Deep sea

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This article describes the habitat of the deep sea. It is one of the sub-categories within the section dealing with biodiversity of marine habitats and ecosystems. It gives an overview about the characteristics, the bottom topography, the adaptations to the environment of the biota and the threats.


The deep sea floor represents the largest habitat on earth. It ranges from the edge of the continental shelf at 200m to the bottom of the ocean. At the edge of the continental shelf is the shelf break, where the gradient of the floor increases down the continental slope. Below the '''continental slope''' lies the continental rise, which has a more gently slope. At around 4,000 meters depth, the ocean floor is reached and extends over the ocean basins at depths of 5,000 meters on average. This is called the abyssal plain. The zone between the continental shelf and the abyssal plain is the bathyal zone. In some places, the sea floor drops again into elongated trenches with depths of 10 to 11 kilometers. This region is the hadal region. The ocean floor is interrupted by mountain chain known as the mid-oceanic ridge system. Other features on the ocean floor are seamounts and hydrothermal vents. [1]

Ocean zones [2]


The temperature of the waters of the deep sea varies from 4°C to -1°C. Exceptions are the Mediterranean Sea with an average temperature of 13°C in the upper 4,000 meters and the Red Sea where the bottom water can reach temperatures up to 21.5°C at depths of 2,000 meters. The lowest temperatures are found in the Antarctic Ocean and are about -1.9°C. The salinity of the seawater is relatively constant. Below 2,000 meters, the salinity is close to 35 ‰ (34.8‰ ± 0.3 ‰) and at the very deepest layers it is 34.65‰. The oxygen concentrations are near saturation except in the oxygen minimum layer between 0.5 and 0.6 kilometers depth. In enclosed basins such as the Black Sea, the water is anoxic below 250 meters. When the deep water is further from its origin the oxygen concentration declines by metabolic processes. This is e.g. the case in the deep North Pacific. The most predictable physical variable is hydrostatic pressure. Per 10 meter the depth increases, the pressure increases by 1 atmosphere ([math]10^5[/math] Pa). [3] In the deep ocean, light is almost absent. It is a dark environment and the organisms are adapted to it. The circulation in the deep sea is explained in the article ocean circulation.

Deep sea bottom

The bottom of the deep sea has several features that make the bottom a more diverse ecosystem. The main features are mid-oceanic ridges, hydrothermal vents, mud volcanoes, seamounts and cold seeps. But features such as canyons can also occur. Special forms of a diverse environment are the carcasses of large animals.

Mid-oceanic ridges

A mid-oceanic ridge is an underwater mountain range and is formed by plate tectonics. It is characteristic of an oceanic spreading center. The uplifted sea floor results from convection currents which rise in the mantle as magma and emerge as lava, creating the new crust upon cooling. It is the boundary between two tectonic plates. The mid-oceanic ridges are connected and form a single global mid-oceanic ridge system that is part of every ocean. The rang is approximately 65,000 kilometers long and the total length of the system is 80,000 kilometers. It can be occupied by a rift valley that is formed by geological forces that pulled apart and broke the solid rock. This process is called faulting. They are active features characterized by frequent, shallow earthquakes, many faults and widespread volcanism. It is also segmented by transform faults, where fractured rocks slide past one another. This causes a zigzag pattern. Hydrothermal vents and cold seeps are frequently found on these mid-oceanic ridges. [4]

Mid-oceanic ridge [5]

Hydrothermal vents

Due to the use of submersibles and more intensive acoustic mapping, unique and often diverse communities are found on features such as hydrothermal vents. These are islands in a sea of mud. The hydrothermal vents are associated with parts of the ocean floor that exhibit high levels of tectonic activity such as mid-oceanic ridges. In these regions, hot magma chambers occur near the seabed and heat up water that has permeated into the ocean floor. The water that has sinks down into cracks of the crust, are free of oxygen and potassium. After this step, calcium, sulfate and magnesium are also removed. When the water sinks deeper into the crust, calcium, sodium and potassium are added from the surrounding crust. At the deepest point, the fluids have reached their highest temperatures (350-400°C). Copper, zinc, iron and sulfur dissolve into the fluids. These hot fluids rise up and are released in the cold, oxygen-rich seawater at the sea bottom. The metals and sulfur combine to form black metal-sulfide minerals. [6] This type of hydrothermal vents is called a black smoker. White smokers also exist and are a little bit cooler.

Hydrothermal vents - Black smokers [7]

A huge biomass of associated organisms can be found. Because of the extreme conditions, the organisms need an alternative method of food supply. This production is autochthonous and related to the supply of reduced compounds from the vents. The primary production is generated by bacteria through chemosynthesis. The bacteria are chemoautotrophic and tend to be members of the most ancient Archaea. They can tolerate extremely high temperatures (called hyperthermophiles (80-115°C) and superthermophiles (>115°C)). Chemosynthetic bacteria can produce their own food from inorganic compounds without sunlight. They use energy derived from chemical reactions that involve substances such as ammonia ([math]NH_3[/math]), sulfides ([math]S^2-[/math]), nitrates ([math]NO_3^-[/math]) and sulfates ([math]SO_4^2-[/math]). The bacteria of the vents use sulfide ions and oxidize it to sulfur and sulfates. The energy produced by this process is used to produce food. [8]

Reaction: [math]CO_2 + H_2S + O_2 + H_2O \rightarrow CH_2O + H_2SO_4 [/math]

Mud volcanoes and seamounts

A mud volcano or mud dome is used to refer to formations created by geo-excreting fluids and gasses. The most abundant gas that is released is methane. Seamounts are mountains rising from the ocean floor that do not reach the water’s surface. They are typically formed from extinct volcanoes, which rise abruptly. It is estimated that there are 30,000 seamounts in the ocean, but only a few have been studied. They are hotspots of marine life. They can have their own localized tides, eddies and upwelling zones.

Distribution of seamounts [9]

Cold seeps

Cold seeps are found along active and passive continental margins related to geological processes such as tectonically induced high-fluid pressures, petroleum or natural gas escape, catastrophic erosion and slides. The source of energy is methane-rich fluids of thermogenic and/or biogenic origin. But production of sulfide by sulfate reduction also plays an important role. Metanotrophic bacteria use methane to produce carbonates by methane reduction. The carbonates can react with calcium to form calciumcarbonate. Sulfate reducing bacteria use sulfate to form sulfides and this sulfide is oxidized by sulfur oxidizing bacteria and they release sulfate again into the seawater.

Overall reaction: <math> CH_4 + SO_4^{2-} \rightarrow HCO_3^- + HS^- + H_2O

Cold seep with tube worms and bivalves [10]


Carcasses of large animals from the overlying water column sink to the bottom. They produce much localized food hot-spots. A similar food fall is the nutritious marine snow. They accumulate in tick patches separated by relatively barren areas.


For the animals of the deeper layers of the water column where there is no plant growth at all, finding food and avoiding predation are the main problems. Many mid-water animals have bizarre body shapes, because they never come into contact with a physical barrier. The most abundant for is more or less spherical, often with long and delicate extensions. Most animals do not need a rigid skeleton because they spend all their time suspended in water. An important adaptation is not to be seen too easily. In the upper parts of the mesopelagic zone, where there is still quite some sunlight, many animals have more or less transparent bodies. An alternative is to have silver sides that act as mirrors and light organs along their bellies. At the deeper layers, the organisms have a dark red, black or brown color. These colors do not reflect the blue color of most bioluminescence. In the mid-water, animals have mouths full of big teeth. In the deeper water, the anglerfish has a fishing rod with bioluminescent lures to attract preys. Many fishes also have large mouths to take up as much as possible. Another adaptation is dial vertical migration of many animals. The general pattern is that animals spend the daylight hours at depth and move upwards during the night. This is to avoid predation and to feed. Bioluminescence is another adaptation. Depending on the depth, organisms use it as camouflage to protect them from predators or to confuse or frighten predators rather than to hide from them. Other animals use it to look for a potential mate. A few deep-sea animals use it together with another light organ that produces deep red light. Other animals cannot see this red light, but the red light will be reflected very efficiently by the red bodies of the specific prey. The blue light is a recognition mark for other members of the same species. An example of such an organism is Malacosteus. [11]


The mid-water communities include representatives of almost every major animal group on Earth, from tiny single-celled organisms only a fraction of a millimeter, through all the invertebrate phyla, to fish several meters long. Most of them have an average size in the range of centimeters. The dominant species are crustaceans, fish and a variety of animals with soft and jelly-like bodies such as jellyfish.

On the deep-sea floor, many mounds and depressions are formed by benthic animals such as worms, mollusks, crustaceans, starfish, brittlestars, shrimps, fishes sea cucumbers ans sea urchins.

Riftia with associated animals [12]

Hydrothermal communities may be hundreds of times more abundant than on the adjacent sea floor. This is because the animals have their own rich source of food. This food source is totally independent of the input from the photosynthesizing plants in the overlying surface waters. Organisms that can be found in association with these vents are bacteria, mussels, cockles, oysters, sea anemones, shrimps, crabs, fish and tube worms. An example of a tube worm is Riftia pachyptila. It has a red plume of gills sticking out of the top of the worm’s white tube. They are also found around cold hydrocarbon seeps. It has no mouth or gut and it has special chemosynthetic bacteria in the tissues of its body called the trophosome. The chemicals that the bacteria need are delivered by the blood of the worm. [11]

Cold seeps have a lower metazoan biodiversity but a higher productivity. This is because of the high biomass of megabenthic organisms. The communities are often dominated by the bivalves that have symbiontic relationships with chemosynthetic bacteria. Organisms that can be found here are tube worms, bivalves, crabs, anemones and soft corals. An example of an organism is the vesicomyid clam that has a symbiontic relationship with sulfur oxidizing bacteria.

Seamounts have high concentrations of plankton. Also fishes are abundant, because of the high concentration of plankton and the constant influx of prey organisms. Marine mammals such as sharks, tuna and cephalopods all congregate over the seamounts to fee. Even seabirds have been shown to be more abundant in the zones with shallow seamounts.


Human activities can cause serious damage to the deep sea. Examples of human activities that have a negative impact are [13]:

  • Fishing activities: overfishing, destructive practices and illegal, unreported and unregulated fishing activities (IUU)
  • Bycath or incidental mortality of non-target species
  • Noise pollution such as powerful sonar systems and airguns
  • Shipping: noise, accidental spills of oil, discharge of garbage, oily wastes, sewage, chemical residues and ballast water, anti-fouling products
  • Submarine cables and pipelines
  • pollution
  • Climate change and ozone depletion


  1. Kaiser M. et al. 2005. Marine ecology: Processes, systems and impacts. Oxford University Press. p.584
  2. ocean_zones.jpg
  3. Gage J.D. and Tyler P.A. 1991. Deep-sea biology – A natural history of organisms at the deep-sea floor. Cambridge University Press. p. 504
  8. Kaiser M. et al. 2005. Marine ecology: Processes, systems and impacts. Oxford University Press. p.584
  10. NOAA
  11. 11.0 11.1 Rice T. 2000. Deep Ocean. The natural history museum, London. p. 96
  12. NOAA
  13. UNEP Regional Seas Report and Studies No.178. 2006. Ecosystems and Biodiversity in Deep Waters and High Seas. p. 60

The main author of this article is TÖPKE, Katrien
Please note that others may also have edited the contents of this article.

Citation: TÖPKE, Katrien (2008): Deep sea. Available from [accessed on 19-08-2019]