Any water in the sea that is not close to the bottom 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 is less light. Depending on the depth, scientists further subdivide the water column, rather like the earth's atmosphere is divided into different layers.
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, 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. However in other contexts, coastal water that is 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 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
Depending on how deep the sea is, there can be up to five vertical layers in the ocean. From the top down, they are:
From the surface ( MSL) down to around 200 m (656 ft).
The illuminated surface zone where there is enough light for photosynthesis. Due to this, plants and animals are largely concentrated in this zone. Nearly all primary production in the ocean occurs here. This layer us the domain of fish such as tuna, many sharks, dolphin fish, and jellyfish.
From 200 m down to around 1,000 m (3,280 ft).
Although some light penetrates this deep, it is insufficient for photosynthesis. The name stems from Greek μέσον, middle. At about 500 m the water becomes depleted of oxygen. Still, an abundance of life copes with more efficient gills or minimal movement. Animals such as swordfish, squids, wolffish, a few species of cuttlefish, and other semi-deep-sea creatures live here.
From 1,000 m down to around 4,000 m (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 hatchetfish) by preying upon others. Giant squid (as well as smaller squids & Dumbo octopuses ) live at this depth, and here they are hunted by deep-diving sperm whales. From Greek βαθύς (baths), deep.
From 4,000 m 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).
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 6,000 m (19,685 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.