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Major Subdivisions of the Ocean

Updated on July 25, 2014

Out of all the places on Earth, the ocean still remains a place of intrigue and mystery. When oceanographers and marine biologists study the ocean, they tend to concentrate within one subdivision at a time. Scientists have divided the ocean up into several major subdivisions in order to discover more of the mysteries the world's oceans hold.

The world oceans have two major divisions. The pelagic realm is in essence the watery realm and the organisms that thrive within. The benthic realm is the ocean floor and the life that resides on the surface or below the ocean floor. These major divisions are then divided both vertically as well as horizontally into several major subdivisions.

The Pelagic Realm

The pelagic realm extends out to all surface water. Below the surface of the water, scientists have divided the ocean horizontally into seven major subdivisions. The water mass that extends to the continental shelf is called the neritic zone. Once past the continental shelf it becomes the oceanic zone.

Photic Zone

To go a step further with the subdivisions, the neritic and oceanic zones are classified vertically as being a part of the photic zone, also referred to as the euphotic zone. This vertical zone extends below the surface to 200 meters. The photic zone is the area of ocean water which is lighted. Depending on the clarity of the water, the lower boundary of this zone has very limited light. The lower boundary of the photic zone is called the epipelagic zone. It is typically measured between 100 meters to 200 meters below the surface. These subdivisions are of major importance to researchers because this is where most of the ocean's primary production occurs.

Aphotic Zone

Continuing to divide the pelagic part of the ocean vertically is the aphotic zone. This encompasses several zones classified horizontally and includes some of the darkest waters of the ocean. Some scientists prefer to recognize a zone between the photic and aphotic zone. They call the subdivision the disphotic zone.

Disphotic Zone

The disphotic zone is a transitory zone. Some light comes through to see by and for organisms to react to but there isn't enough light to be used for photosynthesis. This subdivision can extend down to 1000 meters in certain areas. Even still, many scientists consider the disphotic zone to be synonymous with the mesopelagic zone. The mesopelagic zone is a horizontal classification whereas the disphotic zone is a vertical classification.

Mesopelagic Zone

The mesopelagic zone is measured between 200 meters and 1000 meters. Although the temperatures of these waters vary upon location, it is typically 10 degrees Celsius in the isotherm of 700 meters to 1000 meters in tropic waters. An isotherm is a horizontal area of equal temperature that can occur in any zone.

Bathypelagic Zone

The next subdivision is the bathypelagic zone. This zone, depending on the area, can start anywhere between 700 meters and 1000 meters below the surface. The temperature ranges from a frigid 4 degrees Celsius to 10 degrees Celsius. The next isotherm starts between 2000 meters to 4000 meters below the surface and stays at a steady 4 degrees Celsius.

Abyssalpelagic Zone

The abyssalpelagic zone is measured between 4000 meters to 6000 meters below the surface. This is the water overlying the abyssal plains of the major ocean basins.Deep ocean trenches begin around 6000 meters and extend to an estimated 10,000 meters. This water zone is referred to as the hadalpelagic zone.

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The Benthic Realm

Just like the pelagic subdivisions, the benthic area is also classified into several subdivisions. The littoral zone is the area of the shoreline that extends from extreme high tide to extreme low tide. This zone is also referred to as the intertidal zone and has been well studied.

Sublittoral Zone

The ocean floor under the neritic pelagic zone is called the sublittoral zone. It is also referred to as the shelf zone because it extends out along the continental shelf. This zone is abundance in life. Many of these are interdependent communities such as coral reefs, seagrass beds, and kelp forests.

Bathyal Zone

Once the ocean floor drops down along the continental slope, this becomes the bathyal zone. The bathyal zone extends downward to 4000 meters below the surface. Once past 4000 meters, the ocean floor becomes the abyssal zone as it opens to the broad abyssal plains of the ocean basin. The abyssal zone extends as far down as 6000 meters.

Hadal Zone

The final zone in the benthic realm is the hadal zone. This includes deep ocean trenches and extends past 6000 meters below the surface. There is no light in the area and little is known about the organisms that thrive there.

By dividing the ocean into zones horizontally and vertically, scientists can better study the life and science of the ocean one zone at a time. It also allows them to learn how each zone is vital to the health of the ocean as a whole. Consider learning more about the major divisions of the ocean and how it is studied by reading Marine Biology: An Ecological Approach (ISBN: 0673994511).

© 2014 L Sarhan

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