Organisms living in the deep ocean must survive in a physical environment that is radically different from ocean habitats near the sea surface. Image courtesy of NOAA Okeanos Explorer Program.
As you travel from the surface of the ocean downward to the seafloor, water pressure increases by one atmosphere (or 14 pounds per square inch) for every 10 meters of depth. This means, for example, at 100 meters, you would experience one atmosphere of air pressure and 10 atmospheres of water pressure, for a total of 11 atmospheres. Travel to 1,000 meters below the ocean’s surface and most organisms with gas-filled spaces (e.g., lungs) would be crushed by the pressures that other deep-sea life experience.
Light is virtually absent in the deep ocean, which means that deep-sea organisms cannot rely on vision to find food and mates and to maintain various interspecific and intraspecific associations. Lack of light, of course, also precludes photosynthesis and contributes to a general scarcity of food in the deep ocean. As a result, the overall density of organisms is low in many parts of the deep ocean, and many of these organisms are relatively small with low metabolic rates. Chemosynthetic communities in the vicinity of hydrothermal vents and cold seeps are an exception, since they are not dependent upon photosynthesis as a primary food source.
Most deep ocean waters have sufficient oxygen to support life. Most deep seawater originates at the surface of the Arctic or Southern Oceans, where oxygen-rich cold water sinks and flows north or south into the deep oceans. The oxygen in these waters does not appear to be depleted by the respiration of deep-water organisms, presumably because the overall density of these organisms is quite low and these organisms typically have low metabolic rates (exceptions to this generalization are some species that live in the vicinity of seamounts and deep-sea chemosynthetic communities).
Temperature, too, is relatively stable in the deep ocean, typically about 5° C at 1,000 meters depth, although water temperatures in the vicinity of hydrothermal vents can vary by nearly 400° C (the water does not boil because of the high pressure) in the space of a few meters.