layers of the Ocean
The average depth of the seafloor is 4000 m, but ocean trenches can be as deep as 11,000 m! Sunlight powers productivity in the surface ocean, but it quickly disappears with depth. The loss of sunlight alters the composition of sea life, and each layer contains organisms with special adaptations that optimize energy expenditure. At extreme depths at or near the seafloor, energy based on chemical reactions becomes more important to sustaining life.
OCEAN ZONES BY DEPTH
Epipelagic Zone
The epipelagic zone is the top 200 m of the ocean, where sunlight penetrates and photosynthesis is still possible. Pelagic refers to open ocean where organisms swim, float, or drift as opposed to benthic, which is a seafloor habitat where organisms crawl, burrow, or otherwise anchor themselves in place.
The epipelagic zone is characterized by a high abundance of phytoplankton like the bacteria Prochlorococcus or Synechococcus, as well as other eukaryotic algae that form the base of the food web. Many zooplankton (tiny animals) graze on the phytoplankton and, to avoid predation during the day, perform the largest vertical migration on the planet every day. Zooplankton like copepods, which are tiny crustaceans, are food for nekton, or swimming animals like fish, marine mammals, turtles, and squid.
Mesopelagic Zone
The mesopelagic zone is where things get weird. This zone is also known as the "Twilight Zone". Since sunlight is not strong enough to support photosynthesis here or warm up the water, there is a steep thermocline and biology begins to look quite different. A thermocline is a steep temperature gradient. In this case, it begins to get very cold the deeper you go into the Twilight Zone.
The mesopelagic zone begins around 200 m depth and ends around 1000 m depth. The pelagic organisms begin to decrease because there is less energy here, and fishes begin to take on some creepy characteristics. Squid and crustaceans like to hang out here and provide food for fishes and marine mammals like sperm whales. The midwater fishes are adapted to survive in an environment where food is scarce, so they have developed the ability to trap and keep prey in their mouths to prevent escape. Other adaptations include weird tubular eyes, bioluminescence, and adaptive coloration or transparency.
The Deep Ocean
The layers of the deep ocean include the bathypelagic (1,000 - 4,000 m), the abyssopelagic (4,000 - 6,000 m), and the hadopelagic (6,000 - 11,000 m). The abyssopelagic and the hadopelagic include areas near the seafloor that are below the average depth of the ocean. The abyssal and the hadal zones are locations where there is convergence of two or more crustal plates, where one is being dragged down beneath the other in a process called subduction. The hadal zone is exclusively in deep ocean trenches.
The deep ocean is characterized by pelagic species like deep-sea anglerfish, devilfish, gulpers, and swallowers. Benthic fish and other crustaceans, molluscs, and worms are common at hydrothermal vents and seeps, where chemosynthesis can support a more diverse ecosystem. Check out the page on hydrothermal vents and seeps!