Photic zone

The upper layer of a body of water where sunlight is sufficient for photosynthesis. Its depth varies with water clarity, sunlight, and location.

Photosynthetically active radiation: The portion of light between roughly 400 and 700 nanometers that plants and many microbes use in photosynthesis. This wavelength range supplies the light used to define the zone’s photosynthetic limit.
Primary production: The creation of organic matter from inorganic carbon by photosynthesis or chemosynthesis. Photosynthetic primary production is concentrated where light reaches sufficient levels.
Aphotic zone: The part of a water body where light is too weak to support photosynthesis. It lies below the light-supported zone and lacks photosynthetic production.
Water transparency: The degree to which light can pass through water without being absorbed or scattered. Transparency varies over time and controls how deep the photic zone extends.
Light attenuation: The reduction of light intensity as it travels through a medium, caused by absorption and scattering. Attenuation determines how quickly usable sunlight disappears with depth.
Phytoplankton: Microscopic photosynthetic organisms that drift in aquatic environments. These organisms account for much of the zone’s open-water photosynthesis.
Dysphotic zone: The dimly lit region of a water body where sunlight is detectable but insufficient for photosynthesis. It separates productive surface waters from the lightless depths in some classifications.
Turbidity: The cloudiness of water caused by suspended particles and other substances that scatter light. Runoff and resuspension can increase turbidity and shrink illuminated habitat.
Secchi disk: A circular disk lowered into water to estimate its transparency from the depth at which it vanishes from view. Its visibility depth provides a practical clue to light penetration.
Seagrass meadow: A shallow coastal ecosystem dominated by flowering plants adapted to life underwater. Seagrasses require enough bottom light, limiting meadows to illuminated waters.

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