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Nahle, Nasif. 2008. Marine Environments. ©Biology Cabinet Organization. New Braunfels, TX. Obtained (day) (month) (year),

By Nasif Nahle Sabag

The sea is very important for life on Earth. Theoretically, the animals evolved in oceans and eventually they emigrated to lands and freshwater environments. In addition, the oceans act like the main regulators of the terrestrial climate and temperature. I can assure, without fear to be wrong, that without the oceans the animal life would not be possible on our planet because more than 85% of the oxygen breathed by plants and animals, aquatic and terrestrial, is produced by the vegetal plankton (phytoplankton) that dwells in the surface of the oceans (neritic environment). The oceans are also the main source of biotic energy for the survival of human beings.

For the reasons referred in the previous paragraph, it is important that the biologists know the organization of the oceans. To understand the organization of the marine biomes, I have prepared the following scheme on the demarcation of oceanic environments. I am not going to complicate to the reader with specific data of the oceans that perhaps we will never know in the course of our lives:




Let us considered firstly the surface of the sea in the horizontal perspective. The sea surface extends down to two meters in depth. The zone is marked by the penetration of incident infrared solar radiation. Below the surface, we find a subdivision of the ocean in zones that have been determined by the intertidal area and the topographic features of the sea bottom.


1. Littoral Environment: It is the region comprehended between the limits of the high tide and the low tide. Their extension and depth are variable. It is the region that marks the coastal line. This zone is rich in living beings. It is called also intertidal zone.

2. Pelagic Biome: It extends from the line of the low tide towards the open sea. Its depth is variable. The pelagic environment is classified in two important regions:

a) Neritic Biome: It includes the volume of water between the line of the low tide and the end of the continental shelf. Its extension and depth are variable, but in general we can say that its extension to the open sea (in the surface) is 150 km., and about 150 meters in depth, although there are oceans which neritic biome is more than 1500 meters. The life develops optimally in this environment because its waters are warm and rich in nutrients. Most of the aquatic multicellular plants develop in this environment. The neritic biome is the richest in living beings zone. Some authors include the intertidal zone into the Neritic biome.

b) Oceanic Biome: It extends from the end of the continental shelf towards the open sea. Like the neritic biome, the extension and depth of the oceanic biome are variable. The Oceanic Biome reaches 14000 meters in depth in the oceanic trenches.


Vertically, or in depth, the marine biomes are subdivided in two main zones:

1- EUPHOTIC ENVIRONMENT: It is delimited by the penetration of the light. Its inferior boundary is the plane where the light is absorbed absolutely. It is subdivided in the following zones:

1. Epipelagic Environment: It extends from the surface of the sea to approximately 100 meters in depth. It is determined by the penetration of light; by this reason, the epipelagic environment is the richest environment in variety and number of producers or photosynthesizing organisms, as unicellular (phytoplankton) as multicellular plants (alga and superior) and animals. Where the light reaches strongly, the organisms that produce food and oxygen and consequently the consumers that obtain their food of the producers prosper abundantly. For that reason, the epipelagic environment is known like euphotic zone, that is to say, the zone “with light”.

2. Transitional Environment: This environment is placed between the epipelagic or euphotic environment and the aphotic environment (aphotic means “without light”). The light is gradually absorbed as it penetrates the mass of water down to 200 meters in depth. Below this limit, all the light has been absorbed by the water and the following environments are in complete darkness. In the transitional environment, given that only the blue light can penetrate dimly, the amount of producers is too little. Only a few photosynthesizing organisms can be found there. Most of the living beings that live in this environment of transition are carnivorous or detritivores.

2- APHOTIC ENVIRONMENT: It is determined by the lack of light. In natural conditions the darkness is absolute, the meaning for its denomination “aphotic” or “without light”. It is subdivided in the next environments:

1. Mesopelagic Environment: This ambient includes the water volume between 200 meters and 1000 meters in depth. The upper limit is determined by the edge of the continental shelf, on condition that the edge of the continental shelf is immersed into a depth where the light cannot penetrate.

2. Bathypelagic Environment: It is situated between the plane at 1000 meters and the plane at 4000 meters in depth.

3. Abyssalpelagic Environment: It extends from 4000 meters down to the marine floor, including the bottom of the marine trenches.

Now let us consider the marine bottom in its horizontal perspective. The marine bottom is denominated Benthic Environment.


1. Benthic Littoral Zone: Like the littoral environment corresponding to the surface, the littoral benthonic zone extends from the line of the high tide to the line of the low tide.

2. Benthic Sublittoral Zone or Shelf: It is comprehended from the line of the low tide to the edge of the continental shelf. It corresponds to the neritic biome of the pelagic zone.

3. Benthic Bathyal Zone: It extends from the limit of the continental shelf and it descends in declivity to 4000 meters in depth.

4. Benthic Abyssal Zone: It is the marine continuation of the marine bottom from 4000 meters in depth to the bottom of the depressions or trenches.

5. Benthic Hadal Zone: It extends from the edge to the floor of marine trenches.

We can find living organisms in all vertical and horizontal biomes of the oceans; in warm, cold, and even in frozen waters! Living beings prosper also in abyssal and hadal environments. There are animals living on the sea floor (epifauna) and others living within the sea floor (infauna) in all the zones of the oceanic floor (benthonic fauna).

98% of the marine living beings inhabit the benthonic environment. We enclose those organisms into the group “benthos”. From that 98% of living beings inhabiting in the benthonic environment, 1% live in depths of more than 2000 meters.

2% of the marine species live in the open sea, that is to say, in the pelagic zone. Therefore, we call them pelagic organisms.

Taking into account their stratigraphical disposition in the oceans and in fresh waters, we can classify them in the next artificial groups:

Plankton: Microscopic species that live in the surface of the waters and that are displaced by the water currents and movements. Phytoplankton is the vegetal plankton. It is formed mainly by diatoms, chlorophytes, rhodophytes, phaeophytes, etc. Animal Plankton is known like Zooplankton. It includes protozoan, larvae and nymphs of multicellular animals, small crustaceans, worms, rotifers, etc.

Nekton: The species that can swim and propel themselves, even against currents, are known like Nekton. Nekton comprehends macroscopic species that can move by waters without depending on the aquatic currents; for example, fish, amphibians, reptiles, some strictly aquatic birds, like penguins, and some mammals like whales, seals and dolphins.

Benthos: Comprehends the vegetal and animal species that live in the bottom of the water or in the submerged vegetation. The benthos is formed by sessile species, i.e., fixed to the floor or to some substrates, and species with free movements and displacements.

As you can see, the marine biome that interests to biologists is… THE WHOLE SEA.


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