Layers of the ocean

Layers of the ocean

Physical properties, as well as the flora and fauna of the ocean change with depth.

Geography

Keywords

sea, hydrosphere, light absorption, consumer, producer, decomposer, plankton, alga, open sea, coast, deep sea, flora and fauna, salinity, ocean, seawater, water, nature, geography

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Scenes

Layers of the ocean

  • coastal zone - The upper layer of the ocean, to about 200 m depth, close to the coast.
  • open sea - The upper layer of the ocean, to about 200 m depth, far off the coast.
  • deep sea - The layer of the open sea deeper than 200 m.
  • 100 m
  • 200 m
  • 500 m
  • 1,000 m
  • 2,000 m
  • 3,000 m
  • 4,000 m
  • 5,000 m

Marine flora and fauna

  • 100 m
  • 200 m
  • 500 m
  • 1,000 m
  • 2,000 m
  • 3,000 m
  • 4,000 m
  • 5,000 m

Physical properties of sea water

  • 100 m
  • 200 m
  • 500 m
  • 1,000 m
  • 2,000 m
  • 3,000 m
  • 4,000 m
  • 5,000 m
  • 300
  • 400
  • 500
  • 600
  • 700
  • 800
  • nm
  • solar radiation
  • wavelength
  • ultra-violet - Light of about 100–380 nm wavelength, it can penetrate the ocean to a depth of 30 m.
  • violet - Light of about 380–420 nm wavelength, it can penetrate the ocean to a depth of approx. 120 m.
  • blue - Light of about 420–490 nm wavelength, it can penetrate the ocean to a maximum depth of 1000 m.
  • green - Light of about 490–575 nm wavelength, it can penetrate the ocean to a depth of approx. 150 m.
  • yellow - Light of about 575–585 nm wavelength, it can penetrate the ocean to a depth of approx. 50 m.
  • orange - Light of about 585–650 nm wavelength, it can penetrate the ocean to a depth of 25–30 m.
  • red - Light of about 650–760 nm wavelength, it can penetrate the ocean to a depth of 5–15 m.

Absorption of light

The surface of sea water is reached by the entire spectrum of light. Water absorbs light, but it absorbs its components of different wavelengths at a different rate. Infrared rays, which are invisible to the human eye, are absorbed at a depth of 3 m, while ultraviolet light can penetrate sea water to a depth of about 30 m.

Different wavelengths of visible light (wavelengths between 380 and 760 nm) correspond to different colours. Sea water absorbs red light best at a depth of 5-15 m. Orange is absorbed at a depth of 25-30 m, while yellow at about 50 m. Green and purple are absorbed less, light waves of these colours can get to a depth of 100 m. Blue is not absorbed well in sea water, thus it can penetrate to a depth of 200-300 m, but occasionally even to 1,000 m. This is why sea water appears blue.

Light is vital for organisms living both in the coastal zone and in the open sea. These organisms need the energy of light for photosynthesis. As light becomes more and more scarce with depth, photosynthesising plants gradually disappear, which also affects the fauna.

Temperature

The temperature and its fluctuations are more balanced in the oceans than on the continents. The seasonal changes of the temperature of surface waters can be observed in the temperate zones; the temperature of seas is relatively constant in the tropical and polar zones.

There is a distinct layer in the ocean in which temperature decreases more rapidly than it does in the layers above. This layer is called the thermocline.

In tropical seas, the thermocline is permanent and can be found at a depth of 100–500 m. In seas of the temperate zones, the thermocline changes seasonally. In polar seas, rapid changes of temperature are rare.

The temperature of the water below the thermocline is relatively stable, about 2-4 °C.

The range of marine organisms is also affected by the temperature. Species less tolerant of the changes of temperature inhabit tropical, polar seas or the abyssal zones of the oceans. On the other hand, species that inhabit seas of the temperate zone or the coastal zones are more tolerant of the changes of temperature.

Pressure

Hydrostatic pressure results from the weight of a liquid. Pressure is the same in every direction in a fluid at a given depth, it depends only on the density of the fluid and the height of the fluid column. Therefore, the deeper we go in the sea, the higher the pressure. Each 10 metres of depth adds another bar (100 kPa) to the pressure.

At a constant temperature, the volume of a gas is inversely proportional to the pressure applied to it. For instance, if we lower a ball into the ocean, it starts to sink and as it goes deeper down, the pressure increases and its volume decreases.

Organisms living in the ocean search for an optimal zone in terms of pressure. Due to the difference in pressure, many of these organisms developed the ability to adapt to the changing environment, especially marine mammals that breathe air, like seals, dolphins and whales.

Salinity

The average salinity of the oceans is 3.5%, that is, 1 litre of water contains 35 g of mineral salts, mainly sodium chloride.

The salinity of the oceans is more or less constant. The abyssal zone is the most uniformly saline, its salinity is 3.45-3.5%.

Surface salinity is also relatively constant, it varies between 3.4–3.5%. The most uniformly saline ocean is the Pacific Ocean. The Atlantic Ocean is more landlocked and its salinity is higher in the subtropical zone.

Besides climate, the change of seasons, the supply of fresh water and the geographical position all influence the salinity of the seas.

The salinity of warmer seas that have a small supply of fresh water from precipitation or river runoff, is higher. Thus, the more landlocked Mediterranean Sea, for example, has a high salinity.

The salinity of the oceans changes with depth: it can be divided into layers similarly to temperature. Temperature and salinity differences play a major role in generating ocean currents.

The salinity of sea water is crucial to organisms living in the ocean, and a sudden change in it would lead to a dramatic decrease in marine populations.

Animation

  • coastal zone - The upper layer of the ocean, to about 200 m depth, close to the coast.
  • open sea - The upper layer of the ocean, to about 200 m depth, far off the coast.
  • deep sea - The layer of the open sea deeper than 200 m.
  • plankton - Organisms that live and float in the ocean and cannot swim against a current.
  • 0 m = 25 °C
  • 100 m = 23 °C
  • 200 m = 20 °C
  • 500 m = 15 °C
  • 1,000 m = 5 °C
  • 2,000 m = 3 °C - Temperature is nearly constant from here.
  • 100 m 1,000 kPa = 10 bar
  • 500 m 5,000 kPa = 50 bar
  • 1,000 m 10,000 kPa = 100 bar
  • 5,000 m 50,000 kPa = 500 bar
  • 100 m
  • 200 m
  • 500 m
  • 1,000 m
  • 2,000 m
  • 3,000 m
  • 4,000 m
  • 5,000 m
  • 300
  • 400
  • 500
  • 600
  • 700
  • 800
  • nm
  • solar radiation
  • wavelength
  • ultra-violet - Light of about 100–380 nm wavelength, it can penetrate the ocean to a depth of 30 m.
  • violet - Light of about 380–420 nm wavelength, it can penetrate the ocean to a depth of approx. 120 m.
  • blue - Light of about 420–490 nm wavelength, it can penetrate the ocean to a maximum depth of 1000 m.
  • green - Light of about 490–575 nm wavelength, it can penetrate the ocean to a depth of approx. 150 m.
  • yellow - Light of about 575–585 nm wavelength, it can penetrate the ocean to a depth of approx. 50 m.
  • orange - Light of about 585–650 nm wavelength, it can penetrate the ocean to a depth of 25–30 m.
  • red - Light of about 650–760 nm wavelength, it can penetrate the ocean to a depth of 5–15 m.

Narration

The World Ocean covers 71% of the Earth's surface, thereby forming the largest habitat in the world. Since marine habitats can be divided into different zones, environmental conditions in the ocean change from zone to zone, both horizontally and vertically.

Marine habitats consist of three main zones: the coastal zone, the open sea and the deep sea.

The coastal zone is located above the continental shelf, not deeper than 200 m. Its flora and fauna are the richest in varieties of species. The coastal zone is eutrophic because rivers that empty into it are rich in nutrients, and the remains of dead organisms also flow back there. Photosynthesis is possible in the coastal zone due to the presence of light.

The open sea is the upper 200 m layer of the ocean, far from the shore. Light is present in this zone as well, so photosynthesis is possible. However, there are usually not enough nutrients here, because the remains of dead organisms sink to the bottom of the ocean. Thus, life is scarce in the open sea. Plankton prevails near coastal waters or upwellings, where there are enough nutrients. Plankton is a mass of organisms floating in the water, that cannot swim against the currents. Water becomes green in areas where there is an abundance of plankton.

The deep sea is the layer of the open sea below depths of 200 m. This zone has barely been explored. Since light does not penetrate the deep sea below 1,000 m, it is completely dark below this depth. Consequently, plants, the primary producers, are missing. The only creatures that survive here are animals and bacteria that feed on organic matter called marine snow, which falls from the upper layers.

Marine life is also influenced by environmental factors. Water absorbs light, but it absorbs components of different wavelengths at different depths. Blue light is not absorbed well in sea water, so it can penetrate to a great depth, and this is why sea water appears blue.

Light is vital for flora living both in the coastal waters and in the open sea, which naturally need the energy from light for photosynthesis. As light becomes more and more scarce with depth, photosynthesising plants gradually disappear, a development which also affects the fauna.

The temperature and its fluctuations are more balanced in the ocean than on land. The deeper we go, the colder the temperature grows. Water temperature at a depth of 2,000 m and below remains constant at about 2-4 °C.

The range of marine organisms is also affected by the temperature. Species less tolerant of the changes in temperature inhabit tropical and polar seas or the abyssal zones of the oceans. On the other hand, species that inhabit seas in the temperate zone or the coastal zones are more tolerant of these changes.

The deeper we go in the sea, the higher the pressure becomes. At a constant temperature, the volume of a gas is inversely proportional to the pressure applied to it. For instance, if we lower a ball into the ocean, as it sinks deeper, the pressure exerted on it increases and therefore its volume decreases.

Organisms living in the ocean search for an optimal zone in terms of pressure. Due to the difference in pressure, many of these organisms have developed the ability to adapt to the changing environment, especially marine mammals that breathe air, like seals, dolphins and whales.

The average salinity of the oceans is 3.5%, that is, 1 litre of water contains 35 g of mineral salts, mainly sodium chloride.

The most uniformly saline ocean is the Pacific. The Atlantic Ocean is more landlocked and its salinity is higher in the subtropical zone. Besides climate, the salinity of the seas is also influenced by the change of seasons, the supply of fresh water and the geographical position. The salinity of warmer seas with a small supply of fresh water from precipitation or river runoff is higher. Thus, the more landlocked Mediterranean Sea, for example, has a high salinity.

The salinity of sea water is crucial to organisms living in the ocean, and a sudden change in it would lead to a dramatic decrease in marine populations.

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