Ocean Zones
Ocean Zones
Water depth vs. light penetration
Bathypelagic, abyssopelagic, and hadalpelagic zones
Ocean zones are regions of the ocean that contain distinctive plant and animal life. They are sometimes referred to as ocean layers or environmental zones. In 1957 Joel Hedgpeth first suggested a system of ecological zonation for the ocean. According to that system, the ocean environment is divided into two broad categories: the benthic realm, consisting of the seafloor; and the pelagic realm, which consists of the ocean waters. Each of these realms is then subdivided into separate zones according to the depth of the water, which strongly influences the types of plant and animal life that live there.
Water depth vs. light penetration
The single most important factor in distinguishing vertical subdivisions of the benthic and pelagic realms is the availability of solar energy. Because it is strongly absorbed and scattered by water, phytoplankton and other suspended particulates, light cannot penetrate deeply into the ocean. Plants require sunlight to carry on photosynthesis, the process that converts carbon dioxide, water, and other nutrients to simple carbohydrates, providing food for themselves and for animals at higher trophic levels. In the open ocean, where light penetrates the deepest, there is sufficient light for photosynthesis at depths aboveabout 650 ft (200 m). This interval from the surface to 650 ft (200 m) is therefore known as the euphotic (“eu-” = good, “photo-” = light) zone, or simply the photic zone. By some estimates, about two-thirds of all the photosynthetic activity that occurs on Earth (on land and in the water) takes place within the euphotic zone.
Below the euphotic zone to a depth of about 3000 m is the layer known as the dysphotic (“dys-” = poor) zone. In these regions light is only about 1% of that at the surface and photosynthesis is rarely possible. Below the dysphotic zone down to the deepest parts of the ocean, light is essentially nonexistent. This layer is called the aphotic (“a-” meaning, without) zone. At one time, scientists thought that very little life existed within the aphotic zone. However, because of research using remotely operated cameras and deep-sea submersibles the existence of a variety organisms living on the deepest parts of the ocean floor has been discovered. Some oceanographers do not distinguish between the dysphotic zone and the aphotic zone since the ecology of these regions is similar.
The benthic realm
The benthic realm includes all of the ocean floor and extends from the coast to the deepest parts of the ocean. The section of the shoreline above the high tide line is known as the supralittoral, supratidal, or splash zone. It is covered by water only during the highest tides of the year. The next lower region of the shoreline, between high and low tide, is referred to as the littoral or intertidal zone. The portion of the seafloor below low water, extending outward to the edge of the continental shelf, is the sublittoral or subtidal zone. Depending on the turbidity of the water, the base of the sublittoral zone commonly corresponds to the base of the euphotic zone. Some classifications use the term littoral to refer to the entire shore zone, from the supratidal to subtidal zones of the continental shelf. The floor of the continental slope, extending from a depth of about 650-13,200 ft (200-4,000 m), is defined as the bathyal zone. The abyssal zone is comprised of flat, nearly featureless expanses of ocean floor at depths ranging from 13,200-20,000 ft (4,000-6,000 m). The deepest parts of the ocean bottom, within the ocean trenches, are referred to the hadal zone.
The benthic realm is a biologically rich environment. It is estimated that up to 98% of all marine species are found on or near the ocean floor. Some of these species are fish or shellfish swimming just above the ocean floor, but most are organisms that burrow in the sand or mud, bore into or are attached to rocks, live in shells, or simply move along the ocean floor.
In the deeper parts of the benthic zone, below the photic zone, plants and therefore herbivores are unable to thrive. However, when organisms from the photic zone die, they rain down on deeper parts of the ocean. This dead organic matter from above supports thriving benthic communities.
The pelagic realm
Scientists often separate the pelagic realm into two regions. The portion of the ocean that overlies the continental shelf, to a maximum depth of about 650 ft (200 m), is known as the neritic or coastal zone. The portion outward from the continental shelf is called the pelagic or oceanic division.
Epipelagic zone
The epipelagic zone, from the surface to 650 ft (200 m), roughly corresponding to the euphotic zone, supports phytoplankton (algae and microscopic plants). They are the primary producers of the ocean, the lowest level on the oceanic food web.
On the next level of the pelagic food web are the primary consumers, mostly zooplankton or microscopic animals. They feed on phytoplankton and, in turn, become food for larger animals or secondary consumers such as sardines, herring, and other small fish. Tuna, bonito, sharks and marine mammals are higher-level consumers, making their diet of smaller fish.
Mesopelagic zone
The open ocean below the photic zone to a depth of 3,000 ft (1,000 m) is known as the mesopelagic zone.
In the mesopelagic zone, a number of organisms survive by spending daylight hours within this zone and then rising toward the surface during evening hours. In this way, they can feed on the phytoplankton and zooplankton in the photic zone while avoiding visual predators during the day. The most common organisms found in the mesopelagic zone are small fish, squid, and small crustaceans.
A number of inhabitants of the deeper dysphotic zone have evolved some interesting adaptations for living in this twilight world. They often have very large eyes, capable of detecting light only 1% as intense as that visible to the human eye. A majority also have light-producing organs that give off a phosphorescence that makes them glow in the dark.
Bathypelagic, abyssopelagic, and hadalpelagic zones
Organisms found in the aphotic zones of the open ocean, the bathypelagic zone (about 3,000-13,000 ft [1,000-4,000 m]), the abyssopelagic zone (about 13,000-20,000 ft [4,000-6,000 m]), and the hadalpelagic zone (below 20,000 ft [6,000 m]), have evolved some unusual adaptations for survival in their lightless environment. In these regions, pressures may exceed 500 atmospheres—500 times that of atmospheric pressure, or the equivalent of several tons per square inch—and temperatures never get much warmer than about 3°C (37°F). Organisms within these zones generally prey on each other and have developed special features such as expandable mouths, large and very sharp teeth, and special strategies for hunting or luring prey.
Recent discoveries
As oceanographers extend their studies to the deepest parts of the oceans, they continually encounter surprises in the kinds of marine life found there. One of the most interesting of these surprises was the discovery of deep-sea vents found near the Gal´pagos Islands in 1977. These vents are located in regions where molten rock lies just below the surface of the seafloor, producing underwater hot springs. Volcanic chimneys form when the escaping super-heated water deposits dissolved minerals and gases upon coming in contact with the cold ocean water. The vents are surrounded by intriguing communities of organisms never seen before the 1977 discovery.
The basis for these non-photosynthesis based communities are bacteria that obtain energy from the oxidation of hydrogen sulfide escaping from the vents— a process called chemosynthesis. These bacteria are the primary producers and are consumed tubeworms,
KEY TERMS
Benthic —Pertaining to the ocean floor.
Chemosynthesis —The chemical process by which bacteria, by oxidizing hydrogen sulfide, serve as primary producers for a marine community.
Consumer —An organism that consumes other organisms as a food source.
Littoral —The part of the benthic realm between high and low water points.
Pelagic —The water portion of the ocean, usually divided into the neritic and oceanic divisions.
Phytoplankton —Microscopic aquatic plants.
Producer —An organism that is capable of utilizing non-living materials and an external energy source to produce organic molecules (for example, carbohydrates), which are then used as food.
Zooplankton —Minute animal life that lives in water.
huge clams, and mussels, and other organisms living around the vents. These communities live in isolation from photosynthetic-based communities and may provide clues to the nature of early life on Earth. Since the initial discovery, hydrothermal vent communities have been found in a variety of locations in the deep ocean.
See also Hydrothermal vents; Photic zone.
Resources
BOOKS
Garrison, Tom. Oceanography: An Invitation to Marine Science. 5th ed. Stamford, CT: Thompson/Brooks Cole, 2004.
OTHER
The Boston Museum of Science. “Oceans Alive: The Living Sea.” 1998. <http://www.mos.org/oceans/life/index.html> (accessed October 25, 2006).
David E. Newton