Pelagic Zone

January 16, 2008 at 1:16 pm

The pelagic zone is the part of the open sea or ocean that is not near the coast. In contrast, the neritic zone comprises the water that is near to (and is significantly affected by) the coast or the continental shelf. The name is derived from the Greek πέλαγος (pélagos), which might be roughly translated as “sea” but is more accurately translated as “open sea.” When used on its own, “pelagic” describes something that exists in the open sea as opposed to inland or coastal waters.

Sub-sections of the pelagic zone

The pelagic zone (also known as the open-ocean zone) is further divided into sections, creating a number of sub-zones, based on their different ecological characteristics (which is roughly a function of depth):

  • Epipelagic (from the surface down to around 200 m) (656 feet) – the illuminated surface zone where there is 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 many sharks,as well as colorful dolphinfish & jellies.
  • Mesopelagic (from 200 m down to around 1,000 m) (3,280 feet) – the twilight zone. Although some light penetrates this deep, it is 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 (from 1,000 m down to around 4,000 m) (13,123 feet) – by this depth the ocean is almost entirely dark (with only the occasional bioluminescent 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 octopi ) live at this depth, and here they are hunted by deep-diving sperm whales. From Greek βαθύς (bathys), deep.
  • Abyssopelagic (from 4,000 m down to above the ocean floor) – no light whatsoever penetrates to this depth, and most creatures are blind and colourless. 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 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.

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. Some define the hadopelagic as waters below 6,000 meters (19,685 feet), whether in a trench or not.

Scale diagram of the layers of the pelagic zone.

Scale diagram of the layers of the pelagic zone.
from answers.com 

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