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The ocean

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The blue planet - Characteristics of water - Ocean currents - Zoogeographic regions - Vertical zones

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The blue planet


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

We call earth "the blue planet". 71 percent of Earth's surface is covered by oceans and freshwater features. Water is the third most common molecule in the Universe (after H2 and CO), the most abundant substance on earth and the only naturally occurring inorganic liquid. Only on earth and no other planet in the solar system water can exist in a liquid state.

Every life form we know has evolved from water. Invertebrates, fish, amphibians, reptiles, birds and mammals - all started their evolution in the ocean. The first fishes in fossil record date back about 500 million years. The first coral reefs only appeared more than 400 million years ago. These early corals are now extinct, the modern forms evolved not until the past 25 million years.

It is not surprising, that the bodies of most plants and animals are 60 or more percent water!

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Characteristics of water

Unusual properties  

All life forms we know require water (H2O) for their biological processes. And although water seems chemically simple - just 2 hydrogen atoms bound to an oxygen - it is in fact an enormously unusual substance. It has often been stated that life depends on just these anomalous properties of water.

Thermal parameter   Water has different thermal parameter than other substances. Most compounds contract as they cool and expand when they heat. Water does this, too, until just before it starts to freeze. Then it does something remarkable - it begins to expand, even when it grows colder. By the time it does freeze it becomes ice which is actually less dense than liquid water. Ice floats on water instead of sinking. If it did not, life on earth would be impossible. If lakes and oceans froze from the bottom up instead the top down no sea-floor or lake bottom ecologies would exist. Once starting freezing, bodies of water would stay frozen solid forever. The freezing from the top down is insulating the water from further freezing and also allowing rapid thawing.
Surface tension   Water has a very high surface tension. It can enter into cracks in rocks and since it expands when freezing these rocks break apart. If water had a lower surface tension, the process by which soil is formed would not occur.
Viscosity   Water is not very viscous, if it had higher viscosity, circulatory systems could not have evolved.
Compressibility   The compressibility of water reduces the sea level by about 40 m giving us 5% more land.


salinity of water and adaption of animals

Salinity in parts per thousand


Water can dissolve more substances than any other solvent. Water is an excellent solvent for substances such as salt. Actually 38 million tons of salt are solved in one cubic kilometer, that's about 25 grams of salt in one kilogram of water.

Water can retain and store large amounts of heat - this large heat capacity of the oceans and seas allows them to act as heat reservoirs such that sea temperatures vary only a third as much as land temperatures. This minimizes extremes in temperature and stabilizes our climate. That is why coasts experience a milder climate than areas that lie more inland. The heat stored in water is also transported to other places around the globe through currents and will warm up colder water.

The salinity of water determines where marine animals can live. Some tolerate only moderate amounts of salt, others more. If the salinity fluctuates much, special ways of regulating their body fluid concentration develop. The starfish needs at least 1.5 percent salinity, some shrimps will tolerate large fluctuations, other animals such as the snapper can life only in open ocean.

Water also dissolves gases such as oxygen. Many microscopic plants and animals depend on this. However the oxygen content of water decreases with rising temperatures so tropical oceans are poorer in oxygen. But here in the tropics where a wide belt of warm water is stretching around the planet we find a complex habitat - the coral reef.

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Ocean currents

Surface currents   Surface currents are driven by the winds. However because of the so called Corolis effect (caused by the rotation of the earth) all winds around the equator blow slightly west. The currents form huge circular movements in the ocean basins. As seen on the map below the Atlantic, Pacific and Indian Ocean each have two such circles, one in the north and one in the south circulating anti-clockwise.

Map of the ocean currents

Map: warm and cold currents on earth

Red = warm currents / Black = cold currents / Orange = water of 20°C or more

Cold and warm currents   Warm and cold water is being transported by the ocean currents around the globe. This movement is called the "Great Ocean Conveyer Belt". Cold water originating from the poles travels mostly along the western side of the continents toward the equator. Heated up warm water flows towards the poles mostly on the eastern side of the continents.
Important ocean currents   South and North Equatorial Currents, Equatorial Countercurrent, Golf current (Atlantic), Canary Current (Europe) Kuroshio (Japan, China), California Current (U.S.A.), South Indian Current (Indian Ocean), Benguela Current (Africa), Antarctic Circumpolar Current.

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Zoogeographic regions


There are several distinct zoogeographic regions, each with specific species living there, but there are also animals living in all warm waters (= circumtropical), such as many oceanic sharks and mackerel.

Map of the tropical zoogeographic reef regions

Map of the zoogeoographic regions of reefs on earth

--- = 20° isotherm / Pink = Red Sea / Dark red = Indian Ocean / Red = Western Pacific / Yellow = Great Barrier Reef / Orange = Central Pacific / Green = Eastern Pacific / Light green = Caribbean  / Yellow-green = Bermudas / Violet = Brazil / Dark green = Eastern Atlantic

distribution of coral reefs around the world

The distribution in the 80 countries and geographical regions where coral reefs are found. The largest concentration of reefs is around Indonesia and Australia. From the World Atlas of Coral Reefs (UNEP)

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Vertical zones

Pelagic: inhabiting the open water, especially the upper layers

Benthic: living on or in the sea floor

Demersal: living near the sea floor


The ocean actually is quite barren with only a small fringe of coastal zones teeming with life. Food is sparse in the open ocean and animals are very dependent on ocean currents which bring food (colder currents) and provide passage.

Some of the most fearsome hunters such as the tiger shark or the mako live in the open water. Many fish try to avoid these predators by waiting during the day in the twilight zone and migrating during the night to the richer waters of the surface (photic zone). The top 30m of the ocean will teem with plankton during the night but at dawn these animals will travel down 200 to 2000m back to the twilight zone. The extent of vertical migration varies between species.

An example is the nautilus (Nautilus pompilius), a cephalopod with a chambered shell. They live in deep water but make a daily vertical migration of several hundred meter. Their shell not only protects the nautilus but also serves to regulate its flotation.

    The deep sea is by far the largest habitat for life on earth but it is quite unknown. At 1000m below the surface of the ocean the pressure increases to 100 times that of the surface. Only creatures that can cope with pressure can live here. Most have excellent vision to find their prey in the gloomy depths. Most are darkly colored, colors can't be seen here so they are not important. Some produce their own light by symbiotic bacteria that produce a bioluminescent glow, for example the frogfish (or anglerfish).

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The blue planet - Characteristics of water - Ocean currents - Zoogeographic regions - Vertical zones

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. Copyright Teresa Zubi