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The Pelagic Zone: The Open Ocean

written by: Terrie Schultz•edited by: Laurie Patsalides•updated: 11/11/2010

The vast open ocean, known as the pelagic zone, is the world's largest habitat, extending around the globe and thousands of feet below the surface.

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    Organisms of the Pelagic Zone

    school of fish 

    The organisms that make their home in the pelagic zone can be divide into three types, phytoplankton, zooplankton and nekton.

    Phytoplankton- these primary producers are the basis of the ocean's food chain. Like plants, phytoplankton are autotrophs, able to produce their own food by photosynthesis, but they also require nutrients such as salts and minerals from ocean water. Some principle types of phytoplankton are cyanobacteria, diatoms and dinoflagellates.

    Zooplankton- these animal-like plankton are heterotrophs, meaning they are unable to produce their own food. Zooplankton include protozoa, krill, and the larval stages of many crustaceans, worms, mollusks, fish and other marine animals. Zooplankton feed on phytoplankton.

    Nekton- organisms that are capable of freely swimming rather than drifting with the current are known as nekton. This category includes crustaceans, mollusks, fish and marine mammals and reptiles.

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    Layers of the Pelagic Zone

    The pelagic zone is an enormous column of water that extends from the surface to the bottom of the world's oceans, away from the shore. It can be divided into several layers, which differ in amount of light, temperature, pressure and dissolved oxygen.

    Epipelagic, or Sunlit Zone- this is the topmost layer of the pelagic zone, from the surface to a depth of about 200 meters (660 ft). It receives enough sunlight to support photosynthetic organisms such as seaweed and phytoplankton, and also has a high level of dissolved oxygen due to the action of the waves. The epipelagic zone is home to the majority of ocean life, including large predatory fish such as tuna and sharks, small forage fish such as herrings, sardines and anchovies, dolphins, whales and other marine mammals, sea turtles, and numerous other species.

    Mesopelagic, or Twilight Zone- ranging from 200 to 1,000 meters (3,280 ft), the mesopelagic zone receives very little sunlight, and photosynthetic organisms cannot survive in this layer. The pressure increases and the temperature and dissolved oxygen levels continue to drop with increasing depth. The fish that live in the mesopelagic zone are predominantly small filter feeders such as lanternfish, and larger predator fish including sabertooth fish. The filter feeders make vertical migrations up to the epipelagic zone by night to feed on plankton, returning to the darkness by day to hide from predators.

    Bathypelagic, or Midnight Zone- between 1,000 and 4,000 meters (13,000 ft) in depth, the bathypelagic zone is a region of total darkness, with a very low level of dissolved oxygen and nutrients, low temperature and extremely high pressure. Adaptations of the inhabitants of the bathypelagic zone include a slow metabolism, bioluminescence, and hinged jaws and distensible stomachs that allow them to swallow prey that is several times larger than they are.

    Abyssopelagic, or Lower Midnight Zone- the abyssopelagic zone, extending down from a depth of 4,000 meters, is similar to the bathypelagic, but more extreme in terms of pressure.

    Hadropelagic Zone- taking its name from the Greek Hades, or underworld, the hadropelagic zone refers to the deep ocean trenches, some of which exceed 9,000 meters (30,000 ft) in depth, and have pressures of 16,000 psi. Average temperatures hover around freezing, with the exception of hydrothermal vents, where water heated by the magma of Earth's mantle boils out. Organisms of the hadropelagic zone are highly specialized and cannot survive if they are removed to shallower water with lower pressure.

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