Open ocean habitat

Fig. 1. Open ocean. Photo credit David Stauffer.

The open ocean, or pelagic ecosystem, comprises marine areas beyond coastal boundaries and above the seabed (Fig. 1). It includes the entire water column beyond the continental shelf, extending from tropical to polar regions and from the sea surface to abyssal depths. The open ocean is a highly heterogeneous and dynamic habitat in which physical and chemical processes strongly influence biological activity, resulting in substantial geographic variability in productivity.

The open ocean plays a major role in regulating Earth’s climate. Ocean currents redistribute heat from equatorial regions to higher latitudes, helping to moderate global temperatures. The ocean is the primary source of atmospheric water vapor and also acts as an important sink for carbon dioxide, absorbing large quantities of CO₂ from the atmosphere and thereby reducing the accumulation of greenhouse gases. In addition, open seas influence regional and local weather patterns; winds originating over the ocean often bring rainfall, while tropical cyclones develop through interactions between warm ocean waters and the atmosphere. The open ocean provides habitat for highly specialized pelagic organisms and serves as an important feeding ground for many other species.

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 indicated.

Zonation

Fig. 2. 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 (Fig. 2):

• The epipelagic zone ranges from the sea surface to a depth of 100-200 metres. This is also the limit of the photic zone. Light penetrates sufficiently into this surface layer to allow for 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.

Characteristics

Currents

Currents are driven by density differences, wind forcing and tides. Density depends on the temperature and salinity of seawater, see Seawater density. Cold, saline water is denser and tends to sink, whereas warm, less saline water remains near the surface. Cooling, warming, evaporation and freezing modify density differences between water masses and thereby influence their motion. Large-scale circulation patterns driven by wind and density differences are strongly influenced by topography; see Ocean circulation.

The largest open-ocean ecosystems are the subtropical gyres of the Atlantic, Pacific and Indian Oceans. They are driven by the anticyclonic wind-stress curl generated by the trade winds at low latitudes and the westerlies at mid-latitudes. Convergence associated with Ekman transport generates large downwelling zones within these gyres. The North Pacific Subtropical Gyre is the largest circulation system on Earth and one of the most persistent, with boundaries that have remained approximately stable for millions of years. It is characterized by weak surface currents, strong stratification and generally low biological productivity, although substantial mesoscale and seasonal variability occurs.

Light transmission and absorption

Fig. 3. Light absorption in the open ocean

Life in the ocean depends directly or indirectly on solar energy. Marine plants and protists use chlorophyll and accessory pigments to capture visible light for photosynthesis. Part of the incoming sunlight is reflected at the sea surface, while the remainder penetrates the water column and is progressively absorbed by water molecules and suspended matter.

Infrared and red wavelengths are rapidly absorbed in the upper meters of the ocean, whereas blue and green wavelengths penetrate much deeper, especially in clear oceanic waters (Fig. 3). This selective absorption of wavelengths gives the open ocean its characteristic blue colour. In very clear oceanic water, less than 1% of surface photosynthetically active radiation remains at depths near 100 m. Based on light penetration, the water column can be divided into two zones. The upper photic zone receives sufficient light for photosynthesis, whereas photosynthesis is generally not possible in the deeper aphotic zone.

Nearshore waters are often more turbid because of high concentrations of suspended particles. In these waters, light penetration may be limited to less than 20 m depth. Scattering by suspended particles shifts the underwater light field from blue toward green and yellow wavelengths.

Stratification

Fig. 4. Vertical profiles of density, temperature and salinity in the ocean. The upper gray fringe represents the mixed surface layer. The profiles based on ARGO observations are typical of the subtropical North Pacific. Image credit: Columbia University.

The water column is stratified by density differences related to temperature and salinity gradients, as explained in Seawater density. The transition zone between the less dense surface layer and the denser underlying water is called the pycnocline. In many regions, the pycnocline coincides with the thermocline, which separates the warm surface layer from colder deep water (Fig. 4).

Wind forcing and convective overturning caused by surface cooling and evaporation mix the upper ocean above the thermocline, creating a surface mixed layer whose temperature and biological communities vary seasonally. Stratification can also result from salinity differences; the corresponding transition zone is called the halocline. High surface salinity results from strong evaporation.

In many subtropical gyres, the upper ocean consists of a warm, well-mixed surface layer, typically 40–100 m deep, overlying colder and denser interior waters. Strong density gradients suppress vertical mixing across the pycnocline and restrict the upward transport of nutrients from deeper waters. Downwelling and stratification both contribute to reduce the supply of nutrients, which become the limiting factor for primary production, leading to oligotrophic conditions and low surface chlorophyll concentrations (Fig. 5).

High chlorophyll concentrations along the eastern margins of ocean basins, particularly off western Africa and South America, are associated with coastal upwelling driven by Ekman transport. In highly productive, decomposition and denitrification of sinking organic matter may lead to oxygen depletion within the thermocline. Major oxygen minimum zones occur in the eastern tropical North Pacific and the Arabian Sea. Both areas are characterized by relatively long residence times due to weak lateral transport and mixing.

In high-latitude oceans, cold surface waters reduce vertical density gradients, allowing deep vertical mixing. This mixing supplies abundant nutrients to surface waters, but phytoplankton growth is often limited by low light availability and, in some regions, by iron limitation. Nevertheless, surface chlorophyll concentrations in high-latitude oceans are generally higher than in the subtropical gyres.

Fig. 5. Composite global maps of satellite-derived surface chlorophyll concentrations. Left: July–September. Right: January–March. The low-chlorophyll dark blue regions correspond to the subtropical ocean gyres. © 2012 Nature Education

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Biology

The epipelagic or photic zone

Fig. 6. Typical depth profiles of light, nitrogen and chlorophyll. The subsurface chlorophyll maximum is around 120 m in the subtropical ocean and around 80 m in subpolar regions. Adapted from © 2012 Nature Education

In large parts of the open ocean, primary production is limited by the availability of inorganic nitrogen. Nitrogen fixation by cyanobacteria is therefore an important source of bioavailable nitrogen. Nitrogen-fixing cyanobacteria have a particularly high iron demand because the nitrogenase enzyme complex requires iron. Iron limitation is especially important in high-nutrient, low-chlorophyll regions such as the Southern Ocean, the equatorial Pacific and the subarctic North Pacific. See also Biogeochemical cycles in the coastal marine environment.

In oligotrophic open-ocean regions, primary production in the sunlit upper layer is constrained by nutrient availability. Near the base of the photic zone, nutrients are supplied by turbulent diffusion from the mesopelagic layer below, where mineralization of sinking organic matter (marine snow) releases inorganic nutrients. Enhanced phytoplankton biomass and productivity therefore often occur near the base of the photic zone, forming a subsurface chlorophyll maximum (Fig. 6). Phytoplankton optimize growth by adjusting resource allocation. Under low-light and nutrient-replete conditions, they preferentially allocate resources to light harvesting at the expense of nutrient uptake.

Despite the enormous size of the open ocean, biomass concentrations are generally low, although species diversity is high. Pelagic primary producers consist mainly of phytoplankton, including cyanobacteria, diatoms, dinoflagellates and coccolithophores. Diatoms are important food sources for herbivorous zooplankton, especially copepods and krill, which in turn are consumed by fishes and other predators.

Many neustonic (not-swimming) organisms living at the sea surface are blue or transparent, providing camouflage and protection against intense solar and ultraviolet radiation. .

The epipelagic biome hosts a specialized fish fauna, including flying fishes and their relatives (needlefishes and halfbeaks), flotsam-associated fishes (such as triggerfishes, filefishes, jacks and dolphinfish), schooling fishes (herrings and anchovies), larval and juvenile fishes of many species, and highly migratory predators such as tunas, billfishes and mako sharks.

Fig. 7a. Neuston Physalia physalis

Fig. 7b. Microphytoplankton

Fig. 7c. Copepod

Fig. 8. Bioluminescence in nudibranchs. Photo credit Steven Haddock

The mesopelagic or dysphotic zone

The mesopelagic or dysphotic zone extends approximately from 200 m to 1,000 m depth and receives only faint light. Oxygen concentrations may become very low in this zone because of bacterial decomposition of sinking organic material from surface water. Seasonal temperature variability is much weaker than at the surface, resulting in relatively stable environmental conditions.

The mesopelagic fauna is dominated by crustaceans such as copepods, shrimps, amphipods, ostracods and euphausiids, together with squids and fishes. Many species are red or black because red wavelengths are rapidly absorbed with depth, rendering these colors effectively invisible. Many organisms possess large, highly sensitive eyes adapted to dim light conditions. Others possess photophores, organs that produce bioluminescence (Fig. 8). In some fishes, the light is produced by symbiotic bacteria living within specialized photophores.

Many mesopelagic organisms undertake diurnal vertical migration. They ascend into the epipelagic zone at night to feed and descend again into deeper waters during daytime.

The bathypelagic or aphotic zone

Fig. 9. The humpback anglerfish or common black devil (Melanocetus johnsonii) is a deep-sea anglerfish occurring in tropical and temperate oceans at depths to 2,000 m. It has a fishing rod with an attached luminous lure in front of the head.

The bathypelagic or aphotic zone extends from approximately 1,000 m to 4,000 m depth. Sunlight no longer penetrates into this region, which is permanently dark and cold. Many species are black or red in color and possess bioluminescent organs. Common organisms include copepods, ostracods, jellyfishes, amphipods, mysids, worms and deep-sea fishes.

Many bathypelagic fishes possess adaptations that enhance prey detection and capture under extremely low-light conditions. They are often small, with bioluminescent organs, large mouths and expandable jaws that allow them to consume relatively large prey. Some species possess luminous lures, as in female deep-sea anglerfishes (Fig. 9). Metabolic rates and body musculature are generally reduced, reflecting the low availability of food. Many deep-sea species are slow growing, long lived and late to mature.

The deeper ocean and seabed ecosystems are sustained mainly by organic matter exported from surface waters, including marine snow, fecal pellets and occasional large carcasses. Turbidity currents may also transport organic detritus downslope into the deep ocean. Bacteria consume dissolved organic matter and are subsequently grazed by heterotrophic nanoflagellates, ciliates and other microzooplankton. This recycling pathway is known as the microbial loop.

Increasing depth exposes organisms to strong physiological stress, especially hydrostatic pressure. Pressure increases by approximately 1 atmosphere for every 10 m increase in depth.

Threats

• Overfishing. Industrial fishing has caused major declines in many populations of large pelagic fishes and sharks. While several shark populations have declined by more than 70% globally, many tuna stocks have stabilized or partially recovered under improved international fisheries management. 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.

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.’

For more details, see Legislation for the sea.

Related articles

Shelf sea exchange with the ocean

Biogeochemical cycles in the coastal marine environment

Ocean carbon sink

Deep sea habitat

Deep sea bottom

Plankton bloom

Ekman transport

Ocean circulation

References

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 (2026): Open ocean habitat. Available from http://www.coastalwiki.org/wiki/Open_ocean_habitat [accessed on 29-05-2026] For other articles by this author see Category:Articles by TÖPKE, Katrien For an overview of contributions by this author see Special:Contributions/Ktopke

Article reviewed by

Job DronkersSee the discussion page