The Pelagic, Photic, and Aphotic Zones The pelagic zone is subdivided along both the horizontal and vertical axes. Horizontally, it contains neritic and oceanic zones, which are delineated by the edge of the continental shelf. The neritic zone is shoreward of the shelf. Along the vertical axis, the pelagic zone is divided by depth. The depth of light penetration defines the top layer, called the epipelagic zone. The majority of the epipelagic zone receives sunlight and lies in the photic zone. The photic zone is divided between the euphotic (lighted), and dysphotic (dimly lit) zones. The aphotic zone and the remaining deeper zones are in constant darkness, and comprise the water below approximately 1,000 meters (3,280 feet).
The basic divisions of the ocean, based on location, are between the top of the water column and the seabed. The water column is called the pelagic zone (from the Greek pelagikos, meaning "sea"), and the sea bottom is called the benthic zone (from the Greek benthos, meaning "depths of sea"). Both of these have subdivisions.
The pelagic zone is divided horizontally into the neritic and oceanic zones. The neritic zone is the water area between the low tide mark on land and the edge of the continental shelf. This region is also referred to as the continental shelf or coastal zone.
The oceanic zone is the open water beyond that. It is further divided into vertical regions called the epipelagic, mesopelagic, bathypelagic, abyssalpelagic, and hadalpelagic zones. These vertical regions can also be labeled sunlit, twilight, midnight, and abyssal (very deep region).
The epipelagic zone is the top layer of the ocean and is penetrated by sunlight (from the Greek epi, meaning "over" or "before"). Think of it as the "sunlit" zone. Below that is the "twilight" or mesopelagic zone (from the Greek mesos, meaning "middle"). Sunlight reaches the mesopelagic zone, but not strongly enough to support diverse marine life. The zones below the mesopelagic are the bathypelagic, abyssalpelagic, and hadalpelagic. The bathypelagic zone (from the Greek bathos, meaning "deep") is the deep, open ocean. The abyssalpelagic zone (from the Greek abyssos, meaning "bottomless") is even deeper. Hadalpelagic (from the Greek Hades: the underground abode of the dead) refers to the deepest water in the ocean trenches. Collectively, these zones are called the "midnight" zone.
The Benthic Zone The benthic zone—the seafloor—is also subdivided. The littoral zone, also known as the intertidal zone, is submerged according to the tides. Past the low-tide mark is the continental shelf. After the continental shelf break is the bathyal zone, which extends down to the bottom of the continental slope. The abyssal zone runs from the base of the continental slope to the beginning of the deep ocean, the deepest parts of which fall into the hadal zone.
Divisions in the benthic zone are based on depth. Moving from shore toward the open ocean, the first zone is the supralittoral zone (form the Latin supra, meaning "upper," and litus, meaning "shore"). In this zone, the benthic flora and fauna are frequently splashed with incoming water, but do not remain submerged. Beyond lies the littoral zone (also known as the intertidal zone), which is the region between the high-tide and low-tide marks. In this zone, the bottom alternates between being covered by water and being exposed.
Beyond the littoral zone is the continental shelf. This area is divided into the sublittoral zone, which is the ocean bottom close to shore, and the outersublittoral zone, which is the ocean bottom out to the edge of the continental shelf. The bathyal zone is the sea bottom along the continental slope down to the deep ocean bottom. The deep ocean bottom is called the abyssal zone. The deepest zone—areas below 6,000 meters (19,685 feet)—is the hadal zone. The bathyal, abyssal, and hadal zones are often called the deep sea floor.
The pelagic zone can be divided vertically based upon a scale of light penetration. The majority of the epipelagic zone receives sunlight and therefore lies in the photic, or sunlit, zone. The photic zone is further divided between the euphotic (lighted) and dysphotic (dimly lit) zones. The aphotic zone is in constant darkness and comprises the seas below approximately 1,000 meters (3,280 feet).