1.3 Salinity variable
Have you ever taken a swim and came outside feeling your skin stretch? Have you ever seen white crystals shine in the sun on a rock by the sea?
Salt is the main ingredient that characterizes marine water, but where does it come from? His presence in the water is due to minerals transported by rivers, to the decomposition of marine organisms and to the activities of submarines volcanoes. There are several kinds of salt in seawater but the most common is sodium chloride, NaCl, which is also used in the kitchen for cooking.
The salinity of the sea (FIG1_SES1.3) can be expressed as the content in grams of dissolved salts in a kilogram of water. As well as the temperature, its variations influence marine life and the movements of the waters (Video1_sez 1.3). Salinity alterations may depend on the degree of evaporation, on precipitations and on the contribution of rivers, particularly in coastal areas or in small and closed seas. The average salinity in the oceans is 35 g/kg but in the Mediterranean it can reach even 39-40 k/kg values, in addition it generally increases with depth and its gradient is called halocline.
Over the last few years, the level of salinity in the Mediterranean seems to be rising, due to the decrease in river fluxes and the increase in temperature leading to an increase in evaporation. This high salinity causes a rise in water density in addition to the alteration of the developmental phases of marine organisms.
Marine animals are adapted to keep their body salts at a constant level so that it doesn’t interfere with the metabolism within cells, but significant changes in salinity can cause problems for some. For example, some marine animals are defined as stenohaline, which means they can’t cope with large salinity fluctuations; others as euryhaline that can tolerate a wide range of salinities.
Salmonids such as salmon and sea trout are good examples of euryhaline organisms. Their migration from sea to freshwater is an essential part of their life cycle: they swim up into the freshwater rivers to spawn and then head back out to the ocean (Fig.2 and Fig.3_SES1.3).
From scientific studies it has been calculated that every year deep waters (under 600 m), are becoming saltier by 0,001 units; this increase alters the habitats and facilitates the transfer of fauna and flora in other areas with unforeseeable consequences.