Open ocean habitat

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This article describes the habitat of the Open oceans. 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, zonation, biology and threats of the open oceans. Some legal aspects are also discussed.


Open ocean. Photo credit David Stauffer.

The open oceans or pelagic ecosystems are the areas away from the coastal boundaries and above the seabed. It encompasses the entire water column and lies beyond the edge of the continental shelf. It extends from the tropics to the polar regions and from the sea surface to the abyssal depths. It is a highly heterogeneous and dynamic habitat. Physical processes control the biological activities and lead to substantial geographic variability in production.


Ocean zonation.

The water column is subdivided into distinct zones by water depth and distance from shore. This is based on water depth and population composition. The distinct zones are:

  • The epipelagic zone ranges from the sea surface to a depth of about 200 metres. This is also the limit of the photic zone. Light penetrates into this surface water and is usually enough for the photosynthesis.
  • The mesopelagic zone lies underneath the epipelagic zone and extends to about 1,000 metres.
  • The bathypelagic zone is the zone between the 1,000 and 4,000 metres.
  • The abyssalpelagic zone extends to a water depth of 6,000 metres.
  • The hadalpelagic zone is deeper than 6,000 metres and is found in deep-sea trenches.



Currents are driven by differences in density, by wind action and by tides. Density is dependent on the temperature and the salinity of the water, see Seawater density. Cold and salty water is dense and will stay near the bottom, whereas warm and less salty water will stay near the surface. Cooling, warming, evaporation and freezing influence density differences between water masses and thus their relative motion. The circulation patterns driven by wind and density differences are strongly influenced by topography. Further information can be found in the article about large scale ocean circulation.


Vertical profiles of density, temperature and salinity in the ocean. The shown profiles based on ARGO observations are typical of the subtropical North Pacific. Image credit: Columbia University.

The water column can be stratified by density, as explained in the article Seawater density. The interface zone between the less dense surface layer and the denser underlying water is called pycnocline. In the oceans, the pycnocline often coincides with the thermocline, that separates the warm, buoyant surface layer from the colder, denser deep layer. The water above the thermocline is well mixed by the wind. Therefore the temperature and salinity in this water layer change with the seasons, as well as the community of organisms. In the temperate latitudes, alternating heat exchange with the atmosphere during summer and winter produces a seasonal summer thermocline at about 100 meters depth. Below the thermocline, the water is isolated from the atmosphere so the temperature and salinity remain stable over the year. This water is relatively less populated with animals. [1] Besides the temperature, stratification can also be caused by differences is salinity. In this case, the transition zone for the salinity is called halocline.

Light transmission and absorption

Light absorption in the open ocean [2]

Life depends directly or indirectly on energy from sunlight. In the ocean, marine plants and protists (eukaryotic, single-celled organisms) use green chlorophyll and a few accessory pigments to capture the visible light from the sun. A large fraction of that sunlight is reflected from the sea surface back to the atmosphere. The remaining light enters the water and is absorbed by water molecules. Approximately 65% of the visible light in water is absorbed within 1 meter of the sea surface. This energy is converted into heat and elevates the surface water temperature. The red and yellow light (longer wavelengths λ) are absorbed by water more readily then the green and blue light (shorter wavelength λ). This is called the selective absorption of wavelengths. It accounts for the blue colour of the open ocean. In very clear water, not even 1% of the light that enters the ocean penetrates to a depth of 100 meters. Due to the characteristics of light transmission, the water column can be divided into two distinct zones. The upper zone is called the photic zone. In this zone, plants receive adequate levels of sunlight and can photosynthesize. The zone below the photic zone is called the aphotic zone. Plants cannot survive in this dark zone. Water in the nearshore zone is generally turbid due to a high concentration of suspended solids. In this zone, light cannot penetrate deeper than 20 meters. The reflection by suspended particles causes a shift from blue to the green and yellow wavelengths.


Photic zone

Despite the enormous size of the open ocean, it does not support a dense population of organisms in its water or on the seabed. This is because it is located far from land, which is the main source of the essential nutrients that organisms need to grow. Although the biomass is limited, species diversity is remarkably high. In the well-illuminated upper zone or photic zone plants can photosynthesize. The pelagic flora falls into two main groups: the unicellular microphytoplankton and the large free-floating macroalgae. Microphytoplankton consists of diatoms, dinoflagellates and coccolithophorids. The larger diatoms are mainly grazed by large zooplankton and these are on their turn consumed by fish. Herbivorous zooplankton consists mainly of foraminifera and radiolarian. These are consumed by larger animals such as copepods. The floating organisms (neuston) at the surface have a blue colour due to the presence of protective pigments. These pigments are able to reflect the damaging part of the light spectrum (UV). This is necessary because the habitat is exposed to high levels of ultraviolet radiation. Organisms in the epipelagic zone such as crustaceans adopt red colours as a mean of camouflage. This is possible because red light is absorbed rapidly and does not penetrate far into this zone. The red colours are effectively invisible at depth and this makes the organisms invisible to others. A more complete overview of the photic zone ecosystem is presented in the article Marine Plankton.

Neuston Physalia physalis[3]
Copepod [4]

Bioluminescence in Nudibranchs. Photo credit Steven Haddock

Dysphotic zone

The dysphotic zone is a zone with very sparse light. The lower limit lies at a depth of approximately 500 to 1,000 meters. In this layer, oxygen levels can be low. These oxygen minima are caused by the bacterial breakdown of material sinking from the sea surface (see also Possible consequences of eutrophication). No seasonal effects of heating and cooling are present. This makes the zone stable and unchanging over time. The most abundant organisms are crustaceans (copepods, shrimps, amphipods, ostracods and prawns). These organisms have red to red-orange pigmentation. Other animals are squids and fishes. Several adaptations have evolved to this zone. The first one is the presence of large size and light-sensing ability of the eyes. Some organisms have additional organs called photophores, which produce bioluminescence. Some fish have bacterial photophores in which light is produced by the metabolic activities of symbiotic bacteria that live in dense concentrations in the photophores. Some species engage in diurnal vertical migration to get food. They swim upward to the photic zone at night to feed and descend again during the day.

Aphotic zone

In the aphotic zone, it is always dark and cold. The zone begins at depths of 500 to 1,000 meters. Many species in this region are coloured red or black. Organisms that occur here are copepods, ostracods, jellyfishes, prawns, mysids, amphipods, worms and fishes. Fishes at mid-depth have adaptations evolved to maximize their chances of capturing their prey. These fishes are small with enormous mouths lined with sharp teeth. Their jaws can be unhinged to eat large preys. They also have bioluminescent organs. Some species have a fishing rod with an attached luminous lure in front of the head. This is the case for the female deep sea angler fish. The body musculature is reduced. The deeper zones are dominated by macrobenthos. The principle food resources on the deep-sea bottom appear to be the slow fallout of fin and coarse organic detritus from surface waters, the settling of large animal carcasses, the sinking of fecal matter and the transport of organic detritus by turbidity currents. Bacteria in this region break down organic matter into simpler inorganic nutrients. [5] The bacteria are consumed by heterotrophic nanoflagellates, which are in turn consumed by ciliates. This food chain is called microbial loop. This loop recycles organic matter that is too small to be consumed by metazoan plankton.

The greater the depth, the more organisms are confronted with increasing physiological stress. One of these stress factors is pressure. Pressure increases by 1 atmosphere for every 10 meters increase in depth. [6]


  • Overfishing. Despite the fact that much of the open ocean is remote from the land, it has not escaped human impacts. To illustrate, 90% of the stocks of large pelagic fish, such as tuna and jacks, have been removed by fishing. The result of this overfishing is that the fishes of higher trophic levels are replaced by fishes of lower trophic levels. This is called fishing down the marine food web. It is ecologically unsustainable. More about this subject can be found in the articles on Effects of fisheries on marine biodiversity and Overexploitation.
  • Pollution. This causes the loss of many species or a degradation of the environment, see the article Coastal pollution and impacts.
  • Alien species. These are species that are introduced from another area and can compete with the indigenous species. This introduction can be done through ballast water from cargo ships or on hulls of vessels. More specific information is given in the article Non-native species invasions.
  • Global warming. The direct effect of global warming is strongest for the animals at the top of the food web and much less for the lower trophic levels. However, the intermediate trophic levels (zooplankton) are strongly influenced by the indirect effect of cascading trophic interactions from the top of the food web. [7]

Legal aspect

In the United Nations Convention on the Law of the Sea (UNCLOS 1982), the definition of the open ocean or High Sea is: ‘The high seas are open to all States, whether coastal or land-locked. Freedom of the high seas is exercised under the conditions laid down by this Convention and the rules of international law. It comprises, inter alia, both for coastal and land-locked States:

  • freedom of navigation
  • freedom of overflight
  • freedom to lay submarine cables and pipelines
  • freedom to construct artificial islands and other installations permitted under international law
  • freedom of fishing
  • freedom of scientific research

No State may validly purport to subject any part of the high seas to its sovereignty. Every State shall effectively exercise its jurisdiction and control in administrative, technical and social matters over ships flying its flag. The States have prohibitions such as the transport of slaves, piracy, whaling and pollution. The States have to ensure the conservation and management of natural living resources. The high seas shall be reserved for peaceful purposes. Every State has the right to ships flying its flag on the high seas. Warships on the high seas have complete immunity from the jurisdiction of any State other than the flag State. Ships owned or operated by a State and used only on government non-commercial service shall have complete immunity from the jurisdiction of any State other than the flag State.’ [8]

For more details, see Legislation for the sea.

Related articles

Deep sea habitat
Deep sea bottom


  1. Pinet P.R. 1992. Oceanography: An introduction to the Planet Oceanus. West Publishing Company. p. 571
  5. Pinet P.R. 1992. Oceanography: An introduction to the Planet Oceanus. West Publishing Company. p. 571
  6. Kaiser M. et al. 2005. Marine ecology: Processes, systems and impacts. Oxford University Press. p.584
  7. Murphy, G.E.P., Romanuk, T.N. and Worm, B. 2020. Cascading effects of climate change on plankton community structure. Ecology and Evolution 10: 2170–2181

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 (2021): Open ocean habitat. Available from [accessed on 13-06-2024]