Layers of the Ocean

Oceanographers (scientists who study the oceans) often divide the ocean into horizontal layers. They use the physical characteristics of the water such as temperature, density, and the amount of light at different depths to classify these layers. The most important factor is the density of the water, which is determined by the combination of salinity (the amount of salt in the water) and temperature. All ocean water is salty, but some contains more salt than others. The water that is saltier is heavier and sinks, while the water that is less salty is lighter and floats. Similarly, warmer water is lighter than colder water, so it floats on top of colder water.

Oceanographers generally categorize the ocean into four layers: the epipelagic zone, the mesopelagic zone, the bathypelagic zone, and the abyssopelagic zone. The word "pelagic" refers to the open ocean, away from the coast. The prefix epi means "surface"; the prefix meso means "middle"; the prefix bathy means "deep"; and the prefix abysso means "without bottom." In addition, the transition zone between the epipelagic and the mesopelagic is often called the thermocline.

Epipelagic zone

The epipelagic zone refers to the surface of the ocean where light penetrates. This layer is also called the photic zone, referring to the light that is found at these depths. Light is extremely important in the ocean. Just like plants on land, phytoplankton (free-floating plants, generally microscopic) require light to grow. Phytoplankton are the base of the food web (the network of feeding relationships in an ecosystem) in the ocean; they produce food by converting the energy from the Sun into energy they need to live and grow. When phytoplankton are eaten by zooplankton (free-floating animals) and fish, this energy is converted into the materials in their bodies. This transfer of energy continues as each predator (an animal that hunts, kills, and eats other animals) eats its prey (animals that are hunted and eaten by other animals), but it all begins with the energy from the Sun.

Sometimes the photic zone is referred to as the surface mixed layer. This layer is in contact with the wind and air above the ocean. The wind acts as a mixer, moving the water up and down throughout the top layer of the ocean. As a result, all of the water in the surface mixed layer has the same density. Because this water is often in contact with the air, it contains many of the gases required for life, such as oxygen and carbon dioxide.

The epipelagic zone extends about 500 feet (150 meters) into the ocean, although this varies depending on location. Only about 2% of the total volume of the ocean falls in the epipelagic zone.

The thermocline Just below the surface mixed layer is a layer of water where the temperature and density change very quickly. This layer is called the thermocline. In warm tropical waters, the thermocline is very abrupt, while in cold polar waters the thermocline is often rather gentle. Below the thermocline, the temperature is always about 40°F (5°C). The thermocline acts as a density barrier between the surface, where there is light and phytoplankton growth, and the deeper layers of the ocean, where food is often scarce.

Upwelling

Phytoplankton need four things to live: water, carbon dioxide, light, and nutrients (substances like nitrogen and phosphorus that are required for growth). In the ocean, getting water is never a problem. There is also a lot of dissolved carbon dioxide in the water. In the epipelagic zone, light is available, so if there are nutrients phytoplankton can grow easily. The problem is that as phytoplankton grow, they use up all the nutrients in the water.

After the nutrients in the epipelagic zone are gone, the phytoplankton and the animals that eat them cannot continue to grow. They will die and sink below the thermocline into the mesopelagic zone. In this part of the ocean, bacteria digest the dead organisms breaking them into the same nutrients required by the phytoplankton in surface waters. Unfortunately, these nutrients are trapped below the thermocline, where there is no light.

In some locations around the world, currents (steady flows of water in a prevailing direction) cause a phenomenon called upwelling. Upwelling brings the water containing nutrients from the deep water up to the epipelagic zone where there is light. In these locations, the combination of nutrients and light results in conditions that are perfect for phytoplankton growth. Important upwellings occur on the western sides of continents and along the equator (the imaginary circle around the Earth halfway between the North and South Poles).

Cannery Row, which was the subject of a famous novel by John Steinbeck (1902–1968), described the enormous sardine fishery off the California coast in the first half of the twentieth century. As a majority of the sardine diet was phytoplankton, this fishery depended directly on the strong upwelling in Monterey Bay.

Mesopelagic zone

The mesopelagic zone extends from about 500 feet (150 meters) to about 3,250 feet (1,000 meters) below the surface. It is often referred to as the "twilight zone" because it is between the epipelagic zone where there is light and the deep ocean where light is absent. The majority of light in this part of the ocean comes from bioluminescence, light that is generated by chemical reactions in bacteria, animals, and plants. Because the epipelagic zone is where phytoplankton grow and where zooplankton and fish feed on phytoplankton, some animals that live in the mesopelagic zone migrate upwards at night to feed. Other animals in the mesopelagic zone rely on food that falls through the thermocline into the mesopelagic zone. Still others have developed special adaptations to prey on the animals that live within the mesopelagic zone. Since food is rather scarce in this zone, predators often have sharp teeth and expandable stomachs to take advantage of anything they encounter, even if it is bigger than they are.

Bathypelagic zone

The bathypelagic zone is the part of the ocean between about 3,250 feet (1,000 meters) and 13,000 feet (4,000 meters) below the surface of the ocean. Light is almost non-existent in this part of the ocean. What little light there is, is generated from bioluminescence from animals and bacteria. The pressure is also extremely great in this part of the ocean. However, marine life still exists here. Fish, jellyfish, mollusks, and crustaceans (aquatic animals having no backbone with jointed legs and a hard shell) all have representatives that live at these extreme depths. Most of these animals are either black or red. Red appears black at these depths, because the only wavelengths of light (wave patterns of light that are perceived by the eye as colors) available are blue light from bioluminescence.

Abyssopelagic zone

The abyssopelagic zone extends from 13,000 feet (4,000 meters) below the surface to the seafloor. In this zone, the waters are nearly freezing and the pressures are immense. Nonetheless, there are some animals that live in this very deep part of the ocean. Squid and jellyfish can be found swimming through the waters in the deep ocean. Often, they have little color but they do have special organs that can produce light. This bioluminescence is used to attract prey and scare away predators. Several kinds of echinoderms (spiny-skinned animals including starfish and sea urchins) are relatively common in the abyssopelagic zone. The basket star has long arms that it waves above the seafloor in order to catch prey. The sea pig is a kind of sea cucumber that digs holes or tunnels in the mud, digesting dead animals and bacteria. Another sea cucumbercalled the flying cucumber has flaps like wings to fly through the deep ocean. Crustaceans can also be found in the abyssopelagic zone. Sea spiders and isopods (of the shrimp family) are often found in these deep regions.

Very deep trenches (narrow depressions or cracks in the sea floor) can extend down to 35,750 feet (11,000 meters) in some parts of the ocean. Trenches are the deepest parts of the ocean and are classified by some oceanographers as the hadal zone.

Juli Berwald, Ph.D.

For More Information

Books

Garrison, Tom. Oceanography. 3rd ed. Belmont, CA: Wadsworth, 1999.

Gross, Grant M. Oceanography: A View of the Earth. 5th ed. Englewood Cliffs, NJ: Prentice-Hall, 1990.

Websites

"The Deep Sea." OceanLink.http://oceanlink.island.net/oinfo/deepsea/deepsea.html (accessed on August 12, 2004).

Knight, J. D. "Layers of the Ocean." Sea and Sky.http://www.seasky.org/monsters/sea7a4.html (accessed on August 12, 2004).

"Ocean Water: Density." Office of Naval Research: Science & Technology Focus.http://www.onr.navy.mil/focus/ocean/water/density1.htm (accessed on August 12, 2004).