What is the water column?

What lies beneath the surface of the ocean?

If the terms "territorial waters", "EEZ" and "high seas" describe a horizontal division of the maritime spaces from the coast to the open sea, what about what lies beneath the surface of the ocean? Are the ocean depths divided into several layers? And what lies between the surface and the "Zone" that describes the bottom and substratum of the ocean? What is the water column? What lives below the surface of the ocean?


What is the water column?

What lies between the surface and the bottom of the ocean is called the water column. The study of the water column provides information on the physical and chemical characteristics of the seawater at a given depth.

The temperature, salinity, light penetration as well as the pH, oxygen content and minerals present in the seawater can then be determined.

The study of the water column also provides information on:

    Living organisms and their environment and where they live, which may change during their life cycle.

    The circulation of water bodies on a large scale and the transfer of matter between these water bodies.


How is the water column divided?

There are 3 main zones in the water column:

    The pelagic zone describes the zone more or less close to the surface

    The demersal zone concerns the zone more or less close to the sea floor

    The benthic zone refers to the sea floor and the sediments.

The pelagic zone is further divided into several sections:

    Epipelagic: this is the zone between 0 and 200 metres where there is enough light for photosynthesis. It has many plants and animals. Approximately 90% of all marine life thrives in this area.

    Mesopelagic: between 200 and 1000 metres, photosynthesis is no longer possible at these depths.

    Bathypelagic: between 1,000 and 4,000 metres, at this depth there is complete darkness and the animals that live there feed on marine snow, i.e. marine detritus from the upper layers.

    Abyssopelagic: from 4,000 to 6,000 metres, down to the ocean floor. There is no more light at these depths, the creatures living at this depth are albino and generally blind.

    Hadopelagic: from the ocean floor to the deepest trench, 11 km down from the surface of the ocean.

    Did you know? The average depth of the ocean is 3,800 metres, whereas the average height of the peaks on land is 840 metres. The water temperature is below 2°C at extreme depths, and every 10 metres, the pressure increases by one bar!

What lives in the water column and at what depth?

 From the surface of the ocean down to the abyss, there is life at every level!

At depths below 200 m, sunlight no longer penetrates the surface of the water, but marine life continues to thrive at abyssal depths.

This means that the water column can measure 11 km and lead from the surface down to the Mariana Trench!

In fact, despite the absence of light and therefore, of plants that form the basis of photosynthesis, underwater life develops and adapts to the extreme conditions. Organic matter that is produced by photosynthesis comes from the surface and is decomposed by bacteria; in addition, microorganisms live in the hydrothermal vents and support an ecosystem that does not depend on photosynthesis.

A pelagic fish is a fish that lives between the surface and the seabed, but does not depend on the seabed for food. It lives far from the coast in the open sea. Mackerel, herring or pilchards are pelagic fish just like tuna and swordfish.

    Their streamlined bodies enable them to move quickly,

    Their blue-green backs protect them from birds and marine predators,

    They live and swim in shoals that are made up of individuals of the same size.

    Fish larvae that are zooplankton are pelagic

Jellyfish, squid, sharks, marine mammals and sea turtles are also pelagic marine animals.

Did you know? During the period when herring were breeding, scientists observed a shoal of 250 million individuals stretching for about 40 km! Pelagic species account for about half of the world's catch.

A demersal fish is a fish that lives above the seabed and depends on it for food. Sea bream, cod, pout, gurnard, hake and whiting are demersal fish, as are groupers, which live on rocky bottoms.

    Their colour varies from grey to red,

    The barbels of cod or pout are used to scour the mud.

A benthic fish is a fish that lives on the bottom and depends on the sediment for food. Rays and skates, sole, shrimp, lobster, mussels and scallops live on the bottom.

    Benthic fish move along the bottom,

    It often has a flattened shape like skate, plaice or sole.

    It sometimes burrows to the seabed to disappear from the eyes of its predators.

    Benthic fish such as sole have a pelagic life stage when they are larvae; they settle on the bottom as adults.

What lives the deepest under the ocean?

Some marine species, such as sea turtles, penguins, whales or sperm whales, are capable of diving into the depths of the ocean.

However, a distinction must be made between animals that depend on the surface and sometimes dive into the depths of the ocean, such as marine mammals, penguins or turtles, and species that live at great depths, such as the giant squid, the vampire squid, the anglerfish or the Dumbo octopus.

It is important to remember that the ocean trenches contain marine life such as shrimps, fish and crabs.

If you want to explore the depths of the ocean and discover marine life from the surface to the deepest oceanic faults, this animation will take you there. 

Among the creatures you'll see are a clown fish and a palette surgeonfish living at a depth of 40 metres; you will know how deep the polar bear dives, as well as the sea lion, the emperor penguin, champion of apnea at 550 metres (can you tell the difference between a penguin and an auk?), the narwhal, which dives to 1,500 metres, or the sperm whale, which stumbles across the giant squid at 2,000 metres.

Chimaeras, vampire squid and Dumbo octopus live in the great depths, and in the deepest part of the ocean lives the Mariana snailfish Pseudoliparis swirei, which has been sighted at depths below 8,000 metres.

The Ocean Mag

haute mer

What are the high seas?

High Seas 6mn

High seas or international waters? Where do the high seas be...

Marine currents, the regulators of the c...

The Ocean and the Climate 6mn

Life on Earth cannot exist without the ocean.

Biodiversity in the open sea: from the s...

Biodiversity 8mn

Off the coast, beyond the territorial waters, lies the high ...