During the slow evolution of our planet, plate tectonics has continuously changed the positions of continents and oceans. At the time of the Pangea, there was only one large ocean surrounding this one continent. Today, according to the classification of the International Hydrological Organization (IHO), there are three oceans. The Pacific Ocean is the largest, its surface area is about half that of the oceans as a whole, and it alone covers one-third of the Earth’s surface. because of its predominance on the surface that the median meridian of this ocean was chosen as the date change line. The Atlantic Ocean is the second largest by area, accounting for about 30% of the total. It is much better supplied with fresh water than other oceans, since it receives flows from large rivers such as the Amazon, Congo and St. Lawrence. The Indian Ocean, the third largest by area, accounts for about 20% of the total. It is almost entirely located in the southern hemisphere, between Asia, Africa and Australia.
Despite this official classification, we mention 5 oceans on our planet with the Antarctic Ocean to the south, which surrounds the Antarctic continent up to about 60 degrees and whose area represents about 6% of the total, and the Arctic Ocean to the north, which is bordered by the lands of Siberia, Scandinavia, Greenland and North America, whose area represents about 4% of the total. The seas are the marine sub-domains, of relatively small sizes- Mediterranean Sea, North Sea Baltic Sea Caribbean Sea and the English Channel.
Density, salinity and temperature of the marine environment
Under conditions of pressure of 1013 hPa, or hectoPascal, temperature of 3.98 °C, zero salinity, the density of the water is exactly 103 kg/m3. The density of seawater varies mainly with temperature and salinity, much less with pressure, which often leads to considering this fluid as incompressible.
The salinity, or mass fraction of salt is expressed in g/kg (gram of salt per kilogram of sea water). In lakes and rivers, salinity is almost zero, rarely exceeding a few units. It can reach and sometimes exceed 50 g/kg in the seas, its average value is around 35 g/kg. It is 12 g/kg in the Black Sea. In the Dead Sea, its very high value, close to 275 g/kg, practically prohibits any animal or plant life.
In summer, the surface water of the warmest seas can reach temperatures of 26 to 30°C, which often leads to cyclones. In the upper layers of the marine environment, surface water, heated by solar radiation, is subjected to constant thermal exchanges by conduction and convection with the atmosphere. Agitation by waves and turbulence then manages to homogenize the temperature in the first tens of meters (between 0 and -50 m). On the contrary, at great depths (below -120 m), exchanges are almost limited to pure conduction and become much weaker yielding a resting marine environment.
Between these two areas, a relatively thin layer (between -50 and -120 m) called the thermocline, where the temperature can vary by about ten degrees between the water above and the water below. The temperature of the water above the thermocline experiences significant seasonal variations, due to variations in sunlight, without any change in the temperature of the deep layers.
There is a vertical movement of water which plays a key role in the deep life of the marine environment by regenerating oxygen by supplying surface water and also bring nutrients from the seabed to the surface. In the Mediterranean, this phenomenon of deep convection is mainly located in the Gulf of Lions, making it a major center of biological activity. However, this convection only occurs when the conditions are adequate.
The Black Sea lacks these which this prevents the penetration of oxygen beyond a depth of 200 m. Only very specific species can live in this marine environment under these anoxic conditions.
The
amount of sunlight that reaches the water in ocean depend mainly on two factors: distance from shore and depth of
water. Oceans are divided into zones based on these two factors. The ocean
floor makes up another zone (benthos) .
Horizontal Divisions- Zones Based on Distance from Shore
The ocean is divided horizontally by distance from the shore. There are three main Horizontal Divisions -the intertidal zone, neritic zone, and oceanic zone.
- Nearest to the shore lies the intertidal zone (also called the littoral zone), the region between the high and low tidal marks. The important feature of the intertidal is change: water is in constant motion in the form of waves, tides, and currents. The land is sometimes under water and sometimes exposed.
- The neritic zone is from low tide mark and slopes gradually downward to the edge of the seaward side of the continental shelf. Some sunlight penetrates to the seabed here.
- The oceanic zone is the entire rest of the ocean from the bottom edge of the neritic zone, where sunlight does not reach the bottom. The sea bed and water column are subdivided further
Vertical Divisions- Zones Based on Depth of Water
The vertical extent of ocean water is the water column Two main zones based on depth of water and based on light penetration, vertically are the photic zone and aphotic zone.
Sunlight
only penetrates the sea surface to a depth of about 200 m, creating the photic
zone ("photic" means light). Organisms that photosynthesize
depend on sunlight for food and so are restricted to the photic zone. Tiny
photosynthetic organisms, known as phytoplankton, supply nearly all of
the energy and
nutrients to the rest of the marine food web and they occupy photic
zone. In the aphotic zone there is not enough light for photosynthesis.
The aphotic zone makes up the majority of the ocean, but has a relatively small
amount of its life, both in diversity of type and in numbers. The aphotic zone
is subdivided based on depth.
Photic zone is further divided into epipelagic, mesopelagic and bathypelagic zones, while, aphotic zone consists of abyssal pelagic and hadal zones.
Composition of seawater
The chemical composition of seawater is quite complex. Most of the chemical elements are found in solution in the form of a complex mixture of anions, cations and molecules.
Some ions come from the dissolution of continental rocks by rivers that carry them to the oceans, where they stay for very long periods of time and where evaporation of water increases their concentration. A significant part of the cations comes from the original ocean floor. And the origin of the chloride ion is often attributed to the degassing of hydrogen chloride from volcanoes, which is soluble in water
In addition to water and salts, there are also various low-concentration molecules, such as boric acid (0.0198 g/kg) and carbon dioxide (0.0004 g/kg), as well as nitrogen and oxygen. The amount of carbon dioxide in seawater is much greater than in the air- the possibility of sequestering carbon dioxide in the oceans, with a view to reducing the content of this greenhouse gas in the atmosphere is a much discussed topic now.
The amazing biodiversity of the marine environment
On Earth, formed 4.6 billion years ago, life appeared in the oceans about 3.8 billion years ago. And it was only very recently, about 400 million years before our era, that it conquered the land. As a result, extremely diverse lifestyles have developed in the oceans, where light only penetrates the upper layers, gradually adapting to the specific conditions of this marine environment.
Only a few species have managed to pass through and differentiate themselves on land, which means that most of the biodiversity on our planet is found in the marine environment.
The most common estimates of the number of living species in the marine environment range from 5 to 10 million, but most of these species, especially those living in deep water, are still unknown. By way of comparison, the number of species living on land is about 1.3 million, including 850,000 varieties of insects
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