The euphotic zone is somewhat arbitrarily

The euphotic zone is somewhat arbitrarily defined as extending from the surface to the depth where the light intensity is approximately 0.1–1% of surface sunlight irradiance, Sony VAIO VGN-NW71FB/N Battery

depending on season, latitude and degree of water turbidity.^[6][7] In the clearest ocean water, the euphotic zone may extend to a depth of about 150 meters,^[6] or rarely, up to 200 meters.^[8]Dissolved substances and solid particles absorb and scatter light, and in coastal regions the high concentration of these substances causes light to be attenuated rapidly with depth. Sony VAIO VGN-NW71FB/W Battery

In such areas the euphotic zone may be only a few tens of meters deep or less.^[6][8] The disphotic zone, where light intensity is considerably less than 1% of surface irradiance, extends from the base of the euphotic zone to about 1000 meters.^[9] Extending from the bottom of the photic zone down to the seabed is theaphotic zone, a region of perpetual darkness.Sony VAIO VGN-NW91FS Battery

Since the average depth of the ocean is about 4300 meters,^[10] the photic zone represents only a tiny fraction of the ocean’s total volume. However, due to its capacity for photosynthesis, the photic zone has the greatest biodiversity and biomass of all oceanic zones. Nearly all primary production in the ocean occurs here. Sony VAIO VGN-NW91GS Battery

Any life forms present in the aphotic zone must either be capable of movement upwards through the water column into the photic zone for feeding, or must rely on material sinking from above,^[5] or must find another source of energy and nutrition, such as occurs in chemosynthetic archaea found near hydrothermal vents and cold seeps. Sony VAIO VGN-NW91VS Battery

These animals have evolved to survive the extreme pressure of the sub-photic zones. The pressure increases by about oneatmosphere every ten meters. To cope with the pressure, many fish are rather small, usually not exceeding 25 cm in length. Also, scientists have discovered that the deeper these creatures live, Sony VAIO VGN-SR11M Battery

the more gelatinous their flesh and more minimal their skeletal structure. These creatures have also eliminated all excess cavities that would collapse under the pressure, such as swim bladders.^[11]

Pressure is the greatest environmental factor acting on deep-sea organisms. Sony VAIO VGN-SR11MR Battery

In the deep sea, although most of the deep sea is under pressures between 200 and 600 atm, the range of pressure is from 20 to 1,000 atm. Pressure exhibits a great role in the distribution of deep sea organisms. Until recently, people lacked detailed information on the direct effects of pressure on most deep-sea organisms, Sony VAIO VGN-SR140D Battery

because virtually all organisms trawled from the deep sea arrived at the surface dead or dying. With the advent of traps that incorporate a special pressure-maintaining chamber, undamaged larger metazoan animals have been retrieved from the deep sea in good condition. Sony VAIO VGN-SR140D/B Battery

Some of these have been maintained for experimental purposes, and we are obtaining more knowledge of the biological effects of pressure.

The two areas of greatest and most rapid temperature change in the oceans are the transition zone between the surface waters and the deep waters, the thermocline, Sony VAIO VGN-SR140D/P Battery

and the transition between the deep-sea floor and the hot water flows at the hydrothermal vents. Thermoclines vary in thickness from a few hundred meters to nearly a thousand meters. Below the thermocline, the water mass of the deep ocean is cold and far more homogeneous. Sony VAIO VGN-SR140D/S Battery

Thermoclines are strongest in the tropics, where the temperature of the epipelagic zone is usually above 20°C. From the base of the epipelagic, the temperature drops over several hundred meters to 5 or 6°C at 1,000 meters. It continues to decrease to the bottom, but the rate is much slower. Below 3,000 to 4,000 m, the water is isothermal. Sony VAIO VGN-SR140E Battery

At any given depth, the temperature is practically unvarying over long periods of time. There are no seasonal temperature changes, nor are there any annual changes. No other habitat on earth has such a constant temperature.

Hydrothermal vents are the direct contrast with constant temperature. Sony VAIO VGN-SR140E/B Battery

In these systems, the temperature of the water as it emerges from the "black smoker" chimneys may be as high as 400°C (it is kept from boiling by the high hydrostatic pressure) while within a few meters it may be back down to 2 - 4°C.

Salinity is remarkably constant throughout the depths of the deep sea. There are two notable exceptions to this rule: Sony VAIO VGN-SR140E/P Battery

In the Mediterranean Sea, water loss through evaporation greatly exceeds input from precipitation and river runoff. Because of this, salinity in the Mediterranean is higher than in the Atlantic Ocean.^[13] Evaporation is especially high in its eastern half, causing the water level to decrease and salinity to increase in this area. Sony VAIO VGN-SR140E/S Battery

The resulting pressure gradient pushes relatively cool, low-salinity water from the Atlantic Ocean across the basin. This water warms and becomes saltier as it travels eastward, then sinks in the region of the Levant and circulates westward, to spill back into the Atlantic over the Strait of Gibraltar.^[15] Sony VAIO VGN-SR140N/S Battery

The net effect of this is that at the Strait of Gibraltar, there is an eastward surface current of cold water of lower salinity from the Atlantic, and a simultaneous westward current of warm saline water from the Mediterranean in the deeper zones. Once back in the Atlantic, this chemically distinct Mediterranean Intermediate Water can persist for thousands of kilometers away from its source.^[16] Sony VAIO VGN-SR165E/B Battery

2. Brine pools are large areas of brine on the seabed. These pools are bodies of water that have a salinity that is three to five times greater than that of the surrounding ocean. For deep sea brine pools the source of the salt is the dissolution of large salt deposits through salt tectonics. Sony VAIO VGN-SR165E/P Battery

The brine often contains high concentrations of methane, providing energy to chemosynthetic extremophiles that live in this specialized biome. Brine pools are also known to exist on the Antarctic continental shelf where the source of brine is salt excluded during formation of sea ice. Sony VAIO VGN-SR165E/S Battery

Deep sea and Antarctic brine pools can be toxic to marine animals. Brine pools are sometimes called seafloor lakes because the dense brine creates a halocline which does not easily mix with overlying seawater. The high salinity raises the density of the brine, which creates a distinct surface and shoreline for the pool. Sony VAIO VGN-SR190EBJ Battery

The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline, at a depth of 1000 fathoms (1800 m) or more. The deepest part of the deep sea is Mariana Trench located in the western North Pacific. It is also the deepest point of the earth's crust. Sony VAIO VGN-SR190EBQ Battery

It has a maximum depth of about 10.9 km which is deeper than the height of Mount Everest.^[18] In 1960, Don Walsh and Jacques Piccard reached the bottom of Mariana Trench in the Trieste bathyscaphe. The pressure is about 11,318 metric tons-force per square meter (110.99 MPa or 16100 psi). Sony VAIO VGN-SR190EEJ/C Battery

The upper photic zone of the ocean is filled with particle organic matter (POM) and is quite productive, especially in the coastal areas and the upwelling areas. However, most POM is small and light. It may take hundreds, or even thousands of years for these particles to settle through the water column into the deep ocean. Sony VAIO VGN-SR190NAB Battery

This time delay is long enough for the particles to be remineralized and taken up by organisms in the food webs.

Scientists at Woods Hole Oceanographic Institution conducted an experiment three decades ago in deep Sargasso Sea looking at the rate of sinking.^[19] Sony VAIO VGN-SR190NBB Battery

They found what became known as marine snow in which the POM are repackaged into much larger particles which sink at much greater speed, 'falling like snow'.

Because of the sparsity of food, the organisms living on and in the bottom are generally opportunistic. Sony VAIO VGN-SR190NDB Battery