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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Pelagic zone | 1/3 | https://en.wikipedia.org/wiki/Pelagic_zone | reference | science, encyclopedia | 2026-05-05T07:36:06.087361+00:00 | kb-cron |
The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word pelagic is derived from Ancient Greek πέλαγος (pélagos) 'open sea'. The pelagic zone can be thought of as an imaginary cylinder or water column between the surface of the sea and the bottom. 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 to stratification 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 by bathymetry (underwater topography) such as the seafloor, shoreline, or a submarine seamount, 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 by topographical constraints. The oceanic zone is the deep open ocean beyond the continental shelf, which contrasts with the inshore waters near the coast, such as in estuaries or on the continental shelf. Waters in the oceanic zone plunge to the depths of the abyssopelagic and further to the hadopelagic. 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). Pelagic life decreases as depth increases. The pelagic zone also contrasts with the benthic and demersal 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 as clams and crabs living in this zone are called benthos. Just above the benthic zone is the demersal zone. Demersal fish can be divided into benthic fish, which are denser than water and rest on the bottom, and benthopelagic fish, which swim just above the bottom. Demersal fish are also known as bottom feeders and groundfish.
== Depth and layers ==
The pelagic zone is subdivided into five vertical regions. From the top down, these are:
=== Epipelagic (sunlight) ===
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 lowest atmospheric pressure on the oceans surface, at 1 atm for every 10 meters. Nearly all primary 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.
=== Mesopelagic (twilight) ===
The thermocline serves as the boundary from the warmer top zone to the much colder mesopelagic zone, which is also located right under the continental shelf. This zone contains a very trace amount of sunlight and has a pressure of about 20 - 100 atm. A variety of creatures live in this zone, including species of swordfish, squid, wolffish and some species of cuttlefish. Many organisms living here have evolved adaptations, such as bioluminescence, due to the lack of sunlight. Some mesopelagic creatures rise to the epipelagic zone at night to feed. Heterotrophic bacteria are among the more abundant organisms in this zone, and they primarily feed and break down falling matter from the upper zone.
=== Bathypelagic (midnight) ===
The name stems from Ancient Greek βαθύς 'deep'. In this zone, the environment is pitch black at this depth and contains no trace of sunlight, apart from occasional bioluminescent organisms, such as anglerfish. The temperature and salinity of this zone is stable. No plants live here. Most creatures survive on detritus known as "marine snow" falling from the zones above or, like the marine hatchetfish, by preying on other inhabitants of this zone.
=== Abyssopelagic (abyssal zone) ===
The name is derived from Ancient Greek ἄβυσσος 'bottomless'. The ocean floor is next to this zone, and it forms volcanos, mountains, and vents from the movement of the tectonic plates. Among the very few creatures living in the cold temperatures, high pressures and complete darkness there are several species of squid; echinoderms including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the sea spider. Many species at these depths are transparent and eyeless.
=== Hadopelagic (hadal zone) ===
The name is derived from the realm of Hades, the Greek underworld. This is the deepest part of the ocean at more than 6,000 m (20,000 ft) . Such depths are generally located in trenches.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. In this zone, there is an increase in temperature from adiabatic heating. Very few creatures live in this zone. Some of the recorded species are coelenterate, polychaetas, amphipods, echinoderms, and mollusks.
== Pelagic ecosystem == The pelagic ecosystem is based on phytoplankton. Phytoplankton manufacture their own food using a process of photosynthesis. Because they need sunlight, they inhabit the upper, sunlit epipelagic zone, which includes the coastal or neritic 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.