photic

Photic is an adjective relating to light. In scientific contexts it is most often used to describe the part of a body of water that receives sufficient sunlight to enable photosynthesis, commonly referred to as the photic zone in oceanography and limnology.

In ocean and freshwater environments, the depth of the photic zone varies with water clarity, sun angle, and turbidity. In clear open-ocean water, sunlight can penetrate to roughly 100 to 200 meters, while in murkier coastal or stratified waters the photic zone may extend only a few meters. The light level within this zone supports photosynthetic organisms and thus underpins primary production.

The photic zone is closely related to, but not identical with, the euphotic zone. The euphotic zone specifically denotes the depth range where light is intense enough for photosynthesis to exceed respiration, supporting net primary production. Below the photic/euphotic boundary lies the disphotic zone, where light is present but too weak for sustained photosynthesis, and beneath that the aphotic zone, where light is essentially absent.

Ecologically, the photic zone hosts most of the ocean’s phytoplankton—the base of the marine food web—and concentrates a wide range of zooplankton and nekton that rely on light-driven processes for feeding and behavior. Seasonal changes in solar input, chlorophyll concentration, and water clarity can shift the depth and productivity of the photic zone, influencing nutrient dynamics and ecosystem structure.