Ocean zones can be visualized as layers of a cake, distinguished by changes in light penetration and temperature. The coastal neritic zone is a nutrient-rich area extending 300 to 600 feet offshore. Sunlit photic layers, including the pelagic, then descend into aphotic darkness and terminate in the abyssal layer.
At points more than six miles deep, the abyssal layer is completely dark. On the ocean floor -- where temperatures reach as low as -3 degrees Celsius -- tubeworms, clams and anglerfish live in the final layer of the Benthic zone.
In the open ocean, the pelagic zone's photic layer descends about 300 feet and ends when direct sunlight stops penetrating ocean water. Plankton thrives in the photic zone, converting light into energy via photosynthesis. Food supplies dwindle in the mesopelagic zone, where bioluminescent species use flashing lights to navigate the twilight waters. While some light is available at this depth, photosynthesis no longer occurs. The pelagic zone ends in the sunless aphotic layer -- itself sometimes divided into the aphotic zone and the abyss.
Numerous marine animals benefit directly and indirectly from plankton in the open ocean. Plankton itself dies when it falls out of the photic zone. To remain in the sun, plankton have developed flat and parachute shapes that increase water resistance, air pockets and various other means of increasing drag. In the middle waters, animals such as copepods and lantern fish rise at night as part of a diurnal vertical migration, only to sink back into the twilight during the day. Giant squid, octopus and lantern fish feed in the dark aphotic zone.
Disruptive environmental events such as tsunamis and oil spills can wreak havoc in the open ocean zone, as destroyed plankton stocks send ripples up the entire marine food chain. On the other hand, due to water's capacity to retain heat, the ocean has a moderating effect on weather patterns, driving seasonal events like El Nino. Phytoplankton affects the atmosphere by absorbing carbon dioxide and releasing oxygen, and supports rich fishing grounds, such as the "upwelling grounds" off South America.