Which aquatic zone receives less than 1% of sunlight?
The aphotic zone lies below the depth at which usable sunlight can support net photosynthesis. In many aquatic classifications, its upper boundary occurs where irradiance has fallen to about 1% of the value at the surface, although the exact depth changes with water clarity, season, and solar angle. Suspended sediments, dissolved organic matter, and plankton absorb or scatter light, so turbid lakes become aphotic at shallower depths than clear oceans. Because photosynthetic production is negligible there, organisms depend mainly on organic particles sinking from illuminated layers, dissolved organic matter, chemosynthesis in special habitats, or predation. This distinguishes the aphotic zone from the photic or euphotic zone, where enough light remains for photosynthesis, and from the neritic zone, which is defined horizontally as seawater over the continental shelf rather than by illumination. Thus, receiving less than approximately 1% of surface sunlight is the operational ecological feature that identifies aphotic water. Ecologically.
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 2-3