Aphotic

As experiment has shown that light waves capable of effecting chemical changes do not penetrate the water deeper than 400 meters and as the other conditions of assimilation are met above this line, it is proposed to distinguish a diaphanic region including the land and the water to a depth of 400 m. and inhabited by assimilating organisms and an aphotic region including the ocean and lakes below the assimilation-line and the dark caves of the crust, where plant life is impossible.

We have bacteria classified as aërobic and anaërobic, according as they thrive with or without access of air. We need to classify bacteria as photic and aphotic.

Photographically the light of the sun is detectable in exceptionally clear-water tropical seas to a depth of about 2,000 feet, but Johnstone places the average depth for all waters at 650 feet, beyond which there is more or less of total darkness, the aphotic realm.

Respiration and organic matter decomposition occur through the water column, and depending on the amount of organic matter present and the extent of wind-mixing, can deplete the dissolved oxygen supply in aphotic (unlighted) layers.

Large systems can always be subdivided. A large lake, for example, will have a surface photic environment, an aphotic system on the lakebed and a coastal system at the land–water interface.

Although algae are frequently found below the photic zone, because of circulation or settling, they are not photosynthesizing under such conditions. In the aphotic, profundal zone food resources are provided exclusively through secondary productivity, consumption of settling detritus (or the organisms that feed on such detritus), and microbial food resources.

The deep aphotic zone accounts for the largest portion (90%) of the marine benthic environment. Owing to the lack of solar energy, vegetation and photosynthetic primary production are largely nonexistent and food webs are mainly driven by heterotrophy. [...] A special mode of nutrition in the aphotic benthic zone is chemoautotrophy. Here chemoautotrophic bacteria and archaea replace photosynthetic primary production by using geothermally produced inorganic energy as a food source near deep sea vents and cold seeps.