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Pelagic zone

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Diagram of the layers of the pelagic zone.
   Enlarge
   Diagram of the layers of the pelagic zone.

   The pelagic zone is the part of the open sea or ocean comprising the
   water column, i.e., all of the sea other than that near the coast or
   the sea floor. In contrast, the demersal zone comprises the water that
   is near to (and is significantly affected by) the coast or the sea
   floor. The name is derived from the Greek πέλαγος (pélagos), which
   might be roughly translated as "sea" but is more accurately translated
   as "open sea."

Sub-sections of the pelagic zone

   The pelagic zone (also known as the open-ocean zone) is further divided
   into 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) - the
       illuminated surface zone where there is enough light for
       photosynthesis, and thus plants and animals are largely
       concentrated in this zone. Here one will typically encounter fish
       such as tuna and many sharks.
     * Mesopelagic (from 200 m down to around 1000 m) - the twilight zone.
       Although some light penetrates this deep, it is insufficient for
       photosynthesis. The name stems from Greek μέσον, middle.
     * Bathypelagic (from 1000 m down to around 4000 m) - by this depth
       the ocean is almost entirely dark (with only the occasional
       bioluminescent organism). 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 live at this depth, and here they are hunted by
       deep-diving sperm whales. From Greek βαθύς (bathys), deep.
     * Abyssopelagic (from 4000 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
       90% 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 6000 meters, whether in a trench or not.

Photic and aphotic zones

   The epipelagic and (arguably) the mesopelagic zones together comprise
   the open ocean's photic zone. The remaining (lower) zones comprise the
   open ocean's aphotic zone.
   Retrieved from " http://en.wikipedia.org/wiki/Pelagic_zone"
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