The aphotic zone is the oceanic zone under the euphotic zone, where light is limited or absent.
According to the main interpretation of this term, the aphotic zone starts as soon as light is insufficient for photosynthesis, i.e. when the ambient light is lower than 1% of the incident light.
Depending on turbidity conditions and therefore light penetration, the upper limit of this zone varies from a depth of a few centimetres to 200 meters.
A later definition of this term, which is more radical, defines the aphotic zone as the portion of the ocean where total darkness reigns. In this case, the upper limit of the zone is located at a depth of between 1,000 and 1,200 meters.
In this light free zone, only heterotrophic organisms can live, except near hydrothermal sources.
The aphotic zone corresponds to the zones located under the epipelagic zone. Depending on the definition, the mesopelagic zone may or may not be included in the aphotic zone. © Finlay McWalter, Wikimedia public domain