concept

Aphotic zone

The part of the ocean where downwelling sunlight has failed entirely — broadly, below about 1,000 m. The sea does not simply darken; it filters first and then goes out. Under the Beer-Lambert law attenuation depends strongly on wavelength, so red and orange are gone within the first tens of metres while a blue-green window near 470–490 nm decays some twenty times more slowly. Below 200 m there is not enough for photosynthesis; below 1,000 m there is none. Every reflection-based signalling system dies there together, and the only way left to say anything is to make light yourself.