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,
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 with a mean depth of 3.68 km and maximum
depth of 11 km. 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. 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.
Depth and layers
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
From the surface down to around 200 m 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
From 200 m down to around 1,000 m The name for this zone stems from Greek μέσον,
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
From 1,000 m down to around 4,000 m The name stems from Greek βαθύς, 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
From around 4,000 m down to above the ocean floor
The name is derived from Greek ἄβυσσος, meaning "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. However, many organisms live in hydrothermal vents in this and other
zones. Some define the hadopelagic as waters below 6,000 m, 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
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
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. Pelagic invertebrates
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 eggs and larger offspring. Pelagic reptiles
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.
References
Further reading Ryan, Paddy Deep-sea creatures Te Ara - the
Encyclopedia of New Zealand, updated 21-Sep-2007 Pelagic-zone Encyclopædia Britannica Online.
21 March 2009. Grantham HS, Game ET, Lombard AT, et al. "Accommodating
Dynamic Oceanographic Processes and Pelagic Biodiversity in Marine Conservation Planning"
PLoS ONE 6(2): e16552. doi:10.1371/journal.pone.0016552. Wrobel, David; Mills, Claudia. Pacific Coast
Pelagic Invertebrates: A Guide to the Common Gelatinous Animals. Sea Challengers and Monterey
Bay Aquarium. ISBN 0-930118-23-5.