disphotic

Disphotic is an oceanographic term describing the twilight zone of the ocean where there is some light but not enough to support photosynthesis. It lies between the photic zone, where light is sufficient for photosynthesis, and the aphotic zone, where light is effectively absent. The term is often used to describe the zone where light is reduced but still detectable, and it is commonly referred to as the twilight zone.

Depth range for the disphotic zone varies with water clarity and location. In open ocean waters, the boundary between the photic and disphotic zones commonly falls around a few hundred meters depth, while the disphotic zone itself may extend roughly from about 200–300 meters down to around 1000 meters. Clearer waters can push the photic boundary deeper and the disphotic zone deeper as well, whereas turbid or coastal waters shorten these depths. The exact thresholds are not fixed and are defined by light levels, usually in terms of downwelling irradiance or photosynthetically active radiation rather than a rigid depth.

Light in the disphotic zone is dim and tends to be blue-green in quality, diminishing with depth. This lighting is generally insufficient for photosynthesis, though some light persists enough for vision and for the behavior of light-sensitive organisms. Organisms in this zone often rely on enhanced vision, bioluminescence, or other senses, and many perform diel vertical migrations across the boundary between the disphotic and deeper zones.

The disphotic zone is important for understanding marine ecology and energy transfer, as it hosts a diverse array of mesopelagic and deeper-dwelling species and reflects how light shapes behavior and distribution in the ocean.