"Pelagic" and "Open sea" redirect here. For the writer, seeVasa Pelagić. For the marketplace, seeOpenSea. For ocean not controlled by any sovereign territory, also called the "high seas", seeInternational waters.
Thepelagic zone consists of thewater column of theopen ocean and can be further divided into regions bydepth. The wordpelagic is derived from Ancient Greekπέλαγος (pélagos)'open sea'.[1] The pelagic zone can be thought of as an imaginarycylinder or water column between thesurface of the sea and thebottom.
Conditions in the water column change with depth: pressure increases; temperature and light decrease; salinity, oxygen, micronutrients (such as iron, magnesium and calcium) all change. In a manner analogous tostratification in the Earth's atmosphere, the water column can be divided vertically into up to five different layers (illustrated in the diagram), with the number of layers depending on the depth of the water.
Marine life is affected bybathymetry (underwater topography) such as the seafloor, shoreline, or a submarineseamount, as well as by proximity to the boundary between the ocean and the atmosphere at the ocean surface, which brings light for photosynthesis, predation from above, and wind stirring up waves and setting currents in motion. The pelagic zone refers to the open, free waters away from the shore, where marine life can swim freely in any direction unhindered bytopographical constraints.
Theoceanic zone is the deep open ocean beyond thecontinental shelf, which contrasts with the inshore waters near thecoast, such as inestuaries or on the continental shelf. Waters in the oceanic zone plunge to the depths of theabyssopelagic and further to thehadopelagic. Coastal waters are generally the relatively shallow epipelagic. Altogether, the pelagic zone occupies 1.33 billion km3 (320 million cu mi), with a mean depth of 3.68 km (2.29 mi) and maximum depth of 11 km (6.8 mi).[2][3][4] Pelagic life decreases as depth increases.
The pelagic zone also contrasts with thebenthic anddemersal zones at the bottom of the sea. The benthic zone is the ecological region at the very bottom, including the sediment surface and some subsurface layers. Marine organisms such asclams andcrabs living in this zone are calledbenthos. Just above the benthic zone is the demersal zone.Demersal fish can be divided intobenthic fish, which are denser than water and rest on the bottom, andbenthopelagic fish, which swim just above the bottom. Demersal fish are also known asbottom feeders andgroundfish.
From the surface (MSL) down to around 200 m (660 ft)
The illuminated zone at the surface of the sea, and the only zone with sufficient light for photosynthesis. This zone is just above the continental shelf and has the lowestatmospheric pressure on the oceans surface, at 1 atm for every 10 meters. Nearly allprimary production in the ocean occurs here, and about 90% marine life is concentrated in this zone, including:plankton,floating seaweed,jellyfish,tuna,whales,sharks,dolphins, and many more diverse species.[5]
From 200 m (660 ft) down to around 1,000 m (3,300 ft)
Thethermocline serves as the boundary from the warmer top zone to the much colder mesopelagic zone, which is also located right under the continental shelf.[5] This zone contains a very trace amount of sunlight and has a pressure of about 20 - 100 amt.[5] A variety of creatures live in this zone, including species ofswordfish,squid,wolffish and some species ofcuttlefish. Many organisms living here have evolved adaptations, such asbioluminescence, due to the lack of sunlight.[6][5] Some mesopelagic creatures rise to the epipelagic zone at night to feed.[6]Heterotrophic bacteria are among the more abundant organisms in this zone, and they primarily feed and break down falling matter from the upper zone.[7]
From 1,000 m (3,300 ft) down to around 4,000 m (13,000 ft)
The name stems from Ancient Greekβαθύς'deep'. In this zone, the environment is pitch black at this depth and contains no trace of sunlight, apart from occasionalbioluminescent organisms, such asanglerfish.[5] The temperature and salinity of this zone is stable.[8][9]No plants live here. Most creatures survive ondetritus known as "marine snow" falling from the zones above or, like themarine hatchetfish, by preying on other inhabitants of this zone.
From around 4,000 m (13,000 ft) down to above theocean floor
The name is derived from Ancient Greekἄβυσσος'bottomless'. Theocean floor is next to this zone, and it forms volcanos, mountains, and vents from the movement of thetectonic plates.[5] Among the very few creatures living in the cold temperatures, high pressures and complete darkness there are several species of squid;echinoderms including thebasket star, swimming cucumber, and thesea pig; and marine arthropods including thesea spider. Many species at these depths are transparent and eyeless.[6]
The name is derived from the realm ofHades, theGreek underworld. This is the deepest part of the ocean at more than 6,000 m (20,000 ft) . Such depths are generally located intrenches.This zone contains 13 short narrow troughs and 33 trenches. The deepest trenches stretch to 10,924 m deep, while average trenches are usually 5 - 10 kilometers deep. This zone can have an atmospheric pressure of 1,100 atm.[5] In this zone, there is an increase in temperature fromadiabatic heating.[10] Very few creatures live in this zone. Some of the recorded species arecoelenterate,polychaetas,amphipods,echinoderms, andmollusks.[10]
The pelagic ecosystem is based onphytoplankton. Phytoplankton manufacture their own food using a process ofphotosynthesis. Because they need sunlight, they inhabit the upper, sunlit epipelagic zone, which includes the coastal orneritic zone. Biodiversity diminishes markedly in the deeper zones below the epipelagic zone as dissolved oxygen diminishes, water pressure increases, temperatures become colder, food sources become scarce, and light diminishes and finally disappears.[11]
Thorson's rule states thatbenthicmarine invertebrates at low latitudes tend to produce large numbers of eggs developing to widely dispersing pelagic larvae, whereas at high latitudes such organisms tend to produce fewer and largerlecithotrophic (yolk-feeding) eggs and larger offspring.[12][13]
Pelagic fish live in thewater column of coastal, ocean, and lake waters, but not on or near the bottom of the sea or the lake. They can be contrasted with demersal fish, which do live on or near the bottom, andcoral reef fish.[14]
Pelagic fish are oftenmigratoryforage fish, which feed onplankton, and the largerpredatory fish that follow and feed on the forage fish. Migratory fish come up to the more dense prey areas of the pelagic zones to feed, and then descend at night to avoid predators.[15] Examples of migratory forage fish areherring,anchovies,capelin, andmenhaden. Examples of larger pelagic fish whichprey on the forage fish arebillfish,tuna, and oceanicsharks.[16]
Hydrophis platurus, the yellow-bellied sea snake, is the only one of the 65 species ofmarine snakes to spend its entire life in the pelagic zone. It bears live young at sea and is helpless on land. The species sometimes forms aggregations of thousands along slicks in surface waters. The yellow-bellied sea snake is the world's most widely distributed snake species.[17]
Many species ofsea turtles spend the first years of their lives in the pelagic zone, moving closer to shore as they reach maturity.[17]
Some representative ocean animals (not drawn to scale) within their approximate depth-defined ecological habitats.Marine microorganisms also exist on the surfaces and within the tissues and organs of the diverse life inhabiting the ocean, across all ocean habitats. The animals rooted to or living on the ocean floor are not pelagic but arebenthic animals.[18]The yellow bellied sea snake as it glides across the surface of the ocean.The pelagicwandering albatross (Diomedea exulans) ranges over huge areas of ocean and can circle the globe.
Compared to terrestrial environments, marine environments havebiomass pyramids which are inverted at the base.[19][20] In particular, thebiomass of consumers (copepods,krill,shrimp,forage fish) is larger than the biomass ofprimary producers. This happens because the ocean's primary producers are tinyphytoplankton which tend to have a fast life history (arer-strategists that grow and reproduce rapidly) so a small mass can have a fast rate of primary production.[21] In contrast, terrestrial primary producers, such as mature forests, often have a slow life history (areK-strategists that grow and reproduce slowly) so a much larger mass is needed to achieve the same rate of primary production.[22]
Because of this inversion, it is thezooplankton that make up most of the marine animalbiomass. As primary consumers, they are the crucial link between theprimary producers (mainly (phytoplankton) and the rest of the marine food web (secondary consumers).[23]
If phytoplankton dies before it is eaten, it descends from theeuphotic zone down through the pelagicwater column as part of themarine snow, and settles into the depths of sea. In this way, phytoplankton sequester about 2 billion tons of carbon dioxide in the ocean each year, causing the ocean to become a sink of carbon dioxide holding about 90% of all sequestered carbon.[24]
In 2010 researchers found whales carry nutrients from the depths of the ocean back up the pelagic water column to the surface using a process they called thewhale pump.[25] Whales feed at deeper levels in the ocean wherekrill is found, but return regularly to the surface to breathe. There whalesdefecate a liquid rich in nitrogen and iron. Instead of sinking, the liquid stays at the surface wherephytoplankton consume it. In the Gulf of Maine the whale pump provides more nitrogen than the rivers.[26]
Exploring and learning more about the ocean is a main factor to oceanresource management, which sustainably manages how much and how fast we use the oceans resources.Ocean exploration also observes patterns in the oceans weather and climate, and the means why which they were affected. Researchers are better able to understand and see natural phenomena such as earthquakes and tsunamis and react accordingly. Scientists and researchers have developed many methods to sample the oceanbiome, and pelagic fish.[27][15]
This method can be used from a boat to capture organisms like deep pelagic fish. A mesh net is dragged at different depths to collect for recording the captured organisms. This method can produce large amounts of specimen. However it is costly, time consuming, and mostly used by research groups with a lot of support and funding. There are also many fish that are able to out swim the net, which limits data.[15][8]
This method analyzes fish that are detected or present in sound pulses that are emitted from the surface, where the pelagic fish'biomass in the reflected single is analyzed. This method of sampling cannot reach deep depths in the ocean. The pulses cover a broad area of the ocean and causes little harm or distress. The received data from this method is complicated to interpret, due to specific variations of swim bladders in fish, such as having little gas or not having a swim bladder.[15][8]
Remotely operated vehicles (ROC) are used for sampling and examining the deep pelagic sea in ways that other techniques cannot match. An ROV is an unoccupied machine equipped with lights, cameras, sensors, or arms, which allows for detailed and live observations of its surroundings and of pelagic organisms. It can conduct experiments and collect samples.[8] These machine are limited in ground coverage, as well as expensive and hard to control, so few research groups use them. They can also be loud, bright, and big, causing organisms to avoid them.[15]
^Costello, Mark John; Cheung, Alan; De Hauwere, Nathalie (2010). "Surface Area and the Seabed Area, Volume, Depth, Slope, and Topographic Variation for the World's Seas, Oceans, and Countries".Environmental Science & Technology.44 (23):8821–8.Bibcode:2010EnST...44.8821C.doi:10.1021/es1012752.PMID21033734.
^Thorson, G (1957). "Bottom communities (sublittoral or shallow shelf)". In Hedgpeth, J.W. (ed.).Treatise on Marine Ecology and Palaeoecology. Geological Society of America. pp. 461–534.
^Mileikovsky, S. A. (1971). "Types of larval development in marine bottom invertebrates, their distribution and ecological significance: a re-evaluation".Marine Biology.10 (3):193–213.Bibcode:1971MarBi..10..193M.doi:10.1007/BF00352809.S2CID84623588.
^H., Engelhard, Georg; A., Peck, Myron; Anna, Rindorf; Sophie, C. Smout; Mikael, van Deurs; Kristina, Raab; H., Andersen, Ken; Stefan, Garthe; A.M., Lauerburg, Rebecca; Finlay, Scott; Thomas, Brunel; Geert, Aarts; Tobias, van Kooten; Mark, Dickey-Collas (1 January 2014)."Forage fish, their fisheries, and their predators: who drives whom?".ICES Journal of Marine Science.71 (1).doi:10.1093/ice (inactive 13 October 2025).ISSN1054-3139. Archived fromthe original on 19 July 2025.{{cite journal}}: CS1 maint: DOI inactive as of October 2025 (link) CS1 maint: multiple names: authors list (link)
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