Everything about Pelagic Zone totally explained
Any water in the sea, that isn't close to the bottom of the sea, is in the
pelagic zone. The word
pelagic comes from the
Greek πέλαγος or
pélagos, which means
open sea.
It 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, like the diagram on the left. Conditions change as you go deeper down the water column; the pressure increases and there's less light. Depending on the depth, scientists further subdivide the water column, rather like the earth's atmosphere is divided into different
layers.
Description
The pelagic zone occupies 1,370,000,000 cubic kilometres (330,000,000 cubic miles) and has a vertical range up to 11,000 metres (36,000 feet). Fish that live in the pelagic zone are called
pelagic fish. Pelagic life decreases with increasing depth. It is affected by light levels, pressure, temperature and salinity; by the supply of dissolved oxygen and nutrients; and by the submarine topography.
In deep water the pelagic zone is sometimes called the
open-ocean zone, and can be contrasted with water that's near the
coast or on the
continental shelf. However in other contexts, coastal water that isn't 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 sub-surface 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. They are also called
bottom feeders or
groundfish.
Depth and layers
Epipelagic (sunlit)
From the surface (MSL) down to around 200m (656 ft).
The illuminated surface zone where there's enough light for
photosynthesis. Due to this, plants and animals are largely concentrated in this zone. Here one will typically encounter fish such as
tuna and numerous amounts of
sharks, as well as
dolphin fish and
jellyfish.
Mesopelagic (twilight)
From 200m down to around 1,000m (3,280 ft).
Although some light penetrates this deep, it's insufficient for photosynthesis. The name stems from Greek μέσον,
middle. Animals such as
swordfish,
squids,
wolffish, a few species of
cuttlefish, and other semi-deepsea creatures live here.
Bathypelagic (dark)
From 1,000m down to around 4,000m (13,123 ft).
By this depth the ocean is almost entirely dark (with only the occasional
thermoluminescence organism, such as
lanternfish). There are no living plants, and most animals survive by consuming the
snow of
detritus falling from the zones above, or (like the
marine hatchet fish) by preying upon others.
Giant squid (as well as smaller
squids &
Dumbo octopuses ) live at this depth, and here they're hunted by deep-diving
sperm whales. From Greek βαθύς (baths),
deep.
Abyssopelagic
From 4,000m down to above the ocean floor.
No light whatsoever penetrates to this depth. The name is derived from the Greek άβυσσος (
ábyssos),
abyss, meaning bottomless (a holdover from the times when the deep ocean was believed to be bottomless).
Hadopelagic
The deep water in ocean trenches.
The name is derived from the Greek Άιδης (
Haidēs),
Hades, the classical Greek underworld. 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 6,000m (19,685 ft), whether in a trench or not.
The bathypelagic, abyssopelagic, and hadopelagic zones are very similar in character, and some
marine biologists elide them into a single zone or consider the latter two to be the same.
Further Information
Get more info on 'Pelagic Zone'.
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