Aphotic zone

The region of a body of water where light is too faint to support photosynthesis. Its depth varies with water clarity and local conditions.

Photic zone: The sunlit upper layer of water where enough light is available for photosynthesis. Its lower boundary marks the transition into the aphotic zone.
Photosynthesis: The process by which organisms use light energy to convert carbon dioxide and water into organic compounds. The aphotic zone is defined by light levels too low to sustain this process.
Marine snow: A continuous shower of organic particles sinking from upper waters into the deep ocean. It carries energy from sunlit waters into ecosystems without local photosynthesis.
Diel vertical migration: The daily movement of aquatic organisms between deeper waters and the surface. Many organisms leave aphotic depths at night to feed nearer the surface.
Light attenuation: The decrease in light intensity as it travels through water or another medium. Attenuation determines how quickly photosynthetically useful light disappears with depth.
Ocean stratification: The layering of ocean water by differences in density, commonly caused by temperature or salinity. Stratification affects the movement of nutrients and organisms across light-defined layers.
Deep-sea gigantism: The tendency for some deep-sea species to grow larger than related shallow-water species. It is one adaptation associated with life in dark, food-limited waters.
Deep-sea food web: The network of feeding relationships among organisms in the deep ocean. Its energy often arrives as sinking material or derives from chemical sources.
Compensation depth: The depth at which photosynthetic carbon production balances respiratory carbon use. It approximates the deepest level where photosynthesis can sustain net plant growth.
Water column: The vertical body of water between the surface and the bottom of an aquatic environment. The aphotic zone is one region within this vertical habitat.

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Pelagic zoneBroader topic: Much of the deep pelagic zone remains dark and depends on food arriving from above.