A journey of discoveries

Climate change decisively affects the life of our planet, with often dramatic consequences for the economy and survival of populations. Scientific research studies natural phenomena and We have all heard and read news about the Gulf Stream, El Niño and La Niña, hurricanes in tropical areas (hurricanes) and recently of the 'Medicanes'.

To assess the well-being of the seas there are fundamental parameters to be known and analysed, in particular:

Temperature is a parameter used as an indicator of heat.

It is a physical quantity that determines spontaneous heat exchanges between different bodies: heat flows from a body with a higher temperature to a body with a lower temperature.

Heat exchanges are an important interaction between the sea surface and the atmosphere.

Along the vertical profile of the water column the temperature decreases, with generally higher values at the surface. Along this vertical gradient is the thermocline, a layer along which the temperature decreases dramatically, separating a warmer surface zone (mixed layer) from the deeper, cooler zone, along which the temperature remains nearly constant.

Temperature and its vertical profile are influenced by latitude, with temperature increasing at lower latitudes, and by seasons.

Temperature, together with salinity, determines the density of water and consequently characterizes water masses and influences their movements. In addition, it affects other parameters such as the solubility of dissolved gases, e.g., oxygen.

Units of measurement: There are different units of measurement: e.g. degrees centigrade, degrees kelvin. Ideally, one would like to measure an absolute temperature, that is, a temperature whose scale begins at an absolute zero. Since the use of an absolute temperature scale is quite difficult, practical scales are used that are derived from calibrations at well-defined values such as the 'triple point of water' (the best known-but also the triple point of hydrogen, the freezing point of silver or Indium). The practical temperature scale was revised in 1887, 1927, 1948, 1968, 1990).

Average value of the Oceans: ˜3,5°C

Average value of the surface layer of the Mediterranean Sea (0 - 150 m): ˜ 15,4°C

Average value of the middle layer of the Mediterranean Sea (150 - 300 m): ˜ 13°C

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Temperature is an important parameter for the life of aquatic organisms. Each is adapted to live at different temperatures: organisms living at the poles will be adapted to colder temperatures, while those living at the equator to warmer temperatures. In recent decades we are witnessing a global increase in water temperatures. This process has negative effects on so many aspects, from the life of marine organisms to the regulation of climate around the world.

Rising temperatures cause serious damage to those organisms well adapted to stable temperatures, such as corals. These are colonial animals made up of very large numbers of small polyps. To live, corals are in symbiosis with zooxanthellae: single-celled algae that live inside the polyps, where they perform photosynthesis and produce nutrients and oxygen. When temperatures get too high, zooxanthellae are expelled from coral polyps, which bleach and slowly die, affecting entire ecosystems.

Temperature also affects the solubility of oxygen in water, that is, how much oxygen can be present. As the water gets warmer, the solubility of oxygen decreases, so the warming of the sea also results in a decrease in the oxygen available to marine organisms.

On the physical level, on the other hand, as water and air temperatures increase, the melting of ice at the poles also increases, with important global repercussions, such as rising sea levels and decreasing sea salinity.

Salinity indicates the amount of dissolved salts within a water sample.

At the simplest level of definition, salinity is the total amount of dissolved material (measured in grams) in one kilogram of seawater. Thus salinity is a dimensionless quantity. Practical definition that could allow accurate measurement has always been difficult. Early approaches to 'weighing' the amount of dissolved material involved evaporation of water, but it was soon discovered that some of the dissolved material was also carried away by vapors. To avoid this, it was proposed (by intervening chemically) to define salinity as "total amount of solid material (in grams) dissolved in one kilogram of seawater when all carbonate has been converted to oxide, bromine and iodine replaced by chlorine, and all organic material completely oxidized. " Quite a complication from a practical point of view, and so a formula was proposed in 1964 that linked salinity to chlorine content, an element easily measured by chemical analysis. At the same time, work began on formulas that linked salinity to conductivity. These formulas were constantly updated until TEOS 10 was reached. These formulas are based on the principle that as salinity increases, the conductivity of water increases proportionally. So, from the measurement of conductivity it is possible to derive the salinity of a water sample.

There are eleven major components of seawater, and these are the ones that give it its salinity (5 cations, 5 anions and an indissociated element).

Surface salinity follows a latitudinal gradient influenced by seawater evaporation and atmospheric precipitation, with an increase from polar to tropical areas, while in the equatorial belt there is a slight decrease in salinity resulting from increased precipitation.

Freshwater inflow and ice melt are additional factors affecting surface salinity.

Salinity, together with temperature, determines the density of water and consequently characterizes water masses and influences their movements. It also affects other parameters, such as the solubility of dissolved gases, e.g., oxygen, and the freezing temperature of water.

Units of measurement: As with temperature, the scale for measuring sea salinity has changed over time based on measurement technologies and methodologies and the evolution of the practical concept of salinity. The measurement scale has evolved from the 'ppt unit (parts per thousand), to psu (practical unit of salinity) and then to Absolute Salinity (TEOS10 - a pure number) for the reasons that will be given of the 'Description'. A definition of salinity was given in 1889 and published in 1902. In 1964 UNESCO approved a new definition based on 'chlorinity,' and in 1966 the Salinity and Chlorinity Relationship was approved. In 1978 the Practical Salinity Scale (pss) was approved. In 1980 an International Sea Water Equation was defined, and in 2010 the Thermodynamic Equation (TEOS 10) for estimating Absolute Salinity was defined. At the UNESCO meeting held to approve TEOS 10 it was expressly stated that this would not be the last revision of the salinity calculation.

Average ocean value: 34.7 psu

Average value of the surface layer of the Mediterranean Sea (0 - 150 m): ˜ 36.2 psu

Average value of the middle layer of the Mediterranean Sea (150 - 300 m): ˜ 38.4 psu

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Sea density is given by temperature, salinity and depth/pressure. Horizontal differences in density lead to the formation of thermohaline currents. Due to global warming, the salinity of the sea in polar areas is now in danger of decreasing due to increased melting of ice, which consists of fresh water, affecting the density of water masses and consequently thermohaline currents.

Salinity also has an influence on living organisms. Marine organisms do not have an outer epidermis layer like that of organisms living on land, so they have the ability to exchange water through the surface of their bodies. This aspect is certainly an adaptive advantage to the environment in which they live, but if the salinity of the water were to vary too much from what they are used to, they would not be able to survive. Whether salinity increases or decreases, the balance of salts and water within the bodies of marine organisms is disturbed, to the point of causing their death. When salinity reaches very high values, conditions become untenable for life except for a few single-celled organisms adapted to live in extreme conditions. An example of this condition is the Dead Sea: the saltiest sea, or rather lake, in the world, where no life forms visible to the naked eye are present.

Conductivity is a property of seawater. Indeed, positive and negative ions are dissolved in it, making the solution an excellent conductor of electricity. These compounds are the ones that impart salinity to seawater; therefore, the conductivity measurement is used precisely to derive the salinity parameter, parameters that are directly proportional.

Therefore, conductivity is the measurement of the conductance of water by the action of the sensing electrodes on the conductivity electrode. The response signal is expressed in mS/cm. Note that the conductivity of solutions of ionic species is strongly dependent on temperature. 


Units of measurement: mS/cm

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Seawater is an excellent conductor of electricity. In fact, positively and negatively charged compounds are dissolved in it, allowing electricity to flow through the water. Thanks to this property, the conductivity parameter of water can be calculated.  

The compounds that make seawater a good conductor of electricity are precisely those that give water its salinity. The importance of conductivity lies precisely in its link to salinity. In fact, by calculating conductivity one can, through complex formulas, then derive the salinity of water, a very important parameter for the physical and biological balances of water masses (See Salinity).

Conductivity is also used to measure the parameters: nonlinear function (nLF) of conductivity, specific conductance and total dissolved solids.  

Specific conductance is measured using the same technique as conductivity, but the probe uses temperature and conductivity values to generate a compensated specific conductance value at 25 °C. 

The nonlinear function (nLF) of conductivity is defined by the ISO 7888 standard and is applicable for temperature for electrolytic conductivity compensation of natural waters. This convention is typically used in German markets.

This parameter indicates the amount of dissolved oxygen present in one liter of water. 

The concentration of dissolved oxygen is regulated by physical and biological processes.  

Ocean circulation and the interaction between the atmosphere and the sea surface affect the amount of dissolved oxygen in water. A gaseous molecule, oxygen (O2) actually undergoes exchanges between the air and the sea surface by diffusion, following its concentration gradient (a process called ventilation).

Photosynthetic and respiration processes also play a key role in oxygen control. Along the water column we move from areas of net oxygen production, where photosynthetic processes exceed those of respiration, to areas where photosynthetic processes decrease and there is a decline in the presence of oxygen, consumed by the respiration of living things.

Minimum dissolved oxygen concentrations are found around 1000 m depth, where there is a maximum of degradative activity by decompositional bacteria and oxygen-consuming processes. Below 1000 m, oxygen concentration increases again and remains stable due to deep water ventilation by ocean circulation. 

Variables that can affect the amount of dissolved oxygen include temperature and salinity: as temperature, as well as salinity, increases, the solubility of oxygen in water decreases.  

Units of measurement: mg/L or µg/L

Average content in the oceans before the 1980s: ˜ 130 µg/L

Average content in the oceans in the 21st century: ˜ 100 µg/L

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Even in water, as on land, oxygen is necessary for life. Fish, cetaceans, mollusks, crustaceans and many other marine organisms need to breathe oxygen in order to live, just like animals that live out of water. 

Some of the oxygen in the sea is derived from the atmosphere, but some is also produced in the water by autotrophic organisms, namely plants, algae and phytoplankton (very small algae, consisting of as little as a single cell) that carry out photosynthesis.  

Along the water column we move from areas of net oxygen production, where photosynthetic processes exceed those of respiration, to areas where photosynthetic processes decrease and there is a decline in the presence of oxygen, consumed by the respiration of living things. Where the phytoplanktonic stand has more light energy at its disposal it produces more than it consumes, and thus all unused production by phytoplankton can be exported to higher trophic levels. The depth where the amount of production is equal to the amount of respiration is the compensation point: phytoplankton produce as much as is needed for their metabolism.

At greater depths, on the other hand, light decreases and with it the productivity of phytoplankton: the stand thus consumes more oxygen than it produces.

If oxygen in the water were to decline greatly, as is happening according to a 2019 IUCN report, the survival of marine organisms would be jeopardized, affecting biodiversity and the balance of marine ecosystems.  

Chlorophyll a is a green photosynthetic pigment found in all marine plant organisms, large and small. It is a parameter used to assess and quantify phytoplanktonic biomass in the sea. The phytoplanktonic component consists of microalgae, which are autotrophic organisms ranging in size from a few to hundreds of microns. Chlorophyll, a light-absorbing pigment with two absorption peaks, around the wavelengths of 430 nm and 660 nm, is present in these organisms. It is indispensable for carrying out oxygenic photosynthesis, a process by which light energy is converted into chemical energy that can be used by the cell for the biosynthesis of molecules, with concomitant release of oxygen into the environment.  

Variations in chlorophyll values reflect variations in the presence and quantity of microalgae, influenced by light, stressors, and the presence of nutrients, among other factors. 

Unit of measurement: RFU (Relative Fluorescence Unit) or microgram/L. The relationship between the two units of measurement depends on the temperature of the water at the time of measurement.

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Chlorophyll is measured to figure out how many microalgae are present in the water. Microalgae are autotrophic organisms, like plants, meaning that they are able to carry out photosynthesis and thus produce their own nutrients and release oxygen into the environment. In fact, much of the oxygen in the water is precisely produced by marine autotrophic organisms: plants, algae and phytoplankton (very small algae, consisting of even a single cell). 

This oxygen partly remains in the water, while some of it reaches the surface of the sea and passes into the atmosphere: in fact, about 50 percent of the oxygen we breathe is produced right in the oceans! 

Very low chlorophyll values in the sea can therefore also be a problem outside the water: in fact, they indicate a lack or scarcity of microalgae, and thus of the oxygen production necessary for life in and out of the water.  

On the other hand, on the other hand, an excessive increase in chlorophyll values may indicate the presence of algal blooms: in some cases microalgae can proliferate very quickly, giving rise to so-called algal blooms. Although this may seem seem positive for increased oxygen production, algal blooms have negative repercussions on the environment. In fact, as algae die, much of the oxygen in the water is used by bacteria for their decomposition. Oxygen available to other organisms, therefore, decreases rapidly, and many organisms are unable to survive.  

In addition, some species of microalgae that proliferate in these cases release toxic, mucilaginous, and foul-smelling substances into the environment, which also has important repercussions for human well-being and health. 

Phycoerythrin is an accessory photosynthetic pigment belonging to the larger group of phycobilins, pigments found in red algae, cyanobacteria and cryptophytes. It has a red coloration (hence the suffix -erythrin) and a strong yellow fluorescence.  

Since it is a photosensitive molecule, it is possible to measure its presence through the use of fluorescence-based optical methods. It is a parameter used to assess and quantify the presence of microalgae in the sea. 

Phycoerythrin has a peak light absorption between 545 and 566 nm, wavelengths outside the absorption range of chlorophylls and carotenoids. 

Variations in phycoerythrin values reflect variations in the presence and quantity of microalgae, influenced by light, stress, and the presence of nutrients, among other factors. 

Units of Measurement: RFU (Relative Fluorescence Unit)

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To accomplish photosynthesis, plant organisms are able to absorb light through photosynthetic pigments. The main one is chlorophyll a, found in all marine plant organisms, but in order to absorb light at different wavelengths many organisms have developed different pigments, called accessory pigments. One of these is phycoerythrin, found, for example, in cyanobacteria, also called blue algae. 

Measuring phycoerythrin is therefore useful for assessing the presence of microalgae belonging to the cyanobacteria and red algae groups in the sea. An increase or decrease in phycoerythrin values has the same effects as a change in chlorophyll in the sea (See Chlorophyll) 

The pH indicates the acidity or basicity of an aqueous solution, expressed by the decimal cologarithm of the concentration of hydrogen ions.

A pH of 7.0 is neutral; values below 7 are acidic; values above 7 are alkaline.

Seawater is slightly alkaline, generally with a pH currently around 8.1 (a decrease from the past of 0.1), but natural gradients in seawater pH are present.

This parameter is influenced by physical and biological factors: water temperature, springs, and the presence of carbonates cause natural pH variation.

Variations in this parameter are also caused by anthropogenic factors: the increase of carbon dioxide in the atmosphere is the cause of the acidification of the seas, for which there is now an average pH value about 0.1 unit lower than in the past.

The pH of seawater is in fact influenced by atmospheric carbon dioxide, which at the air-water interface is partly transferred to the sea, where it acts on the balance between carbonic acid and bicarbonate ion, causing an increase in H+ and a consequent decrease in pH.

Unit of measurement: Dimensionless

Average content in the oceans: 8.2

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When one hears about the acidification of the seas, it is precisely the pH that one is referring to. The sea had a slightly basic pH, around 8.2 that was kept stable by complex balances. As carbon dioxide in the atmosphere increases, there is also an increase in CO2 in the waters. In fact, carbon dioxide is a gas and, like all gases, it moves from areas where it is more concentrated, in this case the atmosphere, to areas where it is less concentrated, namely the sea surface. Once inside the water, it acts on the balances that keep the pH stable: if CO2 reaches too high amounts it causes a decrease in marine pH, that is, its acidification.

This process has negative effects especially on organisms and substrates formed from calcium carbonate. These organisms include corals, colonial animals that produce an external skeleton formed precisely from calcium carbonate. When these die, their skeletons remain in the environment and over time go on to form the famous coral reefs, important marine ecosystems but also very important substrates for the lives of many people: many inhabited islands or entire nations, such as the Maldives, are founded on coral reefs themselves!

Calcium carbonate, however, is a compound that dissolves with decreasing pH, so acidification of waters can have very serious repercussions for the life of marine organisms, but also for the lives of entire populations.

Turbidity is the indirect measurement of the concentration of suspended solids in water and is typically determined by shining a light into the sample solution and then measuring the light that is scattered by the suspended particles. Turbidity is an important factor in water quality and is a key tool for monitoring environmental changes due to events such as meteorological runoff or illicit discharges. The source of suspended solids varies (examples include silt, clay, sand, algae, and organic matter) but all particles will affect light transmission and result in a turbidity signal. 

Generally, turbidity values are higher in coastal areas, where several factors, including anthropogenic activities, cause greater presence of suspended solids. More nutrients are also present in coastal areas than in offshore (generally oligotrophic) areas, so phytoplankton can proliferate more, decreasing water transparency. Areas with higher turbidity are particularly estuarine areas, where the inflow of river water carries a large amount of suspended solids of different sizes. 

Therefore, light penetration is highest in high water areas and decreases toward the coasts, reaching a minimum in estuarine areas. 

Unit of measurement: NTU (Nephphelometric Turbidity Unit)

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Turbidity is a parameter that indicates the presence of solids in water, and is used to determine its quality.  

For life in water to proliferate, the presence of light is necessary. In fact, microalgae and other plant organisms need light to perform photosynthesis and thus to produce nutrients and release oxygen, which are necessary for the life of all marine organisms. Along the water column, from the surface to the depth, we find the photic zone, where there is light, the dysphotic zone, where light is scarce, and the aphotic zone, where there is no light and is therefore dark. The depth of these zones changes from coast to coast; in fact, where the water is more turbid, light can penetrate less into the water and therefore reaches shallower depths. This occurs naturally by moving from the coast, where there are more suspended solids in the water for natural but also human-influenced reasons, to offshore areas.   

When turbidity reaches values that are too high, there can be repercussions on the health of the water and its organisms: with greater turbidity comes less light penetration and thus less photosynthesis by primary producers. More turbid waters are in fact generally less oxygenated waters, affecting all organisms in that environment.  

Suspended solids are particles present in the water column. They can be inorganic particles (such as silt, sand, and clay) or organic particles (such as microalgae and organic matter). The presence of suspended solids is influenced by natural factors, such as river input, rainfall, wave motion, and winds, or by anthropogenic factors, such as the release of waste materials into the sea, beach nourishment, and coastal urban works. 

The presence of suspended solids increases the turbidity of water, so these are two parameters measured in relation to each other.  

Unit of measurement: mg/L

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Within the water column are suspended solids. These particles may be inorganic, i.e., particles that derive from rocks or sediments, such as sand and clay, or they may be organic, i.e., living particles, such as microalgae, or otherwise related to life, such as waste organisms.  

These solids generally occur naturally in the sea, and are usually present in greater quantities near the coast, especially in estuarine areas, than offshore. Near the coast, in fact, many particles may derive from the land itself, carried into the water by winds or rivers, or they may be resuspended from the seabed by waves.  

Suspended solids can also result from human actions. Before summer, for example, beach nourishments are implemented at many beaches, that is, materials are poured on the beach to increase its size in preparation for summer tourism. These materials also reach the sea water, where because of their small size, they remain suspended in the water column, increasing its turbidity, affecting the balance of marine organisms (See Turbidity).