PELAGIC: CHARACTERISTICS, FLORA, FAUNA - BIOLOGY - 2025

The pelagic, sea or pelagic zone is that column of water that is on the seabed. It includes the neritic zone, located on the continental shelf and whose maximum depth is 200 meters; and also the oceanic zone, which extends from the edge of the continental shelf out to sea.

However, some authors limit the pelagic zone to the oceanic region, thus excluding the neritic zone. Depending on the depth, the pelagic zone can be subdivided into several zones: epipelagic, mesopelagic, bathypelagic, abyssopelagic and hadopelagic, each one with its own well-defined characteristics.

Jellyfish Aurelia aurita, an organism that inhabits the pelagic zone. Taken and edited from: I, Luc Viatour.

The epipelagic zone corresponds to the photic zone, which is the most superficial layer and the one with the highest primary productivity and the highest biodiversity; while the deepest, the hadopelagic has very few species known to date.

characteristics

It represents the entire column of water on the sea floor, which has a high variability in terms of its physicochemical and biological parameters.

In the first meters of the water column, it is well lit, but the sunlight suitable for photosynthesis only reaches up to about 80 meters, while the visible light can reach up to 200 meters deep.

Dissolved oxygen is more abundant in the first few meters, and then falls to a minimum oxygen zone (200 m) and then begins to rise again.

Biological diversity is also higher in the shallower waters, decreasing with depth.

Pressure increases with depth at the rate of one atmosphere of pressure every 10 meters.

The temperature is relatively uniform near the surface. Then it begins to gradually decline with increasing depth and later falls abruptly in the thermocline zone that is located near 150 m depth. Once in that space, it remains relatively stable between 0 and 6 ° C.

Bathymetric division of the pelagic zone

- Epipelagic zone

It extends up to 200 meters deep. It is a well-lit area that corresponds to the so-called photic zone. In this space, photosynthesis is carried out by the phytoplankton, in addition to the benthic photosynthetic producers.

The temperature remains practically stable in the first few meters due to the action of the solar rays and the mixture that occurs thanks to the winds and currents. Then there is a sudden drop in temperature in the thermocline zone.

Flora

The flora of the epipelagic zone is represented by phytoplankton in the first approximately 80 meters of depth, but these begin to become increasingly scarce once they are exceeded. This is due to the quantity or quality of light that reaches these depths and that is insufficient for the requirements of these organisms to carry out photosynthesis.

Phytoplankton is made up not only of unicellular algae, but also of bacteria and other organisms capable of photosynthesis. Phytoplankton species include, for example, Chaetoceros decipiens, Cimbella lanceolata, Ditylium sp., Rhizolemnia (diatoms), prochlorophytes, chrysophytes, chlorophytes and euglenophytes.

Diatom A group of quite representative microalgae within the pelagic phytoplankton. Taken and edited from: fickleandfreckled.

Fauna

The fauna of the epipelagic zone is very diverse and is represented by both microscopic organisms, belonging to plankton, and large organisms such as marine mammals, belonging to nekton.

Among the zooplankton organisms are larvae of practically all the zoological groups existing in the marine environment (meroplankton), copepods, miscidaceans, pteropods, jellyfish, polychaetes and rotifers, among others.

Nekton organisms are those that can swim freely without being carried away by currents and waves. Among them are engulfi, swordfish, barracuda, sharks, tuna, dolphins, squid and also seabirds.

- Mesopelagic zone

It extends between 200 and approximately 1000 meters deep (2000 m according to some authors). It is known as the twilight zone. There is not enough light for photosynthesis, but there is enough for vision by animals.

The temperature in this area oscillates between approximately 5 and 10 ° C, with the highest temperatures found in the lower depths.

Flora

There is not enough sunlight in this area for the plants to carry out photosynthesis, so there is no existence of any organism of these characteristics.

Fauna

Animals in the mesopelagic zone are mostly scotophiles (they like darkness). Mesopelagic zooplankton is more or less similar to epipelagic plankton, being equally dominated by copepods. Ostracods (bivalve crustaceans) are also abundant.

Bristle-mouthed fish (which have enormous numbers of teeth) and lanternfish comprise about 90% of all fish in this area. There are also several species of mesopelagic shrimp.

- Bathypelagic zone

This zone is located immediately below the bathypelagic zone and reaches a depth of approximately 4000 m. The temperature is very constant and is between 0 and 4 ° C.

Flora

Totally non-existent.

Fauna

In this layer, like the layer immediately above, the most frequent organisms are bristle-mouthed fish and lanternfish. Bioluminescent organisms are common, either because they do this themselves or because they are associated with bioluminescent bacteria that live within them.

Giant squid also live in this area, which are preyed upon by sperm whales.

- Abyssal zone

For some authors it is between 4000 and 6000 m deep, however, others place it between 2000 and 6000 m deep. They are cold waters (1 to 4 ° C), poor in oxygen and also have a very high pressure.

Flora

There are no plants in this area due to the lack of light.

Fauna

Deep-sea fish lack a swim bladder and many are completely blind or, conversely, with disproportionately developed eyes. Species that use bioluminescence as a mechanism to attract other specimens of the same species or to attract potential prey are common.

- Hadal area

It is the deepest area known to date. It is located below 6000 meters and represents the so-called oceanic trenches. The pressures in this area are extreme and it is a very little known area.

Flora

Does not exist.

Fauna

The pelagic fauna in this area is practically unknown and difficult to separate from the benthopelagic because it lives very close to the bottom. Most are squid or ratfish belonging to species not yet described.

References

1. R. Barnes, D. Cushing, H. Elderfield, A. Fleet, B. Funnell, D. Grahams, P. Liss, I. McCave, J. Pearce, P. Smith, S. Smith & C. Vicent (1978). Oceanography. Biological Enviroment. Unit 9 The pelagic system; Unit 10 The Benthic system. The Open University.
2. G. Cognetti, M. Sará & G. Magazzú (2001). Marine biology. Editorial Ariel.
3. G. Huber (2007). Marine Biology. 6 ^th edition. The McGraw-Hill Companies, Inc.
4. Piélago. On Wikipedia. Recovered from: es.wikipedia.org.
5. Pelagic environment. Recovered from: ecured.cu.
6. Pelagic zone. On Wikipedia. Recovered from: en.wikipedia.org.
7. Hadal fauna. On Wikipedia. Recovered from: es.wikipedia.org.