Water Depth Vs. Light Penetration
The single most important factor in distinguishing vertical subdivisions of the benthic and pelagic realms is the availability of solar energy. Sunlight obviously cannot penetrate beyond a certain depth in the ocean. Some organisms have, however, evolved to cope with the absence of sunlight at great depths. Plants require sunlight to carry on photosynthesis, the process that converts carbon dioxide, water, and other nutrients to simple carbohydrates, providing food for themselves and for higher organisms. Below a depth of about 650 ft (200 m) insufficient sunlight penetrates to allow photosynthesis to occur. The interval from the surface to 650 ft (200 m) is therefore known as the euphotic ("eu-" = good, "photo-" = light) zone.
From the standpoint of living organisms, the euphotic zone is probably the most important of all oceanic zones since it is the only place in which any significant amount of photosynthesis takes place. By some estimates, about two-thirds of all the photosynthetic activity that occurs on Earth (on land and in the water) takes place within the euphotic zone.
From 650-3,000 ft (200-1,000 m), the layer known as the dysphotic ("dys-" = bad; "-photic" = light) zone, light is very dim (about 1% penetrates) and photosynthesis rarely occurs. Below this depth, down to the deepest parts of the ocean, it is perpetual night. This layer is called the aphotic ("a-" = without; "-photic" = light) zone. At one time, scientists thought that very little life existed within the aphotic zone. However, they now know that a variety of efficient and interesting organisms can be found living on the deepest parts of the ocean floor. (Some classifications include the dysphotic zone with the aphotic zone and refer to the euphotic zone simply as the photic zone.)
Science EncyclopediaScience & Philosophy: Nicotinamide adenine dinucleotide phosphate (NADP) to Ockham's razorOcean Zones - Water Depth Vs. Light Penetration, The Benthic Realm, Epipelagic Zone, Mesopelagic Zone, Bathypelagic, Abyssopelagic, And Hadalpelagic Zones - The pelagic realm