The Abyssal zone (from the 4o C isotherm at 2000 to 3000 meters in depth down to about 6000 meters) is a term in oceanography which originally (before the mid-1800s) meant the entire depth area beyond the reach of fisherman, but later investigations led to its use being restricted to the deepest sea regions that exhibit a uniform fauna and low temperatures.
There is a pronounced drop in the number of species and the quantity of animals as one passes into this zone from the Bathypelagic zone above. This depth zone, also termed Abyssopelagic zone, is distinguished from the overlying Bathypelagic zone (or archibenthal zone) with more varied fauna and higher temperatures.
Eventually an underlying Hadal zone was defined for areas in trenches and deeps below 6000 to 7000 meters (m) depth.
The Abyssal zone is characterized by continuously cold waters of approximately two to three °C throughout the preponderance of its extent.
The upper boundary of the abyssal zone ranges between 2000 to 3000 m, with the position of the 4o C isotherm generally considered the demarcation line. It is the world’s largest ecological unit, with depths exceeding 2000 m comprising over three-quarters of the world ocean.
For tropical and temperate seas the characteristic thermocline presents a vertical profile in which temperature decreases with depth; however, in polar seas, the coldest temperature is typically at the surface. This outcome is caused by direct cooling of the sea surface by intrinsically cold polar air, whose temperature is lower than that implied by extreme latitude, due to the albedo effect of sunlight reflection from snow and ice features. The sea surface cold is further amplified by cooling from sea ice and from evaporative cooling by the extremely cold polar winds, which are abated by few landform topographic features.
The Abyssal zone is generally noted for its lack of nutrients, high oxygen content, and almost total lack of sunlight. In spite of these cold dark conditions, there are a remarkable array of fish, invertebrates, bacteria and other biota that inhabit this zone; examples of the fauna here are sea urchins and sea cucumbers who roam the abyssal plain, which itself is often studded with sea lilies and other deep sea flora. The fauna tend to be translucent, red coloured, or even luminescent carnivores, with much of the food supply falling from the Mesopelagic zone and Bathypelagic zones above. Examples of these curious fauna are luminescent shrimps. Some of the permanent denizens of this deep ocean zone who are able to withstand the extremely high pressures of the Abyssal zone ar Black swallower, Tripod fish, Deep-sea anglerfish and Giant squid. Some larger abyssal fauna have underslung jaws adapted for sifting the benthic sand and mud in order to scavenge for prey.
Deep Ocean Vents
The abyssal zone supports a surprising abundance of lifeforms in the vicinity of deep-ocean vents. The vent efflux is typically at an elevated temperature and/or high concentrations of hydrogen sulfide or other chemicals.
Some of the most striking ecological features are deep hydrothermal vents, often called black smokers. These sea floor hot water vents features emit massive amounts of hydrogen sulfide and other minerals that form a significant base of the Abyssal food chain; in fact, an incredible diversity of bacteria and other extremophiles feed from these nutrient rich vents. In the case of many bacterial species, metabolism of hydrogen sulfide actually is carried out. These bacteria in turn support significant size tube worms, crustaceans and many other types of higher organisms.
Deep ocean trenches and fissures that extend thousands of meters below the ocean floor (for example, the midoceanic trenches or fissures such as the Mariana Trench in the Pacific Ocean) are virtually unexplored. Only the Bathyscaphe Trieste, the unmanned submarine Kaiko and the Nereus submersible vehicle have been able to attain these depths. The world's marine scientists need a major governmental challenge (analogous to outer space exploration} to penetrate this mysterious realm of the Earth, most of whose secrets remain to be unlocked.
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