The photic zone is the area of the marine or lake environment as far as sunlight can penetrate. This zone is divided into two large layers, the euphotic layer or zone, which generally extends up to 80 to 200 meters deep, and the dysphotic zone, immediately below the previous one and which reaches approximately up to 1000 m deep..
The euphotic zone is one in which the sunlight that penetrates is sufficient to support the photosynthetic activity of photosynthesizing organisms. Below this, sunlight still persists, but the quantity and quality of it is insufficient to support photosynthesis.
Divisions of the ocean, photic zone. Taken and edited from: Oceanic divisions.svg: Chris huh.
All photosynthesizing organisms in aquatic environments are located in this space, from members of phytoplankton to macroalgae and marine phanerogams. The greatest diversity of fauna in aquatic environments is also located in this marine strip.
characteristics
This space, which corresponds to the epipelagic zone in the open sea and the neritic zone in coastal environments, is characterized by being well lit. The variation in temperature is very low, giving stability to the water column.
The waters of the photic zone in coastal environments are rich in nutrients thanks to the terrigenous contributions, however, offshore the waters are poorer in nutrients, because the contributions of these are scarcer and depend on complex and infrequent oceanographic phenomena. like ocean eddies.
In this area there is practically all the flora of marine environments, due to their need to use sunlight for their photosynthesis processes. There is also the layer known as minimum oxygen layer, which is that space where the respiration rate of photosynthesizing organisms is equal to their photosynthesis rate.
Due to this, practically all the oxygen that is produced is used by the photosynthesizing organisms themselves, in addition to the oxygen that is consumed by the heterotrophic organisms, for which the partial pressure of this gas falls to its minimum expression.
The light
Light is an electromagnetic radiation whose speed of propagation varies depending on the fluid in which it moves. In a vacuum it propagates at a speed of 2.99 x 10 8 m / s 2 while in the sea this speed is reduced to 2.99 x 10 8 m / s 2.
When sunlight penetrates seawater, it attenuates due to two processes, absorption and diffusion. These two processes depend on the amount of particles suspended in the water column, but in general terms, at a depth of 50 m, the incident radiation from sunlight has been attenuated by 50%.
Incident radiation decreases to 1% when reaching a variable depth depending on the strip, but in the open sea it is around 200 m.
In the euphotic zone, the incident light radiation is sufficient for the photosynthetic process to take place, and that the balance obtained between the oxygen released during photosynthesis and that consumed during cellular respiration is positive.
In the dysphotic zone, the amount of incident light is insufficient for photosynthetic processes, or at least not enough for photosynthesis to take place at a speed equal to or greater than the respiration rate. This light however is sufficient for the vision of the animals.
Flora
Virtually all photosynthesizing organisms are distributed in the euphotic zone because without light there is no photosynthesis and, therefore, autotrophic beings cannot survive.
The flora includes phytoplankton organisms, such as diatoms, cyanobacteria, chrysophytes, euglenophytes, dinoflagellates, among others. It also includes benthic macroalgae such as Ulva, Sargassum or Caulerpa, among others.
There are very few species of seagrass, most of them form seagrass meadows, such as with Zostera, Cymodocea or Posidonia for example. All of them located in the euphotic zone.
Only a few phytoplankton species can eventually be found in the dysphotic zone due to their nictimeral migrations, however they quickly return to the euphotic zone.
Fauna
Plankton
Collage, plankton diversity. Taken and edited from: Kils via Wikimedia Commons.
The zooplankton of the photic zone is represented by organisms that live their entire lives in plankton (holoplankton) and by organisms that spend only part of their life as members of this community (meroplankton).
Within the holoplankton are copepods, ketognaths, some jellyfish, shrimp of the Lucifer genus, rotifers, polychaetes, miscidaceans or ctenophores, among others.
Meroplankton, on the other hand, is represented by the larval stage of benthic species. Among them, for example, parenchymal larvae of some species of sponges, planules of cnidarians, different larvae of crustaceans (zoeas, myscis, phyllosoma, puerulus), mollusks (trocóforas and velígeras), echinoderms (auricularia, doliolaria, brachiolaria, bipinnaria).
Most fish also go through a larval phase that develops in plankton and then completes their life cycle as members of nekton or benthos.
Necton
Nekton organism, Whale Shark, Rhincodon typus. Taken and edited from: Tilonaut, via Wikimedia Commons.
Nekton organisms, which can swim against currents and waves, are larger organisms. Among them are for example different species of pelagic shrimp, as well as squid (cephalopod molluscs).
However, the greatest diversity of nektonic organisms belongs to the group of fish. Among them, some from the coastal or neritic photic zone (anchovies, fish of the Haemulidae family) and other species that are exclusive to the oceanic photic zone (needle fish).
Some species of fish spend their entire lives in the marine environment, while others carry out periodic migrations or only once in their life between marine waters and rivers (catadromes, anadromes, amphydromes).
Reptiles (sea turtles), birds (penguin, gannet, cormorant), and mammals (manatee dolphins) also fall into the nekton category.
Benthos
The primary producers of this marine space are represented mainly by macroalgae, although there are also a few species of higher plants that inhabit the different seas of the world, all limited to the photic zone, such as Thalassia and Posidonia.
Corals are organisms almost exclusive to the photic zone. These cnidarians have a symbiotic relationship with algae called zooxanthellae that live inside them. These algae need the presence of light to be able to photosynthesize.
Biodiversity of the photic zone, coral reefs. Taken and edited from: Wise Hok Wai Lum.
Other benthic species in the photic zone include sponges, anemones, sea fans, polychaetes, snails, bivalves, sea cockroaches, octopuses, urchins, starfish, sea spiders, sea squirts, crabs, shrimp, among others.
Benthic fish spend their lives in direct contact with the seabed, among the benthic species of the photic zone are frogfish, rays, redfish, groupers, moray eels and trumpeter fish, among others.
References
- G. Cognetti, M. Sará & G, Magazzú (2001). Marine biology. Editorial Ariel.
- G. Huber (2007). Marine Biology. 6 th edition. The McGraw-Hill Companies, Inc.
- 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.
- Photic zone. On Wikipedia. Recovered from: en.wikipedia.org.
- Photic zone. Recovered from: esacademic.com.
- J. Castelvi, Ed. (1972). Marine ecology. La Salle Foundation for Natural Sciences.