Marine Life Zones

About this topic

Ocean life is organised by light and depth. The sunlit (euphotic) zone, down to about 200 m, hosts photosynthesising phytoplankton, the base of the food web, and most fish. The twilight zone (to 1000 m) has dim light, and below it the dark, cold, high-pressure midnight and abyssal zones, where animals use bioluminescence, slow metabolism and chemosynthesis.

Horizontally, there are intertidal, neritic (coastal) and open ocean zones. Coral reefs, kelp forests and estuaries are particularly rich habitats.

Imagine descending in a submarine. At 10 m you have lost red light, and by 200 m only faint blue remains. At 1,000 m it is dark apart from living lights, and about 90 percent of deep-sea animals make bioluminescence, used to attract prey, confuse predators or find mates. The anglerfish dangles a glowing lure, and many animals perform a daily vertical migration, rising at night to feed and returning down by day, the largest migration on Earth in terms of biomass.

Coral reefs cover less than 1 percent of the sea floor but support about 25 percent of marine species; they depend on partnership with algae in the coral, and warm water causes bleaching when this fails. Kelp forests shelter fish and absorb carbon, and estuaries where rivers meet the sea are nurseries for young fish. Marine snow, a continuous fall of dead organisms from above, feeds the deep. A common misconception is that the deep sea is empty or lifeless; it is a huge habitat, and most of the planet's living space is in the ocean. Another is that all ocean life needs sunlight.