Habitats by depth

The ocean sunlight zone, also known as the surface zone or epipelagic zone, extends to a depth of 200 metres and makes up a mere 5% of the ocean. It is the brightest and warmest layer ranging from -2 to 37 °C. The light allows plankton, drifting plants, algae, and some bacteria, to photosynthesise, a hugely important function as somewhere between 50 and 80% of the oxygen produced on Earth comes from ocean photosynthesis. Phytoplankton plays an extremely important role in the ocean: in addition to converting sunlight into chemical energy and consuming carbon dioxide and producing oxygen, it is also the lowest point of the marine food chain. The epipelagic zone is where much of the marine life we know is found and where the majority of commercial fishing takes place.

The twilight zone or mesopelagic zone ranges from 200 metres to 1,000 metres and a relatively constant temperature of 4 °C prevails. The light which reaches this zone is minimal but the zone is nonetheless bursting with life, commercial fishing does not reach this depth and only a few predators from the epipelagic zone, such as the sperm whale, dive down to feed here, leaving about 90% of the world’s fish (by weight) to occupy this zone. With little light available, this is where bioluminescent creatures make their first appearance. Many fish, plankton, jellyfish and squid have the ability to make their own light or host bacteria that do, the illumination is often used to defend, communicate, find prey or provide camouflage. Marine life found in this zone ranges from microscopic bacteria or zooplankton to whales. Some remain at these depths their whole lives, other fish, squid and zooplankton embark on a nightly migration (the largest of the planet), swimming hundreds of metres up to the surface at night to feed on phytoplankton and diving back down again to the shelter and protection of the mesopelagic zone before sunrise to avoid becoming prey themselves. This migration supports both the marine food chain and the carbon pump as they transfer carbon dioxide down to the deep ocean when they return to the depths in the early morning hours. About 90% of the carbon remains in the twilight zone, the rest sinks to the ocean floor when animals die or expel carbon-rich faecal matter.

The deep ocean extends from the depth of 1,000 metres down to the ocean floor, making up about 75% of the ocean. The midnight or bathypelagic zone (1,000-4,000 metres), abyss or abyssopelagic zone (4,000 metres to the ocean floor at around 6,000 metres) and the ocean trenches or hadalpelagic zone (down to around 11,000 metres at its deepest point) make up the deep ocean. There is no sunlight (a state known as aphotic), water pressure is high (at the ocean floor this reaches up to 600 times that at the surface) and the temperature remains near freezing. Little is known about the creatures that inhabit this space. Those that do, so-called extremophiles, are highly specialized, they need to be to withstand the enormous pressure and survive on minimal food - marine snow (nutrients sinking from above) is rare. Octopus, squid, some fish and sea cucumbers can be found here. Life does however abound at some sites on the ocean floor: hydrothermal vents found in volcanic areas release boiling water carrying metals such as manganese, cobalt, lithium, iron, zinc and copper. Formed when seawater penetrates cracks in the earth’s crust and is heated, absorbing metals from the surrounding rocks in the process, the water rises to the ocean floor where it meets the near freezing ocean water and forms solid deposits. In just one or two years 9-metre tall chimney-like towers of deposits can form, some spew water filled with black iron sulfide (named black smokers), others emit white coloured elements like barium, calcium, and silicon (white smokers). Around these sites specialized bacteria convert the sulphur compounds and heat into food and energy. As the bacteria multiply they provide food for other animals such as deep-sea mussels, giant tube worms, yeti crabs, and other invertebrates and fish, acting as a magnet for deep sea life.

Habitats by distance from the coast

The ocean can also be divided into zones measured in terms of distance from the shore:

• the continental shelf (up to 350 nautical miles out from the coast, mainly within the epipelagic zone);

• the continental slope (passing through the mesopelagic and bathypelagic zones);

• the continental rise (where the floor levels out at about 6000 metres); and

• the abyssal plain, which makes up about 70% of the ocean floor.

In terms of salinity there are brackish, sea and inline saline zones, each with a different salt level and density, ranging from 30 parts of dissolved salt per thousand parts of water (30 ppt) to 50 ppt, the average being 35 ppt. In parts of the ocean where little rain falls but warm dry winds cause high levels of evaporation, the salinity of the water increases, causing it to become denser (the Mediterranean for example has a level of 38 ppt). Other areas have lots of rain and higher levels of freshwater entering the ocean from rivers or thawing ice, this dilutes the seawater, reduces the salinity and makes it less dense. The ocean around the Arctic has levels of 30 ppt in places.

Information about the division of the ocean into maritime zones based on legal principles can be found under ocean governance and a map of ocean depths relevant to national jurisdiction can be found here.

dive in deeper

The Brilliant Abyss
The Cambridge Festival, 28.03.21

Unexplainable: The Twilight Zone of the ocean
Unexplainable podcast

Why The Deep Ocean Matters
IET EngTalks, 09.12.20

Ocean Twilight Zone
Woods Hole Oceanographic Institute, March 2020