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The marine pelagic zone is the area of the ocean far from the coasts; more precisely, the water masses of the oceanic zone located outside the continental shelf. Also called the open sea, the pelagic zone does not include the seabed. The pelagic term derives from “pielago”, which is the name of that area of the oceans, which in turn derives from the Greek word pelagos , open sea. It is in the marine pelagic zone where the largest species in the terrestrial biosphere develop and where most of the oxygen consumed by terrestrial organisms is produced.
How is the marine pelagic zone composed?
The availability of sunlight is essential for the development of pelagic ecosystems. The development of primary producers, those organisms that are the source of organic matter and energy for ecosystems by capturing sunlight through photosynthesis, is essential for the development of all pelagic organisms. A first differentiation of the marine pelagic zone is based on the penetration of sunlight in the water masses, and therefore in the depth.
The epipelagic zone
The epipelagic zone is the first subzone in this classification, defined as the masses of water that are between the ocean surface and 200 meters deep. Because it is the pelagic subzone that receives the greatest amount of sunlight, it is characterized by the abundance and diversity of organisms.
It is in this environment where plankton develops, marine microorganisms made up of a great variety of species and which constitute the base of the marine food chain. Marine plankton is made up of phytoplankton and zooplankton . Phytoplankton, the primary producers of these ecosystems, are organisms that, like plants in terrestrial environments, capture sunlight and through photosynthesis produce organic matter that stores the energy that higher organisms will consume. And in the same process, phytoplankton produces oxygen, an essential element for life. Marine phytoplankton produce most of the oxygen consumed by other organisms in the terrestrial biosphere.
Zooplankton are the consumers, the predators of the primary producers and other predators at lower trophic levels. Copepods, cladocerans, rotifers, cnidarians, chaetognaths, and euphausiaceae are some species of zooplankton. Krill is a well-known member of marine zooplankton as a staple food for baleen whales, seals, penguins, and other marine animals. Krill is a euphausiacean, a crustacean that constitutes one of the largest biomasses on the planet; a key link in marine food webs, feeding on phytoplankton and transferring its energy to the largest animals on Earth.
An immense diversity of fish make up the complex food webs of the marine epipelagic zone, many of them a very important food source for humans.
The mesopelagic zone
Between 200 and 1000 meters deep the marine mesopelagic zone is defined. Sunlight still penetrates to these depths but its intensity is very dim, insufficient for photosynthesis by primary producers. It is also called the twilight zone, due to the low light intensity. The amount of dissolved oxygen in the water also decreases at these depths as the microorganisms that produce it, phytoplankton, are drastically reduced. These factors constitute a limitation for the development of marine organisms, and the species that inhabit the mesopelagic zone are much less than in the epipelagic zone. Large fish, such as some species of sharks, and the so-called lantern fish, fish that produce light, are some of the animals found in these ocean depths.
the bathypelagic zone
The bathypelagic zone is between 1,000 and 4,000 meters deep. There is no sunlight and the hydrostatic pressure is very high, setting up extreme conditions for the development of life. The fauna is scarce at these depths, made up of organisms such as cephalopods of different sizes and shapes, such as giant squid, and the deep-sea anglerfish ( Melanocetus johnsonii ), a species of fish that has an antenna shape that produces light through bacteria, with what attracts its prey.
the abyssopelagic zone
The abyssal or abyssopelagic zone extends between 4,000 and 6,000 meters deep. It is an area of very cold waters and with a very high hydrostatic pressure, and the few nutrients that can be found come from debris from the upper levels. Only in areas where there are hot springs some species of primary producing bacteria develop. Some species of strange fish have adapted to this area and there are large invertebrates, a phenomenon called abyssal gigantism, such as sea spiders that measure more than 1.5 meters, as well as sponges and sea lilies.
the hadopelagic zone
Ocean depths greater than 6,000 meters are called the hadopelagic zone or hadal zone, taking its name from the Greek god Hades, the god of the dead. These depths are found in the trenches, oceanic areas that constitute a very small surface area of the oceans. The Mariana Trench is the deepest place detected on the Earth’s surface, with a maximum depth of 11,050 meters. Despite the origin of the name of this pelagic zone, even in these extreme conditions living organisms were found; Hundreds of species of microbes, worms, molluscs, crustaceans and fish, sustained, as in the abyssal zone, in the debris that falls from the upper levels and the chemical and biological phenomena associated with hydrothermal vents.
Sources
Britannica. Marine ecosystem | Britannica , 2021.
Inaba K., Hall-Spencer JM Introduction to Marine Biology . In Inaba K., Hall-Spencer J. eds. Japanese Marine Life. Springer, Singapore, 2020. https://doi.org/10.1007/978-981-15-1326-8_1
