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Deep seas 1

Deep sea floor

“You could start now, and spend another forty years learning about the sea without running out of new things to know.” - Peter Benchley (Author of Jaws.) Benchley was being more optimistic than James Cameron, who is quoted as saying “I’m hopeful that we’ll be able to study the ocean before we destroy it.”  (Cameron directed/wrote The Abyss, Titanic,  Avatar series, Terminator as well as other movies.)

The deep sea in still largely unknown. But the services rendered by this region of our planet are vital to our continued survival.


Deep sea structure and terminology

The average depth of the ocean is about 4260 m. At 1000m deep and below, light from the sun is completely absent and temperatures are constantly just above freezing. The pressures at these depths range from about 40 to over 110 times the pressure at sea level.

The diagram below

  • shows the names of the oceans' depth zones. Note the use of the suffix pelagic. Each of the depth zones uses it. e.g. mesopelagic, epipelagic
  •  illustrates the features of the ocean floor. The scales shown on the left and right sides are independent of each other.

  • Abyssal plains cover over 50% of the earth’s surface and are defined as ocean floor between 3,000 and 6,000 m.
  • The mid-ocean ridge is a seafloor mountain system formed by plate tectonics. It typically has a depth of about 2,600 meters and rises about 2,000 meters above the deepest portion of an ocean basin.
  • Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically 50 to 100 kilometers wide and 3 to 4 km below the level of the surrounding oceanic floor.
  • A seamount is an underwater mountain formed by volcanic activity. Due to the steep slopes of seamounts, nutrients are carried upwards from the depths of the oceans toward the sunlit surface, providing food for creatures ranging from corals to fish to crustaceans. It also provides a firm surface to attach to for sessile organisms.

Deep sea biology

Scientists believe that it is possible that as many as 10 million species may inhabit the deep sea – a rich source of biodiversity. Some species have been named and studied, but the majority of species are yet to be discovered.

No photosynthetic organisms can survive where there is no light, nevertheless, the deep ocean hosts a diverse range of organisms, interconnected by food webs. The energy for these life forms is dependent on

  1. daily migration of zooplankton between the epipelagic and the mesopelagic zones (DVM). This behavior takes place every day, in every ocean, and by biomass it is the largest migration on the planet!
  2. the rain of organic matter (often referred to as marine snow)  descending from the open ocean above. The organic matter can be small as algal fragments or as large as a chunk of dead whale (or the faeces of a living whale)
  3. the ability of resident bacteria to perform chemosynthesis
    (This topic is explored in Deep seas 2 )

Services provided by the deep sea

Climate services

Heat storage

The deep ocean system serves as a major heat sink and slows down anthropogenic global warming (according to the IPCC in 2014).  More than 90% of global warming heat ends up in the oceans.

Heat is moved by ocean currents, both local and global, near the surface and in deep water and this affects the regulation of temperature extremes, the stabilisation of global climate patterns and the cycling of gases.

ocean currents
ocean currents

Deep ocean currents are different from surface currents in scale, speed, and energy and are driven by density differences.

Water density is affected by

  • the temperature (colder is denser)
  • salt concentration (saltier is denser)
  • depth of the water

The greater the differences in density between different layers in the water column, the greater the mixing and circulation. Density differences in ocean water contribute to a global-scale circulation system, also called the global conveyor belt. These currents move water masses through the deep ocean—taking nutrients, oxygen and temperature with them.  Cold deep ocean currents carry denser water away from the poles toward the Equator. The ocean’s global circulation system plays a key role in distributing heat energy, regulating weather and climate, and cycling vital nutrients and gases. Geological features of the ocean floor also exert an influence on currents. This is examined in more detail in Deep seas 2.

Green House Gas Regulation

Carbon sequestration by the deep ocean is an important climate change mitigation pathway that relies on an efficient “biological pump” (i.e., the physical process of sinking biologically-produced carbon from the upper ocean into the deep sea). The burial of upper ocean-produced carbon in deep sediments contributes to carbon sequestration and climate regulation because it removes the carbon from the atmosphere for thousands to millions of years.1

DVM plays a major role in the carbon cycle. As animals return to the deep after feeding at the surface, their waste speeds up the rate at which carbon is transported to the ocean floor (a process known as carbon sequestration). During this time of global climate change, this means that shifts in the intensity of DVM behaviors could impact the carbon cycle. We still only know a little about how this phenomenon in different parts of the world is responding to climate change, so this is an important avenue for future research.2

The deep sea is the largest carbon reservoir on Earth, but only part of it is dissolved in the water or locked up in massive deep-sea coral reefs. The majority of the carbon in the deep ocean is in the form of methane hydrate. This is a solid, ice-like form of methane that occurs under very cold and/or high-pressure conditions. When it gets too warm, it becomes unstable and the large chunks of methane ice in the ocean floor go straight from a solid to a gas and bubble up to the surface.3


Fish of the deep sea have been commercially fished since the 1950s. Most of the areas where deep ocean is found are in non-territorial waters and can be exploited by companies based in any nation. Deep sea fishing is usually carried out using trawl nets which can easily damage fragile cold-water coral communities.

A number of species of deep sea fish are of commercial interest as food.

Pharmaceutical services

It was previously thought that corals only lived in shallow tropical water, but recent research suggests that over half of all known coral species live in deep cold waters. Scientists are also discovering more and more species of deep sea sponges. Such organism present a high level of  pharmacognosy potential.

  • Some deep sea coral species produce compounds used in antibiotics and painkillers. Seafan corals contain substances used to treat asthma and heart disease.
  • Compounds found in certain deep-sea sponges are potent immunosuppressive and anti-cancer agents.

Cultural/aesthetic services

Statue of Poseidon
Statue of Poseidon

“The voice of the sea speaks to the soul. The touch of the sea is sensuous, enfolding the body in its soft, close embrace.” – Kate Chopin

“How inappropriate to call this planet Earth when it is quite clearly Ocean.” – Arthur C Clarke

“Why do we love the sea? It is because it has some potent power to make us think things we like to think.” – Robert Henri

“The sea, once it casts its spell, holds one in its net of wonder forever.” –Jacques Yves Cousteau

“The ocean is a central image. It is the symbolism of a great journey.” –Enya

Summary of services provided for us

  • Heat regulation and weather pattern stabilisation
  • Carbon storage
  • Food
  • Deep-sea coral and sponge communities are largely untapped sources of natural products which can be used in medicines, cosmetics and other commercial products.
  • Aesthetic, cultural and recreational services

Threats to the services

Human caused impacts can be grouped into categories:

Climate change - particularly with regard to temperature

A study published in January, 2022 took measurements of temperatures at least 2,000 meters (about 6,500ft) deep and spread across the globe.

Acidity changes

Normal shell/Acid damaged shell
Normal shell/Acid damaged shell

Ocean acidity is changing too rapidly for organisms to adapt to. Acidic water attacks and dissolves calcium in shells, and coral structures. Even if the increased acidity does not kill them initially, they are weakened and more susceptible to predators and disease.  In the past 200 years alone, ocean water has become 30 percent more acidic.
For a brief description of the acidity problems that deep sea organism face, see the article Acidification of the deep Atlantic Ocean is accelerated by ocean circulation.

Noise pollution

Until recently, it has been assumed that noise pollution couldn’t penetrate to extreme depths. The distance that sound can travel in the ocean is a function of both pressure and temperature. It is also influenced by geological features such as trenches and seamounts. Most marine animals rely on sound and hearing in some form for survival e.g. to find mates, to congregate for spawning events, to stun prey organisms, for navigation. Noise can travel remarkably far in the ocean, much farther than it would on land. A 2017 study found that hydrophones lowered 11 kilometers (6.8 miles) into the Mariana Trench — the deepest oceanic trench in the world — picked up the noise of passing ships. Not enough research has been has yet been carried out to definitively assess the effects of human intrusion into the natural soundscape, but we do know that sea organisms have become quieter over recent times.

One of the issues with assessing the impact of noise pollution is that different sounds and frequencies affect animals in different ways, said Rebecca Dunlop from the University of Queensland's School of Veterinary Science. It does make it difficult because there are so many variables you have to consider, such as what the animals can hear and at what point that will have an effect," Associate Professor Dunlop said.4

Resource exploitation (Overfishing)

Because deep-sea species live in rarely disturbed environments, they tend to be slow growing, long lived and late to sexual maturity. For example, the orange roughy lives to over 100 years old and does not begin breeding until it is somewhere between 20 and 40 years old. This makes them exceptionally vulnerable to extinction, because they cannot breed fast enough to maintain numbers when they are heavily fished.
For other types of resource exploitation, see  The Deep Seas 2. (coming soon)

Waste dumping

You would think that the deep seas would be fairly pristine considering considering it has  been off-limits to all but a few adventurous scientists. But when an expedition to examine  whether certain human created chemicals had reached the bottom of the Mariana trench, they not only found that chemical pollutants had reached there, but also an empty can of Spam and can of Budweiser beer. The Mariana Trench is the deepest point of the ocean floor but it was found that small crustaceans living at this depth were heavily contaminated.

When a POP-contaminated animal dies in the open ocean, the carrion slowly drifts down to the bottom, nibbled at the whole way by decomposers until a few contaminated flakes finally reach the seafloor, where they more or less stay. “When it gets down into the trenches, there is nowhere else for it to go,” said expedition lead Alan Jamieson, of Newcastle University. That’s how the [POPs got into the crustaceans] living at depths heretofore considered beyond the reach of human influence.5

As well as the items mentioned above,

  • high levels of degraded plastics have been discovered
  • numerous items from  war activities - e.g. bombs and bullets
  • sunken ships such as The Titanic, and planes lost at sea
  • radioactive materials from nuclear reactors

What can we do to retain these services?

  • Any actions that will mitigate global warming will diminish the necessity for the deep oceans storing more heat and will help correct acidification
    e.g. ending the use of carbon dioxide emitting fossil fuels, cutting down on consumption, protecting/re-invigorating areas such as mangrove forests, sea marshes and kelp beds
  • End deep sea fishing subsidies.
  • Include  noise pollution in environmental assessments of human impacts on marine ecosystems
  • Invest in noise abatement technologies, e.g. ships powered by electricity would make much less noise
  • Reduce consumption of discretionary items, particularly plastics.
  • Educate people about waste dumping at sea and use stronger disincentives to enforce dumping regulations.

Dig deeper


  1. Ecosystem Services of the Deep Ocean []
  2. What is vertical migration of zooplankton and why does it matter? []
  3. Climate change: The deep ocean protects us from ourselves — for now []
  4. Noise pollution is penetrating further into our oceans, endangering marine animals []
  5. Scientists find incredibly high levels of pollution in the Mariana Trench, including low-sodium Spam []
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