Abyssal zone

The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. 'Abyss' derives from the Greek word ἄβυσσος, meaning bottomless. At depths of 3,000 to 6,000 metres (9,800 to 19,700 ft), this zone remains in perpetual darkness. It alone makes up over 83% of the ocean and covers 60% of the Earth. The abyssal zone has temperatures around 2 to 3 °C (36 to 37 °F) through the large majority of its mass. Due to there being no light, there are no plants producing oxygen, which primarily comes from ice that had melted long ago from the polar regions. The water along the seafloor of this zone is actually devoid of oxygen, resulting in a death trap for organisms unable to quickly return to the oxygen-enriched water above. This region also contains a much higher concentration of nutrient salts, like nitrogen, phosphorus, and silica, due to the large amount of dead organic material that drifts down from the above ocean zones and decomposes. The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. 'Abyss' derives from the Greek word ἄβυσσος, meaning bottomless. At depths of 3,000 to 6,000 metres (9,800 to 19,700 ft), this zone remains in perpetual darkness. It alone makes up over 83% of the ocean and covers 60% of the Earth. The abyssal zone has temperatures around 2 to 3 °C (36 to 37 °F) through the large majority of its mass. Due to there being no light, there are no plants producing oxygen, which primarily comes from ice that had melted long ago from the polar regions. The water along the seafloor of this zone is actually devoid of oxygen, resulting in a death trap for organisms unable to quickly return to the oxygen-enriched water above. This region also contains a much higher concentration of nutrient salts, like nitrogen, phosphorus, and silica, due to the large amount of dead organic material that drifts down from the above ocean zones and decomposes. It is the deeper part of the midnight zone which starts in the bathypelagic waters above. The area below the abyssal zone is the sparsely inhabited hadal zone. The zone above is the bathyal zone. The deep trenches or fissures that plunge down thousands of meters below the ocean floor (for example, the midoceanic trenches such as the Mariana Trench in the Pacific) are almost unexplored. Previously, only the bathyscaphe Trieste, the remote control submarine Kaikō and the Nereus have been able to descend to these depths. However, as of March 25, 2012 one vehicle, the Deepsea Challenger was able to penetrate to a depth of 10,898.4 meters (35,756 ft). Without plants, a cornerstone of any ecosystem, a unique ecosystem forms. Organisms that live in this zone cannot rely on plants or herbivores to be the bedrock of the ecosystem, instead the species that call the abyssal zone home resort to the only remaining way of acquiring energy. They must feed on each other or the dead organic material, detritus, that falls into the abyssal zone from the levels above it. It is only because of this fallen material that life can exist at this level, to begin with. It replaces plants as the bedrock of the ecosystem while invertebrate decomposers take the place of herbivores as the primary consumer. The biomass of the abyssal zone actually increases when closer to the seafloor as most of the decomposing material and decomposers live there. Since most of the resources or nutrients are present near the seafloor, that area can hold more biomass because it is able to support a more complex food web than the upper areas of the abyssal zone. The sea floor of the abyssal zone consists of or is layered by different materials depending on the depth of the sea floor. If the seafloor is around 4000m below sea level, the seafloor usually consists of calcareous shells of foraminiferan zooplankton and phytoplankton. At depths greater than 4000m below sea level, the seafloor lacks these shells, as they dissolve once they reach a depth greater than 4000m. This leaves behind a seafloor consisting mostly of brown clay and the remaining silica from dead zooplankton and phytoplankton. In some areas of this zone, organisms are able to sustain themselves off of hydrothermal vents. Some bacterial species use the vents to create and use chemical energy in order to produce their own food. For example, many of these organisms convert hydrogen sulfide to sulfate in order to produce chemical energy. They then take that energy and synthesize the carbon-based compounds they use as food. These organisms are then preyed upon by other organisms, meaning that the bacteria can also take the place of plants as part of the bedrock for this ecosystem.   Organisms that live at this depth have had to evolve to overcome challenges provided by the abyssal zone. Fish and invertebrates had to evolve to withstand the sheer cold and intense pressure found at this level. They also had to not only find ways to hunt and survive in constant darkness but to thrive in an ecosystem that has less oxygen and biomass, energy sources or prey items, than the upper zones. In order to survive in a region with so few resources and low temperatures, many fishes and other organisms developed a much slower metabolism and require much less oxygen than those from the upper zones. Movement for many animals living here is also very slow, which allows them to conserve energy.  Their reproduction rates are also very slow in order to decrease competition and conserve energy. The animals living here typically have flexible stomachs and mouths so that when scarce food items are found they can consume as much as possible. Other challenges faced by life in the abyssal zone are the pressure and darkness caused by the zone’s depth. Many organisms living in this zone have evolved to minimize internal air spaces, such as swim bladders. This adaptation helps to protect them from the extreme pressure, which can reach around 11,000 psi. The absence of light also spawned many different adaptations, such as having large eyes or the ability to produce their own light. Large eyes would allow the detection and use of any light available, no matter how small. Another eye adaptation is that many deep-sea organisms have evolved eyes that are extremely sensitive to blue light. This is because as sunlight shines into the ocean, the water absorbs red light, while blue light, with its short wavelength continues moving down to the waters depths. This means that in the deep ocean, if any light remains then it is most likely blue light so animals wanting to capitalize on that light would need specialized eyes tuned to use it. Many organisms use other specialized organs or methods for sensing their surroundings, some in conjunction with specialized eyes. The ability to make their own light is called bioluminescence. Fishes and organisms living in the abyssal zone have developed this ability in order to not only produce light for vision, but also to lure in prey or a mate and conceal their silhouette. Scientists believe that over 90% of life in the abyssal zone use some form of bioluminescence. Many animals that are bioluminescent will produce blue light since it moves farther underwater than other colored lights, as explained earlier. Due to this lack of light, complex designs and bright colors are not needed. Most fish species have evolved to be transparent, red, or black in order to better blend in with the darkness and not waste energy on developing and maintaining bright or complex designs. The abyssal zone is surprisingly made up of many different types of organisms, including microorganisms, crustaceans, molluscan (mollusks and squids), different classes of fishes, and a number of others that might not have even been discovered yet. Most of the fish species in this zone are characterized as demersal or benthopelagic fishes. Demersal fishes are a term that refers to fishes whose habitat is very close to or on the seafloor, typically less than five meters. Most fish species fit into that classification because the seafloor contains most of the abyssal zone’s nutrients so the most complex food web or greatest biomass would be in this region of the zone.