The Deep Sea

The Earth is a Water World

The oceans cover 70% of the world’s surface, and despite what you have seen on nature documentaries, most of the ocean is empty, dark, cold, and void of mammalian life. However, unlike the land, the ocean exists in three dimensions. With the deepest point in the ocean at 36,198 feet (11,033 m,) and the average depth of the ocean over 12,000 feet, the ocean’s make up something close to 99% of the Earth’s livable habitat. Scientists have been able to look at the ocean in a vertical profile and have found that it is divided into a series of layers (based on temperature, light penetration, and the presence of a bottom). These layers are in many respects independent of each other, creating a series of unique ecosystems at each depth. Nearly 90% of the ocean’s space is found below 1000 meters.  That means that 90% of the ocean, is uniformly cold, dark, and under extreme pressure.

The Ocean is Layered

Water’s unique properties create a situation in the deep ocean in which waters with different temperature and salinity have different densities, effectively forming a layer cake. There are five distinct layers in the ocean, and creatures that inhabit one specific layer rarely move to another.

Epipelagic (sunlight zone) – Typically the first 200 meters of the open ocean where light is abundant.  Algae is able to photosynthesize, and provides a solid base for the ocean food chain. The water in this layer, is well mixed by waves and wind. As a result, oxygen is also abundant.  Most the marine life that we are familiar with from fishing or watching documentaries is found in the epipelagic.

Mesopelagic (twilight zone) – Typically between 200 and 1000 meters. There is light available but not very much, certainly not enough for photosynthesis. This is a zone of great transitions including sharp changes in temperature (known as the thermocline), salinity (known as the halocline) and density (known as the pycnocline.). Species that reside here are highly migratory.  They will spend their evenings feeding near the surface and then dive to darker depths at the onset of day light. The fish and planktonic organisms that make these journeys are part of the largest migration on Earth. The combination of high oxygen use, and zero oxygen production, leads to the mesopelagic having very low levels of oxygen. Where oxygen reaches its lowest levels is known as the oxygen minimum zone.

Bathypelagic (midnight zone) – This layer begins at 1000 meters or where complete darkness begins. Typically the water is uniformly cold (around 4 degrees C,) and pressure is intense. Organisms that live here are often slow growing and slow moving.  They may be mushy or gelatinous in texture to deal with the pressure and the the fact that there are no solid surfaces here. Species here may have tidy eyes, and often produce bioluminescent light themselves.

Abyssopelagic (abyssal zone) – This is the bottom of the ocean, which begins on average at 4000 meters. The presence of a rocky (hard) or silty (soft) bottom means that there is a higher biomass of living organisms that the layer above it. Rather than relying on producers to form the base of the food pyramid, organisms living in the abyssal zone are completely dependent on marine snow, the mostly organic detritus that falls from oceanic layers above.

Hadopelagic (Hades zone) – The hadopelagic zone exists only in long but narrow topographic V-shaped canyons at the edges of active margins. Not very much is known about the hadopelgic. Marine life abundance generally decreases with depth, but there is life even hear. Most of it benthic invertebrates, all of it evolved to handle intense pressures.

Most of the Ocean is Dark and Cold and Under Extreme Pressure

In the clearest parts of the ocean, light can penetrate to almost 1,000 meters, but in most places, light effectively penetrates to only 200 meters. The epipelagic, or sunlight zone, is home to most of the ocean’s producers and hence most of its living things, but over 90% of the ocean’s volume is below this – trapped in complete darkness.

Life in the deep sea is also cold. Above 200 meters, oceanic water is heated by the sun or alternatively chilled by cold surface temperatures. Oceanic temperatures across the globe vary based on latitude and currents. The surface temperature in the Arctic Ocean during an oncoming winter will be well below zero degrees celsius. Alternatively, surface temperatures near the equator may near thirty degrees celsius (85 degrees Fahrenheit.) Wind and surface currents stir the water column to nearly 1000 meters, leaving temperatures generally warmer than the deep waters. Once you go below 1000 meters, temperatures become a nearly uniform four degrees celsius.

Much of the ocean is also under bone-crushing pressure. The pressure you and I experience at sea level is referred to as one atmosphere. As you swim down to the bottom of a pool you may notice the pressure increasing – you can feel it in your ears. In fact, for every ten meters of depth the pressure increases by one atmosphere. Pressure at the deepest point in the ocean is over 1100 times greater than at the surface. Significantly increased pressures can cause deformations in the large organic molecules that make up cells. We are still not sure how deep sea species can endure these deformations, but it is possible that they are using small organic molecules called “piezolytes” to provide some type of protection to the larger molecules of their cells.To learn more about the physical features of the deep ocean its really worth checking – The Deep Sea.

Parts of the Deep Ocean Lack Oxygen

Sinking currents of cold surface water from the poles will often lead to oxygen rich waters along the ocean bottom. However, there exists a region between 500 and 1000 meters known as the oxygen minimum zones. At these depths, there is no light and no oxygen produced from photosynthesis. There is also very little oxygen input from the surface and as a result the levels of oxygen may be far below what would be ideal for life. Life does exist here, but perhaps only in transient form.

It’s hard to grow shells in the deep sea

Calcium Carbonate (or Calcite) is one of the primary materials used by marine organisms to make hard parts like shells. This is true of plankton as well as larger organisms like mussels. It is a truly important chemical to life in the ocean, and the bulk of the ocean’s supply is locked up in living or once living organisms. Carbonate oozes (the remains of the shells from once living organisms) cover about half of the world’s sea floor. These oozes, however, are present only above a depth of 4,500 meters. Below this depth the intense pressure causes the calcite to dissolve. This depth is named the Calcite Compensation Depth (or CCD), and it represents the level at which the rate of carbonate accumulation equals the rate of carbonate dissolution. Below this depth shelled organisms cannot exist.

Adapting to Life in the Darkness

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Life in the deep sea survives only because it has adapted over the millennia to the truly extreme conditions mentioned above. Due to the difficulties in exploring such habitat, we know much less about the organisms that live there than those that live at the surface. What we do know, is that there is an incredible diversity of life. Almost every voyage to the deep discovers new species. We also know that most of the living things in the deep are able to produce their own light through a process called bioluminesce. We also know that physical conditions generally result in living things that are small, gelatinous, and often ghostly or grotesque in its appearance. Organisms like the giant squid are certainly exceptions. Many of the fish and invertebrates of the deep have lost hard structures like bones or shells and taken on gelatinous body forms. In a world that is constantly sinking and there are no other hard surfaces to protect yourself from, shells are nothing but dead weight. To see some more examples of deep sea species, check out Nautilus Live – Gallery or NOAA Okeanos Explorer – Image Gallery.

By IFE, URI-IAO, UW, Lost City Science Party; NOAA/OAR/OER; The Lost City 2005 Expedition. (NOAA Photo Library: expl2230) [CC-BY-2.0 ( or Public domain], via Wikimedia Commons

Image: Wikimedia Commons

The Deep Ocean Contains the World’s Most Extreme Geology

If we could drain the oceans, we would see some amazing geology; the Earths’s tallest mountain (Mauna Kea, Hawaii, rises 33,474 feet from its base on the ocean floor), the longest range of mountains (the Mid-Atlantic Ridge, at almost 40,000 miles long), and the deepest canyon (Mariana Trench in the western Pacific). We would also see huge pancake-flat basins, violently erupting volcanoes, and nearly every other geological feature found on the continents but magnified in size and scope.

Exploring the Deep Sea

On January 23, 1960, the crew of the Trieste reached the bottom of the Challenger Deep in the Pacific Ocean’s Mariana Trench and set a deep diving record — 35,810 feet. Many oceanographers believed this record would never be broken. Pressures at this depth are well over 1000 times greater than at sea level, but Canadian film maker, James Cameron, broke that record in a solo dive during the spring of 2012. He did so using a custom-built submersible that was unlike anything that had been used before. Breaking a tradition of solid steel balls with small thick glass windows, Cameron’s sub is sleek and streamlined. Click to read more about other deep sea submersibles.  You can also study the depth limits of several submersibles and some deep sea fish at the Smithsonian’s “How Deep Can They Go” page.

A much easier way to explore the deep sea is to use a robotic submarine such as NOAA’s Hercules ROV.  Another way to see what’s going on down in the deep is to attach a camera to a marine organism naturally designed to handle depth much better than we do. Researches have attached cameras to sperm whales and penguins, both to study the animals themselves as well as make observations of other deep sea life.

Chemosythetic Communities

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Image from:

In 1977, scientists diving off of the Galapagos Islands made one of the most amazing discoveries in the history of oceanography. There are places in the bottom of the ocean that defied the rules of basic biology. Before this discovery, it was assumed that all living things used photosynthesis, depending upon the sun for energy. What was discovered on this dive were communities that were deriving energy from a process called chemosynthesis. As we have explored more of these, it turns out that deep sea hydrothermal vents and cold methane seeps are rich with living organisms, almost all of which have an existence completely separate from the surface.  Scientists discovered species so bizarre whole new phylum’s had to be created to categorize them.  Most of these communities were dominated by giant tube worms that had neither mouths, nor guts, nor anus. In other words, they did not eat.  Instead, they housed a symbiotic relationship with bacteria that convert hydrogen sulfide spewing out of the vents to energy. Whole ecosystems around these vents were based on a food web that had nothing to do with the sun. Click here to learn more about the organisms that live in these extreme deep sea communities.

Hydrothermal Vents – These are deep volcanic hot springs, mainly along the mid ocean ridges. They can have temperatures that exceed 400 degrees C. The water that comes out of them is mineral-laden and provides abundant energy to the organisms that live around it.

Cold Hydrocarbon Seeps- These are mostly along continental margins, where gases (methane and hydrogen sulfide) and oil seep out of the sediments and also provide abundant energy.

Questions to Research

  1. Read through the animated story of the William Beebe and Otis Barton making their first dives in the submersible Bathyshere at Nonsuch Island.  These guys pioneered the science of deep sea research. Describe what they accomplished and what they learned.
  2. Describe three difficult physical conditions that life has to adapt to in order to survive in the deep ocean.
  3. Explain what and where the oxygen minimum zone is, then identify it in this graph.
  4. Researchers using recorded video from robotic submersibles found some unusual things about what deep sea organisms eat and what eats them. Explain two surprising facts from the publication.
  5. Use the links in the “Adapting to Life in the Darkness” section and provide example of a mesopelagic, abyssal pelagic, and hadal species. You may also check out some of the logs from Okeanos Explorer.
  6. Together in class, we will watch this brilliant explanation of chemosynthetic tube worms by Ed Yong and PBS. After we do, explain what is so unique about the feeding strategies of deep sea tube worms.
  7. If organisms can not produce shells below the calcite compensation depth (4500 meters in the Pacific,) what do organisms (that might normally produce shells) look like at depths below the CCD?
  8. Read the Washington Post article about bioluminescence. Explain three possible reasons why many deep sea species produce their own light.
  9. Check out this NPR story about what marine biologists found when they sent a series of probes to the bottom of the Mariana Trench. Describe what they found at the very bottom. You may also want to check out this article about the fish found in the Mariana Trench (great pics and video.)
  10. Check out the BBS’s “Deep Surprises.” What was the geological and biological surprises found at deep sea hydrothermal vents. Use the reading above to help you answer this question, and check out a YouTube video from a NOAA ROV.