as
bottom feeders and groundfish.
Depending on how deep the
sea is, the pelagic zone can extend over up to five horizontal layers in the
ocean. From the top down, these are:
Epipelagic (sunlight)
From the surface (MSL) down to around 200 m (660 ft)
This is the illuminated zone at the surface of the sea
where enough light is available for photosynthesis. Nearly all primary production in the
ocean occurs here. Consequently, plants and animals are largely concentrated in
this zone. Examples of organisms living in this zone are plankton, floating seaweed, jellyfish, tuna, many sharks
and dolphins.
Mesopelagic (twilight)
rom 200 m (660 ft) down to around 1,000 m
(3,300 ft)
The name for this zone stems from Greek μέσον (meson), meaning "middle".
Although some light penetrates this second layer it is insufficient for
photosynthesis. At about 500 m the water also becomes depleted of oxygen.
Organisms survive this environment by having more efficient gills or by
minimizing movement.
Examples of animals that live here are swordfish, squid, wolffish and some species of cuttlefish. Many organisms that live in this zone are bioluminescent. Some creatures living in the mesopelagic zone
rise to the epipelagic zone at night to feed.
Bathypelagic (midnight)
From 1,000 m (3,300 ft) down to around
4,000 m (13,000 ft)
The name stems from Greek βαθύς (bathýs), meaning
"deep". At this depth, the ocean is pitch black, apart from
occasional bioluminescent organisms, such as lanternfish. No living plant life exists here. Most animals
living here survive by consuming the detritus falling from the zones above, which is known as
"marine snow", or, like the marine hatchetfish, by
preying on other inhabitants of this zone. Other examples of this zone's
inhabitants are giant squid, smaller squids
and the dumbo octopus. The giant squid is hunted
here by deep-diving sperm whales.
Abyssopelagic (lower midnight)
From around 4,000 m (13,000 ft) down to above
the ocean floor
The name is derived from Greek ἄβυσσος
(ábyssos), meaning "bottomless" (a holdover from the times
when the deep ocean, or abyss, was believed to be bottomless).
Very few creatures live in the cold temperatures, high pressures and complete
darkness of this depth. Among the species found in this zone are several
species of squid; echinoderms including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the sea spider. Many of the species living at these depths are
transparent and eyeless because of the total lack of light in this zone.
Hadopelagic
The deep water in ocean trenches
The name is derived from the realm of Hades,
the underworld in Greek
mythology. This zone is mostly unknown, and very few species are known to live
here (in the open areas). However, many organisms live in hydrothermal vents in this
and other zones. Some define the hadopelagic as waters below 6,000 m
(20,000 ft), whether in a trench or not. The bathypelagic, abyssopelagic,
and hadopelagic zones are very similar in character, and some marine biologists combine them into a single zone or consider
the latter two to be the same. The abyssal plain is covered with soft sludge composed of dead
organisms from above.
Pelagic ecosystem
The pelagic ecosystem is
based on the phytoplankton which occupy the start of the
foodchain. 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.
Pelagic birds, also called oceanic birds, live on the open sea,
rather than around waters adjacent to land or around inland waters. Pelagic
birds feed on planktonic crustaceans, squid
and forage fish. Examples are the Atlantic puffin, macaroni penguins, sooty terns, shearwaters, and procellariiforms such as the albatross, procellariids and petrels.
The term seabird includes birds which live around
the sea adjacent to land, as well as pelagic birds.
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 live on or near the bottom, and reef fish, which are associated with coral reefs.
These fish are often migratory forage fish, which feed on plankton, and the larger 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.
Some examples of pelagic invertebrates include krill,
copepods, jellyfish, decapod larvae, hyperiid amphipods, rotifers and cladocerans.
Thorson's rule states that
benthic marine invertebrates at
low latitudes tend to produce large numbers of eggs developing to
widely-dispersing pelagic larvae, whereas at high latitudes such organisms tend
to produce fewer and larger lecithotrophic (yolk-feeding) eggs and larger offspring.
Pelamis platura, the pelagic 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 pelagic sea snake
is the world’s most widely distributed snake species.
