So, saltwater fish drink water copiously while excreting excess salts through their gills.Freshwater fish do the opposite by not drinking but excreting copious amounts of urine while losing little of their body salts.Chemical elements in sea water do not exist on their own but are attracted to preferential ions of opposite charge: sulphur will occur mainly as sulphate, sodium as sodium chloride, and so on.Salinity and the main salt ions The salinity of sea water (usually 3.5%) is made up by all the dissolved salts shown in the above table.Dissolved gases in seawater The gases dissolved in sea water are in constant equilibrium with the atmosphere but their relative concentrations depend on each gas' solubility, which depends also on salinity and temperature.As salinity increases, the amount of gas dissolved decreases because more water molecules are immobilised by the salt ion.In order to understand the sea, some of its chemical properties are important.
Also landlocked seas like the Black Sea and the Baltic Sea, have differing concentrations.
The Red Sea located in the desert region but almost completely closed, shows the highest salinity of all (40ppt) but the Mediterranean Sea follows as a close second (38ppt).
Lowest salinity is found in the upper reaches of the Baltic Sea (0.5%).
The density of 35ppt saline seawater at 15ºC is about 1.0255, or s (sigma)= 25.5. The relationship between temperature, salinity and density is shown by the blue isopycnal (of same density) curves in this diagram.
In red, green and blue the waters of the major oceans of the planet is shown for depths below -200 metre.