Seawater

The main characteristics of seawater are:

  • salinity: it refers to the total salt content in 1000 grams of seawater and it has a value of around 35 grams. The percentage of the different substances that are present in the solution depends on the river contribution, on chemical reactions that occur in sea sediments, on volcanic activity and on the decomposition of organisms. In fact, the quantity of salt is stable only at a certain depth, while on the surface and coastal areas it is also subject to seasonal variations;
  • melted gases: oxygen and carbon dioxide are necessary for the life of water organisms. Oxygen is largely present on the surface, since water is in contact with the atmosphere and where photosynthetic organisms live, and deep underwater, where water temperature is very low.  Carbon dioxide is a very soluble gas that easily spreads from the atmosphere into sea water, transported by river water to the sea, and which derives from decomposing organic materials;
  • the temperature: as well as mitigating the climate of coastal regions, temperature influences the chemical and physical characteristics that are responsible for the vertical movement of water masses. On the most superficial layer (50-200 meters) the temperature is similar to the superficial one; on the thermocline layer (200-1000 meters) the temperature rapidly diminishes; on the deep layers it keeps on diminishing, but very slowly. The thermocline is an important surface as far as the spreading of organisms in the oceans is concerned. It represents an obstacle for many animals, plants, and tropical algae that need a temperature of 15 – 20°C;
  • brightness: it depends on the ability of the light to penetrate in the water and light up only the superficial part even though the water is clear. This area is called photic zone (0-200 meters of depth), and it is where most of marine life and phytoplancton are concentrated.

The wind and the waves
Wave motion is caused by the wind and its action on the sea surface. In the open sea a floating object is lifted and lowered when a wave passes, but it does not move laterally since during the wave motion only the shape of the wave is transmitted. Therefore the water keeps still: individual water particles move according in a circular direction without moving from their original position. The wave motion does not spread deep underwater; on the contrary a submarine at a certain depth can quietly move even if there is a storm on the surface.
The Beaufort Scale describes the wind force according to 12 levels: level 0 refers to calm conditions, when the sea has no waves at all; level 6 refers to a fresh wind that forms big waves with white foam crests; level 12 indicates the presence of a hurricane with the air full of foam and splashes, and the sea is totally white.
Near the coast the waves break because the water depth diminishes and the particles do not manage to keep their circular movement.
Often coasts are not straight and the waves break first on promontories and then on bays. This provokes water movements parallel to the coast that originate currents called coastal drift currents.  If the seabed has sand mounds submerged under the water, suck currents might form, that drag off-shore also the most export swimmers.
Tides
Tides are periodical lowering and lifting of water. They are due to the gravitational attraction of the Moon and the Sun. In the Mediterranean sea the sea range varies from 20 to 50 centimetres, but there can be different ranges according to the seabed morphology.
The tide can create some particular phenomena. In some estuaries in the North Sea and in the English Channel the water manages to go upriver against the current, with repercussions on the river navigability. Instead, when the wind persistently and strongly blows towards the coast, it can make the sea level lift, even more than tides: “the high water” in Venice, “storm surges” in the North Sea that provoke damages on Dutch and German coasts, “raz de marée” on French coasts. In particular, the “high water” in Venice is due to various factors: sirocco wind that raises the water by up to 90 cm, tides that can have a maximum height of 60 cm, and seiches, the presence of low pressure and seasonal variations that can reach 20 cm.

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