Benthic zone

The benthic zone is the ecological region of the bottom of a body of water such as the ocean floor or the bttom of a lake, including the sediment surface and some sub-surface layers. Organisms living in this zone—that is, on or in the bottom of the body of water—are called benthos. The adjective benthic refers to something connected with or occurring on the bottom of a body of water (CSC 2008). In contrast, the pelagic zone is the descriptive term for the ecological region above the benthos, including the water-column up to the surface.

Description and types

The benthic zone refers to both freshwater, brackish, and saltwater environments. Depending on the water-body, the benthic zone may include areas that are only a few inches below water, such as a stream or shallow pond, or may be at the bottom of an ocean below a 4,000 meter (13,000 foot) column of water.

Benthic habitats are very diverse, depending upon their depth and location and have distinct biological, physical, and geochemical characteristics (CSC 2008). The superficial layer of the soil lining the given body of water is an integral part of the benthic zone, as it influences greatly the biological activity which takes place there. Examples of contact soil layers include sand bottoms, rock outcrops, coral, and bay mud.

Marine benthic environments are divided further into zones based on their depth. From the shallowest to the deepest, these include:

• the nearshore and estuarine zones (less than 200 meters deep);
• the bathyal zone (200 to 2,000 meters);
• the abyssal zone (2,000 to 6,000 meters); and
• the hadal zone (over 6,000 meters deep) (CSC 2008).

Estuarine and nearshore benthic habitats include such as coral reefs, kelp forests, shellfish beds, shallow submerged mudflats, rocky hard-body habitats, and rippled sandflats (CSC 2008).

The abyssal zone—the pelagic zone from 2,000 to 6,000 meters in depth—remains in perpetual darkness and never receives daylight. It contains the very deep benthic communities near the bottom of oceans. The abyssal zone has temperatures around 0 to 3°C but 13-15°C in the Mediterranean sea. Abyss is from the Greek word meaning bottomless sea.

The area below the abyssal zone is the sparsely inhabited hadal zone, with the name derived from Hades, the classical Greek underworld. The zone above is the bathyal zone. These three zones belong to the deep-sea realm. The hadal, abyssal, and bathyal zones are in such relatively deep, pressured areas of the ocean, these areas and their organisms are difficult to observe and map. The deep trenches or fissures that plunge down thousands of feet below the ocean floor—for example, the midoceanic trenches such as the Mariana Trench in the Pacific—are almost unexplored. Only the bathyscaph Trieste and the remote control submarine Kaiko have been able to descend to these depths.

The pelagic zone, the open-ocean zone that includes the water-column above the benthos to the surface, is similarly divied into subzones, such as the epipelagic (surface to around 100 meters), mesopelagic (twilight zone from 200 meters to around 1,000 meters), bathypelagic (about 1,000 meters to 4,000 meters), abyssopelagic (4,000 meters to above the ocean floor), and hadopelagic (deep water in trenches).

Organisms

Microphotograph of typical benthic animals

Benthos are the organisms which live in the benthic zone, and are different from the elsewhere in the water column. Many are adapted to live on the substrate (bottom). In their habitats they can be considered as dominant creatures. Many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column. The pressure difference can be very significant (approximately one atmosphere for each 10 meters of water depth).

Because light does not penetrate very deep ocean-water, the energy source for the benthic ecosystem is often organic matter from higher up in the water column which drifts down to the depths. This dead and decaying matter sustains the benthic food chain; most organisms in the benthic zone are scavengers or detritivores.

They generally live in close relationship with the substrate bottom; many such organisms are permanently attached to the bottom. Generally, these include life forms that tolerate cool temperatures and low oxygen levels, but this depends on the depth of the water. For information on animals that live in the deepest areas of the oceans see aphotic zone.

The abyssal zone is theIts permanent inhabitants – for example, the Black swallower, tripod fish, deep-sea anglerfish, and the giant squid – are able to withstand the immense pressures of the ocean depths, up to 775 kilograms per square centimeter (76 megapascals or 4.92 long tons force per square inch). Many abyssal creatures have underslung jaws to sift through the sand to catch food.

Seagrass growing off the coast of the Florida Keys.

Giant clam along the Great Barrier Reef.

Benthos are the organisms which live on, in, or near the seabed, also known as the benthic zone. Although the term derived from the Greek for "depths of the sea", the term is also used in freshwater biology to refer to organisms at the bottoms of freshwater bodies of water, such as lakes, rivers, and streams.

Animals belonging to the benthos are sometimes referred to as zoobenthos, while plants are referred to as phyto-benthos.

Benthos can be categorized according to size:

• macrobenthos, size > 1 mm
• meiobenthos, size < 1 mm and > 32 µm
• microbenthos, size < 32 µm

• epibenthos lives on top of the sediment
• hyperbenthos lives just above the sediment

Meiofauna are small benthic invertebrates that live in both marine and fresh water environments . The term Meiofauna loosely defines a group of organisms by their size, larger than Microfauna but smaller than Macrofauna, rather than a taxonomic grouping. In practice that is organisms that can pass through a 1 mm mesh but will be retained by a 45 μm mesh, but the exact dimensions will vary from researcher to researcher. Whether an organism will pass through a 1 mm mesh will also depend upon whether it is alive or dead at the time of sorting.

The term meiobenthos was first coined in 1942 by Mare, but organisms that would fit into the meiofauna category have been studied since the 18th century. A good comprehensive text on meiofauna is Introduction to the study of meiofauna by Higgins and Thiel (1988).

Benthic organisms play an important role as a food source for fish and some also for humans.

Examples of benthic organisms are sea stars, oysters, clams, sea cucumbers, brittle stars and sea anemones.

References

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• Benthos

See also

• Armor (hydrology)
• Deep sea
• Littoral
• Profundal zone
• Limnetic zone
• Lake stratification
• Benthos
• Stream bed
• Water column
• Intertidal zone
• Aphotic zone
• Photic zone
• Neritic zone
• Pelagic zone

References

• Fathom
• Data Archive for Seabed Species and Habitats from the UK Marine Data Archive Centre

NOAA Coastal Services Center http://www.csc.noaa.gov/benthic/start/what.htm Benthic Habitat Mapping Search Benthic Site: What is Benthic Habitat? National Oceanic and Atmospheric Administration

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