SEA WATER

The chemical-physical characteristics of seawater, such as salinity, density, temperature, pressure, colour, and transparency, influence some phenomena that occur in it and also the life of the organisms that live there

Salinity indicates the quantity of salts (present in ionic form) dissolved in marine waters and coming from the constant supply of saline substances by the rivers that flow on the land and also by the submarine volcanoes. Salinity is expressed in grams of salts dissolved in 1 kg of water and has an average value of 35% (per thousand): that is, if 1 kg of seawater is evaporated, 35 g of salts remain. 

Among the numerous salts dissolved in sea waters, the most abundant is sodium chloride, the common table salt; followed by the magnesium, calcium and potassium salts.

The pressure exerted by the water, called hydrostatic pressure, increases with increasing depth, with an increase of about 1 atmosphere for every 10 m. Very high pressures are recorded on the ocean floor; however, the animals that populate the abyss are not "crushed" by it, since they compensate for the high pressure with an equal pressure exerted by their internal liquids.

The colour of seawater, mainly light blue - blue, with variations to green and purple, is due to the diffusion of light produced by the water molecules themselves, which are so small that they absorb radiation with wavelength longer (such as red) and let those with shorter wavelengths (such as blue) pass. Colour variations can also be determined by the presence of organic and inorganic particles suspended in the water.

The transparency of the water, that is the ability to be penetrated by light, is almost total in the first 50 m of depth (photic zone), while it gradually decreases up to 200 m (aphotic zone), beyond which all radiations are absorbed.

Like any liquid, the free surface of the seas in contact with the atmosphere tends to be perpendicular to the force of gravity at every point, practically coinciding with that of the geoid, a term used to indicate the shape of the Earth. The level of this surface, defined as mean sea level, constitutes the reference for the altimetric measurement of the land and also of the sea depths; it is assumed as a result of the average of measurements normally carried out over a period of twenty years using fixed instruments, called hydrometers, placed in some points of a coast.

Sea currents are constant water movements, comparable to large rivers that flow through the oceans at speeds between 2 and 10 km / h and are distinguished from the surrounding waters by temperature and salinity.

The currents can have different origins. They can be due to the combined action of winds and atmospheric pressure differences or be triggered by the tides or even depend on the differences in the density of seawater, caused, for example, by the different heating of the various parts of the oceans and different salinity values.

Sea currents play a very important role in the transfer of heat from tropical regions to polar regions, exerting a mitigating function of the climate: in fact, by transporting hot water from low to high latitudes, they raise the temperature of the atmosphere, to which they yield part of their warmth.

Waves are irregular movements of the sea surface. They do not produce horizontal displacements of water, but only an oscillation, along a circular or elliptical orbit, of the water particles. They can originate due to the force of the wind on the sea surface (but also as a result of tides, seismic movements, submarine landslides or violent atmospheric perturbations) and can also propagate thousands of kilometres away from the place of origin (windless waves).

When the wind blows on the sea, it exerts pressure on the exposed surface proportional to the intensity with which it blows. Since the liquid is incompressible, it depresses, giving rise to a hollow (or belly) wave. The particles formerly contained in the cable move upwind and give rise to a raised ridge. In persistence of the wind, cables and crests follow each other parallel, creating a train of waves (forced waves).

In the Mediterranean Sea, the maximum amplitude of the tides is on average 45 cm, unlike in the countries in northern Europe where it can exceed 10 meters. The tidal fluctuations are of a mixed semi-daily type, with two maximums and two minimums during the day. They follow each other with different values during the month with minor and major tides. In Italy, the tides have an average periodicity of 6 hours between low and high tide and the excursions are quite limited, ranging on average between 30-70 cm

Marine waters also contain numerous dissolved gases, the same ones that form the atmosphere: among them, of fundamental importance for the respiration of living organisms is oxygen.

Marine waters also contain numerous dissolved gases, the same ones that form the atmosphere: among them, of fundamental importance for the respiration of living organisms is oxygen.

The temperature varies according to latitude, season and depth. The maximum temperature is recorded on the surface, in the equatorial zones, and decreases about 1° C for each degree of increase in latitude. The temperature also decreases with depth, since infrared radiations, those that heat the waters, penetrate only into surface waters (not deeper than 10 m); between 200 and 1000 m, there is a sharp decrease in temperature: this layer is given the name of thermocline; the temperature then stabilizes in depth on values ​​close to 0 ° C.