Marine Science Chapters


The World Ocean: Classification of Marine Environments

Because of the variations in the three dimensional ocean and the changes in light, there are two primary ways of classifying marine environments. One way is by space and the other way is by light.

Spatial Classification
Spatial classification of the marine environment. (GA image)
Spatially, the marine environment is divided into the pelagic division and benthic division.

Being pelagic means to be in the water, surrounded by water at any depth. The pelagic division above the continental shelf is distinguished as the neritic province from the pelagic area above the open ocean (abyssal plain, oceanic ridges/rises, and trenches) which is call the oceanic province. Pelagic organisms that can swim relatively well are called nektonic pelagic (or just nektonic) whereas those that cannot swim, or are feeble swimmers, are called planktonic pelagic (or just planktonic).

Being benthic means to be on the bottom or on a solid surface. Most benthic habitats are associated with the bottom of the ocean and are distinguished by depth as follows: inner shelf (closest to the continent), outer shelf (along the outer edge of the continental shelf next to the continental slope), bathyal zone (on the continental slope averaging between 200 to 3-6,000 meters), abyssal zone (on the abyssal plain, the relatively flat deep-sea area that may be 3-6,000 meters deep), and hadal zone (on the bottom of the trenches - below 6,000 m to 11,000+ meters). Benthic is also used to describe organisms that live on other organisms, like the barnacles that live on some species of whales. The whales are pelagic but their barnacles are benthic.

Some species of marine life can be both pelagic and benthic at the same time. An example of this is the many flatfish (like halibut, sole, and flounder). They tend to rest on the bottom (and are thus benthic at that time) but can swim through the water in search of food (and are pelagic while swimming).

Some species of marine life can be both pelagic and benthic at different times of their life. This is common for most of the benthic invertebrate animals. They begin life as pelagic (water dwelling) babies but settle after a period (often weeks or months) and become benthic adults. Most seastars, sea urchins, snails, clams, crabs, lobsters, corals and sea anemones have this life style.

Classification by light
Classification of the marine environment by light. (GA image)
Using light the marine environment is divided into the photic zone and aphotic zone. The photic zone is the area where there is enough light for photosynthesis to occur. It changes with the seasons, latitude, time of day, clarity of the water, as well as with the weather. In general the photic zone would not be found below 200 meters and would normally be well above this. The aphotic zone is below the photic zone and is the place where there is no photosynthesis.

With all of these classifications there are many ways to describe the marine environment. The oceans are one of the most diverse environments on Earth with incredibly interesting life forms. Many people say we know more about the surface of the moon than we do about the ocean floor and this is proved by the constant new discoveries made by marine scientists.

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(Revised 6 August 2007)
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