The importance of light in the ocean is reflected by the description of the ocean’s vertical zones of the water column in terms of how much light these zones receive. The ocean is generally divided into three named zones: the photic (or epipelagic), twilight (or mesopelagic), and midnight (bathypelagic) zones.
The upper 200 meters (656 feet) of the ocean is called the epipelagic or photic zone. Sunlight penetrates this zone sufficiently to support the growth of phytoplankton and/or macro algae (i.e., plants that need sunlight to make food and survive), providing the bulk of ocean primary production (food). Organisms in the zones below are dependent on what food drifts down from above, ranging from tiny clumps of bacteria and dead algae to occasional bonanzas like a dead whale.
The area between 200 and 1,000 meters (656 and 3,280 feet) is the mesopelagic or “twilight” zone. Light intensity in this zone is severely reduced with increasing depth, so light penetration is minimal. About 20 percent of primary production from the surface falls down to the mesopelagic zone. Consequently, the density or biomass of mesopelagic zone occupants is lower than at the surface, and mesopelagic organisms have an interesting variety of mechanisms that help them find food as well as avoid being meals for other species.
Sunlight does not penetrate the eternal darkness below 1,000 meters (3,280 feet), an area known as the bathypelagic zone. Popular sources sometimes refer to this as the midnight zone. The only light available at these depths is generated by organisms. About five percent of the primary production from the surface makes it to the bottom of the ocean. Less food means lower biomass. Occasionally, large items like dead sharks or whales reach the seafloor, but generally food is scarce.