A hydrothermal vent in the deep sea

The deep sea, or deep layer^[1] is the lowest layer in the ocean, existing below the thermocline. Little or no light penetrates this area of the ocean, and most of its organisms rely on falling organic matter produced in the photic zone for subsistence. For this reason life is much more sparse, becoming rarer still with increasing depth. The other essential ingredient for life is oxygen, which is brought to the ocean's depths via the thermohaline circulation. There is, however, another very different part of the deep sea food chain discovered in the late 1970s, which is rooted in chemosynthesis rather than photosynthesis.

The ocean depths are one of the most hostile environments for life, and represent the least explored of all the world's ecosystems. Cold, dark and almost inaccessible to humans, it is perhaps the closest environment on earth to outer space.

Physical and chemical characteristics

Pelegic zones

The deep ocean is not well mixed, consists of horizontal layers of equal density, and is often as cold as -1 to 4°C (32 to 37°F). It is characterized by a nearly constant temperature and a positive sound speed gradient caused by pressure.^[1] Pressure increases by 1 atm with every 10 meters. Water at a depth of 4 km for example exerts 400 times more pressure than that at the surface.

Light dims by 90% every 75 meters; by 200m, the waters are dimly lit and by 1000m they receive no light at all. For comparison, the human visual threshold occurs at 650m. Ninety percent of the total volume of Earth's oceans is found in the deep ocean. The deepest waters on Earth lie 10,912m below the surface in the Mariana Trench.

Biology

Regions below the epipelagic are divided into further zones, beginning with the mesopelagic which spans from 200 to 1000m below sea level, where a little light penetrates while still being insufficient for primary production. Below this zone the deep sea proper begins, consisting of the aphotic bathypelagic, abyssopelagic and hadopelagic. Food consists of falling organic matter known as 'marine snow' and carcasses derived from the productive zone above, and is scarce both in terms of spatial and temporal distribution.

Adaptations of midwater fish

The midwater fish have special adaptations to cope with these conditions - they are small, usually being under 25cm; they have slow metabolisms and unspecialized diets, preferring to sit and wait for food rather than waste energy searching for it. They have elongated bodies with weak, watery muscles and skeletal structures. They often have extendable, hinged jaws with recurved teeth. Because of the sparse distribution and lack of light, finding a partner with which to breed is difficult, and many organisms are hermaphroditic.

Because light is so scarce, fish often have larger than normal, tubular eyes with only rod cells. Their upward field of vision allows them to seek out the silhouette of possible prey. Prey fish however also have adaptations to cope with predation. These adaptations are mainly concerned with reduction of silhouette, a form of camouflage. The two main methods by which this is achieved are reduction in the area of their shadow by lateral compression of the body, and counter illumination via bioluminescence. This is achieved by production of light from ventral photophores, which tend to produce such light intensity to render the underside of the fish of similar appearance to the background light.

Exploration

Main article: Deep-sea exploration

The deep sea is an environment totally inhospitable to humankind, and it should come as no surprise that it represents one of the least explored areas on Earth. Pressures even in the mesopelagic become too great for traditional exploration methods, demanding alternative approaches for deep sea research. Baited camera stations, small manned submersibles and ROVs (remotely operated vehicles) are three methods utilized to explore the ocean's depths. Because of the difficulty and cost of exploring this zone, current knowledge remains limited.

References

External links

• "Temperature of Ocean Water", University Corporation for Atmospheric Research, University of Michigan.
• Creatures of the Deep: Photo gallery and a Flash video of creatures from the deep ocean.

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