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, 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 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 area below the abyssal zone is the sparsely inhabited hadal zone. The zone above is the bathyal zone.

Trenches

The deep trenches or fissures that plunge down thousands of meters below the ocean floor 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.

Ecosystem

Without producers, the cornerstone of most ecosystems, a unique ecosystem forms. Rather than relying on producers to form the base of the food pyramid, organisms living in the abyssal zone must feed on the dead organic detritus that falls from oceanic layers above. The biomass of the abyssal zone actually increases near the seafloor as compared to areas above as most of the decomposing material and decomposers rest on the seabed.
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 the products of hydrothermal vents. Some bacterial species use the vents to create and use chemical energy to produce food. For example, many of these organisms convert hydrogen sulfide to sulfate to produce chemical energy. They use that energy to 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.

Biological adaptations

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. To survive in a region with so few resources and low temperatures, many fish and other organisms developed a much slower metabolism and require much less oxygen than those in upper zones. Many animals also move very slowly to conserve energy. Their reproduction rates are also very slow, to decrease competition and conserve energy. Animals here typically have flexible stomachs and mouths so that when scarce food items are found they can consume as much as possible.
in the Mid-Cayman Rise. The shrimp are almost entirely blind, surviving at the interface of cold, deep seawater and supercritical hydrothermal fluid.
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 75 MPa. 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 water's 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 not only to 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 produce blue light, since it moves farther underwater than other colors of light, 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 so they better blend in with the darkness and don't waste energy on developing and maintaining bright or complex designs.

Animals

The abyssal zone is surprisingly made up of many different types of organisms, including microorganisms, crustaceans, molluscan, 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. 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.
For benthic organisms in the abyssal zone, species would need to have evolved morphological traits that could keep them out of oxygen-depleted water above the sea floor or a way to extract oxygen from the water above, but also, allow the animal access to the sea floor and the nutrients located there. There are also animals that spend their time in the upper portion of the abyssal zone, and even sometimes spending time in the zone directly above, the bathyal zone. While there are a number of different fish species representing many different groups and classes, like Actinopterygii or ray-finned fish, there are no known members of the class Chondrichthyes, animals such as sharks, rays, and chimeras, that make the abyssal zone their primary or constant habitat. Whether this is due to the limited recourses, energy availability, or other physiological constraints is unknown. Most Chondrichthyes species only go as deep as the bathyal zone.      
As with all of the rest of the natural world climate change has negative effects. Due to the zone’s depth, increasing global temperatures do not affect it as quickly or drastically as the rest of the world, but the zone is still afflicted by ocean acidification. Along with climate change and ocean acidification, pollutants, such as plastics, are also present in this zone. Plastics are especially bad for the abyssal zone due to the fact that these organisms have evolved to eat or try to eat anything that moves or appears to be detritus, resulting in most organisms consuming plastics instead of nutrients. Both ocean acidification and pollution are decreasing the already small biomass that resides within the abyssal zone. Another problem caused by humans is overfishing. Even though no fishery can fish for organisms anywhere near the abyssal zone, they are still causing harm. The abyssal zone depends on dead organisms from the upper zones sinking to the seafloor, since their ecosystem lacks producers due to lack of sunlight. As fish and other animals are removed from the ocean, the frequency and amount of dead material reaching the abyssal zone decreases. A future problem for the abyssal zone could be deep sea mining operations. The talks and planning for this industry are already underway. This could be disastrous for this extremely fragile ecosystem since the ecological dangers from mining for deep sea minerals are many. Mining could increase the amount of pollution in not only the abyssal zone, but in the ocean as a whole, and would physically destroy habitats and the seafloor. This industry represents a looming threat to the abyssal zone and the rest of the ocean's inhabitants.