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Why does photosynthesis stop below certain depths in the ocean?

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Diagram showing ocean zonation by depth and region, with labeled layers: littoral, sublittoral, bathyal, abyssal, and hadal along the seafloor, and epipelagic, mesopelagic, bathypelagic, and abyssalpelagic in open water. The littoral zone is at the high-tide mark. Vertical scale marks photic and aphotic zones from surface to 10,000 meters. — Marine ecosystem Zonation of the ocean. Note that in the littoral zone the water is at the high-tide mark.

Photosynthesis in the ocean stops below certain depths primarily because of the rapid attenuation of sunlight as it penetrates the water column.1

As light travels through the ocean, its intensity decreases due to absorption by water molecules, dissolved substances, and suspended particulates, as well as scattering. While the uppermost layer, known as the euphotic zone, receives sufficient light to support photosynthesis, this region typically extends only to depths of about 80 to 100 metres.123

Beneath this lies the disphotic zone, where light is present but insufficient for photosynthesis to exceed the rate of respiration. Eventually, light is completely extinguished, leading to the aphotic zone—a region of perpetual darkness where photosynthesis cannot occur at all. The specific depth at which these zones occur varies based on local factors such as water turbidity, cloud cover, and the angle of the Sun.231