The photic zone receives light while the aphotic zone does not, and the size of the aphotic zone affects primary productivity. The aphotic zone begins where less than 1% of light penetrates and varies based on turbidity. The deep ocean aphotic zone is home to bioluminescent animals and is relatively deserted compared to the photic zone.
The photic zone refers to the portion of the water column in a lake, sea, or ocean that receives light, while the aphotic zone is the portion of the water that does not. Since light is required for photosynthesis, the primary productivity of a water body (the amount of biomass generated directly from solar energy) is directly proportional to the size of the aphotic zone. Formally, this zone begins where less than 1% of light penetrates.
Especially in the ocean, the part of the water that gets no light is a very deep area. It extends from 3,000 to 15,000 feet (0.9 – 4.6 km) deep. A variety of unusual animals live here, including giant squid, vampire squid, gulper eel, anglerfish, and many more. Many animals in this area are bioluminescent, meaning they are able to generate their own light.
Where the aphotic zone begins depends on the number of particles suspended in the water, a quality called turbidity. It varies significantly by season, whether it has rained recently, the type and location of the body of water, and other factors. Turbidity can be roughly measured with a simple device called a Secchi disk, which consists of a disk covered in an alternating black and white pattern. The puck is put on a rod and lowered into the water. The point where the pattern becomes impossible to see tells the viewer how cloudy the water is. For more accurate measurement, a device called a nephelometer is used.
In the world’s oceans, aphotic zones are relatively deserted compared to photic zones. For comparison, a person can imagine taking a giant parasol and using it to enclose everything in a patch of forest – eventually, most life forms in the shaded part of the forest would die. In the oceans, however, debris that rains down from above provides a food source, so some life can survive. Other animals spend time in this zone but ascend to the photic zone to feed.
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