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The Definition of the Aphotic Zone

The ocean is divided into three main zones that are separated by the amount of light that reaches them: the photic zone, the dysphotic zone and the aphotic zone. As sunlight diffuses through the water, its intensity decreases. This is primarily due to the presence of particulate matter in the water column which scatters the light, thus diffusing it. The deeper the light must travel, the more suspended particulate matter light hits, and thus the dimmer the overall light. In addition, the density of water is greater than air, which attenuates light with depth. Beyond a certain depth, there is no light. This is called the aphotic zone.

The Photic Zone
- The surface of the ocean to approximately 660 feet deep is called the photic zone. "Photic" means "of light." It is also sometimes referred to as the sunlight or euphotic zone. Here, there is enough light for photosynthesis, the conversion of light energy into food and oxygen in plants and algae called phytoplankton. Animals, in turn, feed on the plants and phytoplankton. Since there is an abundance of food in this zone, roughly 90 percent of all life in the ocean is found in the photic zone.
The Dysphotic Zone
- Light penetrates into the dysphotic, or twilight zone, but not enough for photosynthesis to occur. This zone extends from the bottom of the photic zone, at 660 feet, to about 3,300 feet deep. Squid, octopus and hatchet fish are a few of the organisms found living in the dysphotic zone. Some animals in this zone exhibit bioluminescence: they produce their own light by biochemical reactions in their bodies. Since light and prey are scarce here, most animals capture their food by waiting for it to drift by or by stalking it, rather than hunting and chasing it.
The Aphotic Zone
- In the aphotic, or midnight zone, there is no light, so there is also no plant life, since plants require sunlight for photosynthesis. However, there is life here. When organisms from the photic zone die, they fall to the aphotic zone where they are used as food for the animals that dwell in the depths. This includes larger organisms such as fish and mammals, but most of the food descends as dead tiny or even microscopic organisms called plankton. The water here is extremely cold, and the pressure is great, so organisms living here have evolved to deal with these extreme conditions.
The Trench Zone
- The aphotic zone is sometimes divided into the midnight zone and the trench zone. The trench zone is where deep sea trenches extend to extreme depths in certain areas of the ocean. It is part of the aphotic zone, as no light can ever reach here. The deepest trench known is the Marianas trench, which reaches to 35,840 feet deep. Life exists in the trenches as well, where animals live associated with deep sea hydrothermal vents and produce food from chemosynthesis.

Nature