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Ocean Layers: Types – Density – Depth

by Sekar Puspa

Earth is a water planet. It is because water surface on earth reaches 71 percent. The rest totaling 29 percent consists of land in kind of continents and islands. Moreover, 96.5 percent of water surface is saline water, and 3.5 percent is freshwater in lakes and rivers as well as frozen water in glaciers and the polar ice caps. Of 3.5 percent freshwater, 69 percent takes the form of ice. If melting, the altitude of earth’s sea levels would rise to 2.7 km. Freshwater exists beneath earth’s surface as well. The volume of groundwater amounts to 1,386 million cubic kilometers (km3) in volume. The water is also in earth’s atmosphere for evaporation and transpiration. Therefore, water has very meaningful in the planet, particularly saline water in ocean.

Why ocean water is salty? At least there are three processes of saline water in ocean. Firstly, as freshwater from rain falls on soil and rock, it dissolves some of the dissolved salt called weathering. When we drink we cannot taste the salt for its low concentration of the salt. Secondly, evaporation and river inflow for thousands years collect salt in ocean water. Thirdly, it occurs due to hydro-thermal vents when minerals from earth’s crust on the ocean floor dissolve into sea water. These make the salt content of ocean water is relatively stable.

The differences between Ocean and Sea

Moreover, we have to differentiate between ocean and sea. An ocean is wider than a sea partially enclosed by land. The location that meet an ocean and land called as sea. For example, it is Java Sea. However, there is some debate on the extension of  Java Sea whether it is part of the western Pacific Ocean or Indian Ocean. In Encyclopedia Britannica, it is called the extension of the western Pacific Ocean. Meanwhile, there are some experts call it as the extension of Indian Ocean.


ocean layers, oceansOceanographers divide ocean into layers like a cake. It has significant meaning because ocean has different water density. It means that water weight per unit volume has difference of spaces. Its between water molecules and the water density is different from one layer to another in ocean. Then, It increases along with increasing depth of the ocean. The density of ocean water gets influence from its temperature. The relationship between density and temperature is relatively inverse.Then, it also occurs the inverse. The water density has its maximum,vat the temperature of 4 degree Celsius for pure water.

On the other hand, the water density of ocean has close relationship with salinity as well. Ocean water is more dense than pure water for its salt in it. It is why the amount of salt in water is called salinity. If pure water density reaches 1000 kg/m3, ocean water density at the surface is about 1027 kg/m3. Firstly, the density and salinity will increase when the ocean depth increases. It also occurs for evaporation. Secondly, the density and salinity will decrease if freshwater increases from rivers and lakes as well as the melting of polar ice.

Which one more affects the water density? Temperature or salinity? We can say temperature has more effect to the water density. In the bottom of ocean the water temperature definitely decreases for low sunlight, and the water density increases. As a result, why oceanographers divides ocean into some layers.

There are five ocean layers usually called zones. The name of the ocean zones are defined from the ocean surface. The first zone is epipelagic zone, the second is mesopelagic zone, the third is bathypelagic zone, the fourth is abyssopelagic zone and the last one is hadalpelagic zone. With the water depth, it also gets influence from pressure. The deeper you swim into ocean, the greater water pressure pushes down on you. Therefore, we should discuss one by one the ocean layers for better understanding about ocean nature.

Epipelagic Zone

From the above discussion, water temperature and salinity have great effect to ocean water. There are two important concepts in relation to temperature and density. There are Thermocline and Pycnocline.

In the thermocline, temperature changes drastically between Epipelagic zone and the lower zone called Mesopelagic zone). In the range of under 3,300 feet (1,005.84 meters) to 13,100 feet ( 3992.88 meters) in depth, water temperature is relatively constant. Its called Thermocline zone. When the depth of under 13,100 feet (under 3992,88 meter), the water temperature is in the level of near freezing and above the freezing point of water. Moreover, the freezing point of saline water is at -21.1 degrees Celsius and salinity rate reaches 23.3 percent (by weight). Therefore, Thermocline relates to temperature. However, the Thermocline zone does not occur in polar water because there is no permanent Thermocline.

Moreover, Pycnocline also determines the ocean zones. It is boundary and it separates two ocean zones with different density. There are surface water and deep water.  Pycnocline is important because it prevents vertical currents from deep water to surface water. Temperature or salinity influence the formation of Pycnocline. Then, in deep water, temperature or salinity do not change drastically and on the contrary in surface water these occurs per season. Due to the prevention of vertical current from deep water, Pycnocline likes a boundary, and it functions as barrier for surface processes. As a result, Pycnocline relates to Halocline or salinity gradients and Thermocline or temperature gradients.

Sunlight effect

Sunlight has great influence in the Epipelagic zone. It is why oceanographers call it Epipelagic zone, sunlight zone or Euphotic zone. Average depth is around 660 feet (200 meters). Temperature degree reaches 104-27 degree Fahrenheit (from 40 degree to -2.8 degree Celcius). It can occur photosynthesis in the zone. The photosynthesis influences total production of oxygen in the earth. Therefore, food chain occurs in the zone, and sea plants to carnivora and omnivore animals live here.

Phytoplankton does not spread on all oceans. It lives near land because it has high nutrients. Phytoplankton bloom lives in area with full of nutrients. Bloom attracts small animals. The higher Phytoplankton bloom lives the more many small animals feed on.

The combination of wave, wind stress on ocean surface, temperature, salinity and density makes Epipelagic zone a mixed-layer. In the mixed layer, the density from the depth of 25 meters to 200 meters is similar. If the density is unstable, the denser surface water will sink to more than 1000 meters. Moreover, physical, chemical and biological processes occur in the mixed layer, such as ocean circulation, carbohydrate dissolution and Phytoplankton production.

Various Fishes

Human can do fishing activities because food chain occurs in the Epipelagic zone. The fish includes coastal fish. There are various fishes here from demersal fish, forage fish to apex predator oceanic fishes, such as blue-fin tuna and oceanic sharks. However, there are some fishes living below the Epipelagic zone. The number is relatively small and it does not include common fishing activities.

Meanwhile, fishermen and scuba divers should understand the surface mixed-layer. It also becomes attention among oceanographers, marine biologists and climatologists.


[tr][td]Depth [/td] [td]3,300 feet (1,003.84Meters) – 13,100 Feets (3992.88 Meters)[/td][/tr]

[tr][td]Salinity Rate[/td] [td]Relatively Constant[/td][/tr]

[tr][td]Temperature[/td] [td]40°C – (-)2.8°C[/td][/tr]

[tr][td]Water Temperature[/td] [td]23.3%[/td][/tr]

[tr][td]Zone[/td] [td]Vertical Pelagic[/td][/tr]


Mesopelagic Zone

Mesopelagic zone differs from Epipelagic zone. Mesopelagic zone is also called twilight zone. The light is dim. There is no food chain here because no plants grow. If there is a food chain, animals in the zone eat food falling from the above zone, such as decayed matter and algae from surface.The depth of Mesopelagic zone extends from 660 feet (200 meters) to 3,300 feet (1,000 meters) below the surface ocean. The rare light influences the life in the zone. Animal eyes have larger size and upward oriented for finding food in a kind of silhouettes of other animals.


Sunlight influences temperature in the zone as well. Thermocline occurs in the zone. As the water temperature decrease, saliny and density increases. The temperature is also relatively stable, and it differs from the mixed layer. The temperature at 660 feet (200 meters) differs from that at 3,300 feet (1,000 meters) reaching 4-8 degree Celcius. It influences animals which live in the Mesopelagic zone. Moreover, water pressure in the zone is higher than in epipelagic zone, i.e. from 300 pounds per square inch (psi) to 1500 psi.  Some animals do migration called diel migration from Epipelagic zone to Mesopelagic zone in daytime and it returns to Epipelagic zone in night. It also occurs for hunting prey, such whales and large fish. On the other hand, some animals from Mesopelagic zone hunt prey in the Epipelagic zone.

On the other hand, the denser surface water will sink to more than 1000 meters. Then, with the surface water which sink, it takes oxygen to dissolved into Mesopelagic zone. So,it will be useful for the life of animals in the zone. However, if there is no sunlight reaching the zone, there is no plant and photosynthesis. Meanwhile, number of animals living in the zone is rare. The characteristics of animals in the zone include large eyes, large teeth and jaws, small and thin size and dark color. Moreover, there is a small fish which has organ in its body to produce light around its body called bio-luminescent animal. It has function for helping them find food, mate and confuse predators. Sperm whales and squid do migration from Mesopelagic zone to the below one called Bathypelagic zone.


[tr][td]Depth [/td] [td]660 feet (200 meters) – 3,300 feet (1,000 meters) [/td][/tr]

[tr][td]Salinity Rate[/td] [td]33%[/td][/tr]

[tr][td]Temperature/td] [td]4°C – 8°C[/td][/tr]

[tr][td]Water Temperature[/td] [td]Relatively Constant[/td][/tr]

[tr][td]Zone[/td] [td]Vertical Pelagic[/td][/tr]


Bathypelagic Zone

Due to no light, Bathypelagic zone is dark and it has high pressure. It influences animals living here. This zone extends from 3281 feet or 1000 meters to 13,124 feet or 4000 meters. In Honduras the depth is only about 550 meters and it is near to land. The depth of 4000 meters reaches the ocean floor. If we find light here, it comes from animals with light organ in their bodies. The pressure in the zone is higher than two zones above it, and it reaches 5,850 pounds per square inch. The water temperature reaches 35.6 – 40 degree Fahrenheit  or 2-5 degree Celsius.


Moreover, the downward movement from two above zones make circulation chemistry and respiration in bathypelagic zone. The movement is due to animal migration, gravity with denser saline water from surface and dissolved organic carbon. Finally, icy water in Bathypelagic zone comes from the poles. It moves to the sea floor slowly. The downward movement of icy water takes temperature, salinity and dissolved oxygen to the zone.

Like in Monopelagic zone, photosynthesis does not occurs here for no plant and primary productivity. Small animals get feed by consuming others as food sources. Small animals in Bathypelagic zone are similar to in Mesopelagic zone. It functions as growth, reproduction and maintenance. On the other hand, for adaptation, small animals have high water in body. These also have weak and flabby muscles. As a result, the bone is relatively less dense and they move slowly. Dominant colors of the animals are black and red. It can be seen from crustaceans with red color in body. Olfactory hairs and antennae are also seen in crustaceans’ body.


General characteristics of small animals live in Bathypelagic are almost similar to those in Mesopelagic zone. Huge mouths, sharp teen and expandable stomachs are their adaptation to the environment. Some have no eyes and it does not need in the dark zone. They have strong gills as well. It functions for oxygen extraction from water. in addition, they adapt to darkness, cold temperature, low biomass and intense pressure. Activities in pole area affect the life of small animals in Bathypelagic zone.

There is the deep scattering layer or the sound scattering layer between Mesopelagic zone and Bathypelagic zone. It has the depth of 900-1200 feet or 270-360 meters. In the layer fish, squid and crustaceans do migration to above zone. The vertical migration is carried out for feeding in shawl-lower water. It occurs at night. Biologists call it the deep scattering layer because it is formed by the migrated small animals up in evening and down in the morning. Then, the migration to upper surface in evening and down to their origin zone in the morning. As well as its carried out for feeding in surface waters at night and avoiding their predators. It occurs all oceans through the world. The migration has significant impact to the global carbon cycle.


[tr][td]Depth [/td] [td]900 feet (270 meters) – 1,200 feet (360 meters) [/td][/tr]

[tr][td]Salinity Rate[/td] [td]3.47%[/td][/tr]

[tr][td]Temperature/td] [td]4°C[/td][/tr]

[tr][td]Water Temperature[/td] [td]2°C-5°C[/td][/tr]

[tr][td]Zone[/td] [td]Vertical Pelagic[/td][/tr]


Abyssopelagic Zone

Abyssopelagic zone is also called abyssal zone or abyss. Its from the Greek word means “bottomless sea”. The difference between Bathypelagic zone and Abyssopelagic zone relates to water temperature of 39.2 – 35 degree Fahrenheit or 4 – 2 degree Celsius. As well as the darkness and similar environmental condition. It indicates the temperature near freezing point. Then, the depth of 13,124 feet or 4000 meters to below 19,700 feet or 6,000 meters. It reaches ocean basin. The pressure reaches 11,000 pounds per square inch. It is high pressure, and there is almost no small animals which live here.

The depth of Abyssopelagic zone reaches 13123,36 feet or 4000 meters. That’s why, the depth makes Abyssopelagic zone as the widest living environmental in the water planet. Moreover, the width reaches over 300,000,000 square kilometers or about 3/4 of total ocean area and 60 percent of earth’s surface.

If small animals live here, they do make adaptation, such as dragging jaws. It will make mouth open. Then, it can feed mussels, shrimps and microscopic organism without seeing. Their eyes is not functionable at all for darkness in the zone. Moreover, the small animals have organ producing light or called bio-luminescent animals, such as squid and fish. The squid is a kind of deep-water squid. While, body is transparent.

Meanwhile, Photophores are used to lure prey and deter predators. The others is octopus, basket star, sea-pig and sea-spider. Then, it indicates the most small animal which live here are invertebrates, such as jellyfish, mollusks, and crustaceans. However, there are some exception, such as giant squid and angler fish. On the other hand, fish living in the deepest water of 27,460 feet (8,372 meters) is found in Puerto Rico Trench.


[tr][td]Depth [/td] [td]13,124 feet (4000 meters) – 19,700 feet (6,000 meters). [/td][/tr]

[tr][td]Salinity Rate[/td] [td]34.6%[/td][/tr]

[tr][td]Temperature/td] [td]4°C[/td][/tr]

[tr][td]Water Temperature[/td] [td]39.2°C-35°C[/td][/tr]

[tr][td]Zone[/td] [td]Vertical Pelagic[/td][/tr]


Hadalpelagic Zone

Hadalpelagic zone is located below Abyssopelagic zone extending from 19,686 feet o 6000 meters to the deepest ocean, such as deep trenches and canyons. The deepest ocean point is found in Mariana Trench off the coast of Japan. The depth reaches 35,797 feet or 10,911 meters. The water temperature is above freezing and the pressure is eight tons per square inch. Some invertebrate creatures live here, such as starfish and tube worms, tiny isopods.

However, there is a deep-sea sediment community. They eat from falling parts of decayed matter and algae called marine detritus as the source of energy and nutrients. Moreover, another deep-sea sediment community feeds on bacteria around openings called hydro-thermal vents. It is like symbiotic life between host and the bacteria. Then, the host and bacteria use mineral-rich emission as their food source.

About 80 percent of all ocean waters are in deep zone from Bathypelagic, Abyssopelagic and Hadalistic zone. Density is relatively stable throughout these zones particularly from Abyssopelagic and Hadalpelagic zone. Factors that influence water density are temperature, salinity and pressure. As water temperature decreases, water density, salinity and pressure increase.


[tr][td]Depth [/td] [td]19,700 feet (6,000 meters) to deepest ocean. [/td][/tr]

[tr][td]Salinity Rate[/td] [td]36%[/td][/tr]

[tr][td]Temperature/td] [td]Freezing point[/td][/tr]

[tr][td]Water Temperature[/td] [td]10°C[/td][/tr]

[tr][td]Zone[/td] [td]Horizontal Pelagic[/td][/tr]


The zones of oceans have influence from temperature, density, salinity to pressure. There are two significant concepts for the difference of these zones. Then, Sunlight and heat influence the life of ocean animals in the zones. The most various ocean animals are in Epipelagic zone. The food chain occurs here. Human gets benefit from the food chain, like fishing activities. However, the zones below epipelagic zone get influence from the scarcity of light. However, the understanding on the ocean layers is relatively few. Many experts does not yet explore the zone in maximum particularly in the deeper water.

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