Marine Waters seawater

Oceans and Seas

Water is present on Earth in three physical states: liquid (in the seas, oceans, lakes, rivers, etc.), solid (in glaciers) and gaseous (water vapor in the atmosphere). All the waters present on Earth constitute a sort of envelope which is given the name of hydrosphere: most of the hydrosphere consists of oceans and seas, which together occupy over 70% of the earth's surface. More precisely, the term ocean indicates a large expanse of water that surrounds and separates the emerging masses of the continents, while the term sea is reserved for more limited extensions of water masses, generally surrounded by emerging lands or in any case adjacent to them. (in some cases the term sea indicates closed basins, for example the Caspian Sea, which do not communicate directly with the open sea).

The science that studies oceans and seas from a morphological, chemical, physical and biological point of view is oceanography: it deals with the morphology of the ocean floor, with the nature and type of sediments deposited there, with the physical and chemical characteristics of waters and their movements, as well as the interactions between the oceans and the atmosphere. Oceanographic explorations have also made it possible to gather a lot of information on the structure of the ocean floor, which has contributed to the formulation of the theory of plate tectonics.

The morphology of the ocean floor

Proceeding from the coast line towards the bottom, the continental shelf extends for a more or less short stretch (generally up to the isobath of 200 m), followed by a more or less inclined slope (with an average slope from 3 ° to 5 °), which descends into the abyssal plains (over 3000-5000 m deep;). The latter, in turn, can be crossed by real mountain ranges (oceanic ridges), corresponding to the upwelling areas of the magmas, or punctuated by isolated reliefs, sometimes emerging to form single islands or archipelagos. Abyssal plains can also be interrupted by deep wells, or chasms (which descend to a depth of 6000-11000m), corresponding to large lithospheric fractures or subduction lines at the edges of two opposing plates. The sedimentation of materials takes place continuously on the seabed.

Marine sediments

Within the marine and oceanic masses, there is continuous sedimentation of materials, mainly consisting of the remains of organisms (generally with calcareous or siliceous shell or skeleton, when they are not completely dissolved by seawater between 3500 and 5500 m of depth), which accumulate on the bottom mixing with debris coming from continental slopes (where they arrive brought by watercourses). Coarser sediments (sands) prevail near the continental slope, while the finer ones (clays) are deposited in the deeper areas (together with materials of cosmic and volcanic origin), where concentrations of so-called nodules of manganese, pebbles with a flattened shape and an average diameter of about 5 cm, which are found in abundance on vast expanses of the ocean floor: they are particularly rich in manganese as well as iron, but may also contain copper, nickel and other metals. Even if their recovery is currently very expensive, the nodules could become the main source of supply of many metallic minerals in the next decades.

The accumulations and organic remains (derived largely from areas rich in plankton, together with organisms that allow themselves to be passively transported by currents and waves) give rise to extensive formations of calcareous globigerine muds (unicellular protozoa) and siliceous radiolarian muds ( unicellular protozoa) and diatoms (unicellular algae). Continuous sedimentation activity in the oceans underlies the formation of sedimentary rocks.

The chemical and physical characteristics of seawater

There are many chemical-physical characteristics of seawater that influence some phenomena that occur in it and the life of the organisms that live there; among these, we note the salinity, density, temperature, pressure, colour, and transparency of the water.

Temperature

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. The differences in temperature and density at different levels can cause the formation of convective motions at various latitudes, with consequent mixing of the waters, which affects the distribution of organisms.

On the daily scale, unlike what happens in the soil that heats up and cools intensely during the day cycle, the surface water masses warm up and cool down slowly.

The amplitude of the diurnal temperature range varies with factors depending on the geographical conditions of the site and the degree of stability of the water stratification, but also based on local factors such as cloud cover, humidity and wind.

In particular, from the analysis of the daily temperature range of the surface temperature of the sea and the air above, it was shown that the maximum variation in the Tyrrhenian Sea was in the order of 0.3 ° C for water and about 1, 8 ° C for air. In general, the temperature of the sea water, during the day, undergoes variations of the order of tenths of a degree; the maximum temperatures occur around 3.00 pm. It is also observed that in the open sea the average daytime excursion does not exceed a few tenths of a degree, while near the coast there are higher values, even doubled.

In any case, the maximum water temperature values are obtained when the sky is clear, the air is calm and the maximum solar height; the minimums when the sky is overcast, the agitated water, the minimum solar height and the diurnal thermal oscillation of the air considerably exceeds that of surface water.

Over the course of the year, temperatures follow the variations in solar declination, increasing with high sun, decreasing with low sun on the horizon.

In the seas surrounding Italy, subjected to continental influences, the annual temperature fluctuations reach about 16 ° C in the Tyrrhenian Sea and 22 ° C in the Adriatic, compared to an average temperature of 19.5 ° C and 18.2 ° C respectively. In any case, at the same latitude, the marine thermal fluctuations are much lower than those of continental areas.

Pressure

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. With regard to the ability to withstand variations in hydrostatic pressure, organisms more tolerant to variations are defined barophiles or extremophileColour

Colour

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.

Changes in Sea Level

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.

Over time the average sea level has varied repeatedly, especially during the Quaternary era, following frequent climatic changes and alternating glaciations, with decreases (regressions) of more than 100 m from the current level during the last glaciation ascents (transgressions) of a few tens of meters in correspondence with the interglacial periods. In historical times, in the last two millennia, in the Italian coasts, a general rise in the average sea level of 1-2 m has been ascertained.

Sea Currents

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 can develop both on the surface (surface currents) and in depth (deep currents) and are divided into:

• warm currents, if they have a temperature higher than that of the surrounding waters (affecting the western side of the continents);

• cold currents (affecting the eastern side of the continents) in the opposite case .

Taken together, the currents establish an oceanic circuit, that is, they produce a cyclic motion of marine waters, which travel along long closed paths within the same oceanic basin. The masses of water in the polar latitudes have a higher density due to the strong cooling and tend to precipitate in depth; expanding on the seabed, they move towards latitudes with higher temperatures. The sinking of polar water superficially draws other water from lower latitudes. Warm surface waters from low latitudes float to the surface and are thus transported to higher latitudes, where they cool down, become denser and sink, thus repeating the cycle.

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. Thus, for example, during the winter the ports of Norway are free of ice, precisely because the warm Gulf Stream laps the Norwegian coasts bringing the warm waters of the central Atlantic there; on the other hand, the Canadian peninsula of Labrador, at slightly lower latitudes than Norway, has ports closed by ice for most of the year, because no warm current comes to lap its coasts and is instead affected by the influence of the cold Labrador Current which carries the cold arctic waters southwards. The presence of currents can be identified through artificial satellites equipped with devices sensitive to infrared rays (thermal radiation) emitted from the surface of the oceans: these devices record the temperature, and therefore the path of the currents, distinguishing the areas of the oceans with different temperatures.

Surface Currents

The primary causes of the superficial circulation of water are the winds, which drag the masses of water to the surface of the oceans in their movement, and the energy of the Sun, which heats the areas between the tropics more; the waters are set in motion by the energy that the wind, by friction, gives to the surface layer of the waters themselves. Surface currents are limited to the first 200 m of depth.

The course of surface currents is influenced by the Coriolis force: ocean currents are deflected to the right, with respect to their ideal direction of movement, in the northern hemisphere and to the left in the southern hemisphere; the path of currents can also be influenced by the presence of obstacles, such as oceanic ridges and continents.

The best-known surface currents include the Gulf Stream, which is formed from the north-equatorial current, whose engine is the tropical trade wind. It originates in the Gulf of Mexico, at the confluence of the Antillean current with that of Florida. The current first runs along the southeastern coast of the United States then crosses the North Atlantic, laps the coast of the British Isles, the Scandinavian Peninsula and heads towards Iceland. Along the way, an arm bends south, towards the Canaries, then west and returns to the Gulf of Mexico, closing the circuit. Along the eastern coasts of the continents, particularly along the coasts of California and Peru, when the wind blows towards the Equator parallel to the coast it causes a movement of the surface water towards the open sea (due to the Coriolis effect): the mass of water that moves away from the coast is replaced by water that rises from the underlying layers, causing the phenomenon of the rising of deep waters (also known by the English term of upwelling), which is of great biological importance. In fact, the deep waters that are recalled to the surface are rich in nutrients that lay on the bottom and are thus introduced into the biological cycle of marine life.

Deep Currents

Deep ocean circulation essentially depends on the force of gravity and is caused by differences in water density; since cold and salty water is denser and therefore heavier than warmer and less salty water, it tends to sink and flow under the lighter one: currents of this type are called gradient currents or density currents. For this reason, deep oceanic circulation is also called thermohaline circulation (from the Greek thermós, hot and alós, salt), which is caused by the different temperature and salinity. The time required for the waters to descend deeply and then rise to the surface varies on average from 500 to 2000 years.

Deepwater masses are classified according to the region of origin and the depth at which they flow: density currents affect in particular the Mediterranean and the polar regions, where cold and salty surface waters have high density and sink up to reach the ocean. From the Arctic and Antarctic polar zones, a flow is created on the ocean floor towards lower latitudes.

The waves

Waves are irregular movements of the sea surface, which are not felt at depths greater than 200 m. The waves 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).

The following parameters can be recognized in a wave

• wavelength (L), the distance between two crests or two successive cables;

• wave height (H), the difference in height between the top of a ridge and the bottom of the cable;

• wave period (T), the time that elapses between the passage in the same point of two crests or two successive cables;

• wave depth, lowest point below the surface where the movement of water is felt (equal to L / 2).

In the open sea, where the influence of the seabed is not felt, the motion propagates downwards by friction between the individual particles, which make circular orbits "stacked" on the same vertical, with an increasingly smaller diameter going down to the depth, up to the extinction. The waves do not involve the movement of water masses, since the particles, in their circular motion, periodically return to the starting point (oscillation waves).

If the coasts are low, so much so that the depth of the wave becomes greater than the depth of the seabed, the circular orbits described by the particles deform becoming elliptical. The base of the wave is slowed by friction and lags behind the crest. The phenomenon is accentuated until the latter overturns on itself creating the breakers of the beach (translation wave). The actual movement of masses of water, called undertow, is manifested by a movement of advance and withdrawal of the wave on the shoreline. The kinetic energy of a wave, a function of speed, is able to perform a job that grows as the mass of the wave increases and that on the coast produces an erosive process or causes the accumulation of new material taken from the sea in another point of the coast.

Wave types

There is no real classification of waves. However, in addition to the waves mentioned above, the genesis and physical specifications allow us to distinguish:

• dead or long waves that propagate in the absence of wind. Their origin can be very distant. Among the best known we remember those that come from the Antarctic seas and which, due to the effect of the earth's rotation, invest the western coasts of the continents in the southern hemisphere up to the equatorial calm zones;

• storm waves, raised by hurricanes and cyclones. Equipped with high energy, they take on very variable directions in relation to the irregularity of the wind. They can have destructive effects both on the coasts and on ships in the open sea;

• tsunami; they are atypical, very high waves, caused by a tsunami that shakes the seabed and transmits energy to the water above: these waves hit the coast with great violence, due to both the mass of the water involved and the speed of movement. From this fact and from their scarce predictability, derives the great destructive capacity of the tsunamis, particularly feared along the coasts with strong seismic risk.

Tides

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, and the tidal fluctuations are of a mixed semi-daily type, with two maximums and two minimums during the day, which they follow each other with different values during the month with minor and major tides.

With regard to the basins that bathe the Italian coasts, 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

In the Adriatic, which is a semi-closed basin, the highest tide values in the Mediterranean are recorded. In the upper Adriatic the tidal amplitudes can even exceed one meter and, in particular astronomical and meteorological conditions, such as strong sirocco wind, exceptional sea level rises can occur, which cause in the Venetian lagoon, both for the orography of the area both for the known problems of subsidence, the invasion of the sea of large terrestrial spaces for a few hours, giving rise to the well-known phenomenon of high water in Venice.

The statistical study of the characteristic directions of the waves is called "wave climate"

The Italian wave climate has two main behaviors:

unidirectional (as in the case of La Spezia) or distributed over two or more directional sectors (such as Ancona or Civitavecchia). The Tyrrhenian Sea shows a certain unidirectionality of events, while at least bimodal climates are always present in the Ionian and Adriatic.

Furthermore, the Tyrrhenian Sea is subject to higher maximum wave heights than those present in the Adriatic, this finds reason not only in the intense wind regimes but also in the greater extension of the areas in which the wind is able to give energy to the waves.

Generally, in the western Italian seas (Tyrrhenian, Ligurian, Strait of Sicily, central Mediterranean) storm surges are more intense and more frequent than when they occur along the eastern coasts (Adriatic and Ionian Seas).

According to the average number of storm surges per year and the maximums recorded heights it is possible to distinguish three zones:

• The Adriatic Sea, characterized by 12-15 episodes per year and heights of return between 5-6 m;

• The Ionian Sea, characterized by 8-15 annual events with heights of return of about 6 m;

• The sea of Sardinia, the sea of Sicily, the Tyrrhenian Sea and the Ligurian Sea, characterized by 12-20 episodes per year and return heights over 6,8m.

Storms are more frequent in the western Italian seas.

ECOLOGICAL STATE OF THE SEA

The concept of ecological quality of the coastal marine environment was introduced at a regulatory level by the Water Framework Directive 2000/60 / EC, implemented by our country with Legislative Decree 152/2006. Over the last few decades, the awareness has emerged that the pressures on natural marine resources and the demand for marine ecosystem services are often too high and that therefore there is the need to reduce their impact on marine waters, regardless of where their effects are manifested. The marine environment constitutes a precious heritage that must be protected, safeguarded and, where possible, restored in order to maintain biodiversity and preserve the diversity and vitality of seas and oceans that are clean, healthy and productive.

On 17 June 2008 the European Parliament and the Council of the European Union issued the Framework Directive 2008/56 / EC on the strategy for the marine environment, subsequently implemented in Italy with Legislative Decree 190 / 2010.

The Directive is based on an integrated approach and aims to become the environmental pillar of the future maritime policy of the European Union.

The Directive sets the objective of Member States to achieve good environmental status (GES, "Good Environmental Status") for their marine waters by 2020. Each State must therefore implement a strategy for each marine region.

The Directive has divided the European marine waters into 4 regions: the Baltic Sea, the Northeast Atlantic Ocean, the Mediterranean Sea and the Black Sea, and for some of these it has provided for a further subdivision by identifying sub-regions. In the Mediterranean, three sub-regions have been identified: a) the western Mediterranean, b) the Adriatic Sea, c) the Ionian Sea and the central Mediterranean.

Italian waters belong to all three sub-regions. Given the cross-border nature of the marine environment, Member States are called upon to cooperate to ensure that related strategies are developed in a coordinated manner for each marine region or sub-region. Furthermore, in order to ensure healthy and productive clean marine waters, it is essential that these strategies are coordinated, coherent and integrated with those provided by existing Community legislation (such as transport, fishing, tourism, infrastructure, research) and international agreements.

Member States have to develop a marine strategy based on an initial assessment, on the definition of good environmental status, on the identification of environmental targets and on the establishment of monitoring programs.

The good environmental status of marine waters means the ability to preserve ecological diversity, the vitality of the seas and oceans so that they are clean, healthy and productive by maintaining the use of the marine environment at a sustainable level and safeguarding the potential for uses. and the activities of present and future generations.

Sustainable Development Goal n. 14

The defence of Oceans Seas and its use according to sustainable criteria is clearly strictly bounded to Sustainable Development Goal n. 14.

Credits:

Created collaboratively by Luisa Ambrosino and Ester Gasparro.

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