Abyssal zone

The abyssal zone or abyssopelagic zone is a deep part of the pelagic zone of the ocean. It lies between 4,000 and 6,000 meters (13,000 and 20,000 feet) below the surface, where it is always dark. This zone covers a huge area—about 83% of the ocean and 60% of Earth's entire surface.

Because there is no sunlight, plants cannot grow here, and there is very little oxygen. Most of the oxygen in this zone comes from ice that melted long ago in the polar regions. The water is very cold, staying around 2–3 °C (36–37 °F), and the pressure is extremely high, up to 76 MPa (which is like 11,000 times the normal air pressure you feel on land).

The abyssal zone is an important part of the ocean. It has lots of nutrients such as nitrogen, phosphorus, and silica, which come from dead animals and plants that sink down from the shallower parts of the ocean. Below the abyssal zone lies an even deeper area called the hadal zone, which is home to only a few special kinds of life. Above it is the bathyal zone.

Trenches

Main article: Hadal zone

Deep trenches in the ocean go down thousands of meters below the ocean floor, such as the Mariana Trench in the Pacific. These areas are mostly unexplored. Only a few special vehicles, like the bathyscaphe Trieste, the remote control submarine Kaikō, the Nereus, and the Deepsea Challenger, have ever reached these deep depths.

Ecosystem

The abyssal zone is a deep part of the ocean where there is very little light. Most of the animals here rely on food that falls from the layers above, called marine snow. Near the seafloor, there is more life because this is where a lot of the dead material ends up.

The seafloor changes depending on how deep you go. Above 4000 meters, it is often covered with the shells of tiny sea creatures and plants. Deeper than that, the shells dissolve, leaving a mix of clay and tiny bits of dead sea creatures. Special bacteria near underwater hot spots called hydrothermal vents help support many kinds of life, much like plants do in shallower waters.

Biological adaptations

Organisms that live in the deep ocean's abyssal zone have evolved to survive in very cold, dark, and high-pressure conditions. They have developed slower metabolisms and move slowly to save energy, as food is scarce there. Many of these animals have flexible bodies to eat as much as they can when food is found.

To cope with the deep sea's darkness and pressure, many creatures have large eyes to see any tiny bits of light, and some can even make their own light through a process called bioluminescence. This light, often blue, travels farther in the deep sea than other colors. Because there is no light, most deep-sea animals are transparent, red, or black, which helps them blend in without needing bright colors. They have also adapted to the pressure by minimizing air spaces in their bodies, such as not having swim bladders, which helps them stay safe under the immense weight of the water above.

Animals

The abyssal zone is home to many different kinds of creatures, including tiny organisms, crustaceans, molluscs, and various types of fish. Most of these fish live close to the ocean floor, where they can find food. They depend on nutrients that fall down from the layers above, like the remains of animals that die and sink.

Some interesting animals in the abyssal zone include the tripod fish, which stands on long bony rays and uses its fins to catch food. The Dumbo octopus, named for the ears on its head, hovers above the sea floor and uses its arms to move around. The cusk eel, one of the deepest-living fish, can be found at amazing depths and uses special organs to sense its surroundings. The abyssal grenadier has huge eyes to see in the dark, and the Mariana snailfish lives in the deepest parts of the ocean, where few other creatures can survive.

Environmental concerns

Climate change affects the abyssal zone, mainly through ocean acidification. Pollutants like plastics are also found here, which can harm the creatures that live in the deep sea.

Human activities, such as overfishing in deeper waters, can reduce the amount of food that reaches the abyssal zone. In the future, deep sea mining may cause serious problems by polluting the water, destroying habitats, and disturbing the sea floor. These changes could take a long time to recover from because deep-sea creatures live for many years and reproduce slowly.