Salinity of ocean-water is important to oceanographers in so many ways. Because the components of salinity have a density greater than that of water, the density of the sea-water increases with increased salinity.
Physical oceanographers are much concerned with the relationships that exist among water density, temperature and salinity. It is a well known fact that the spatial variation in water density is an important factor affecting the ocean-water circulation in the form of currents of different types.
Evaporation is closely linked with the degree of salinity. The higher rate of evaporation brings about greater degree of salinity in ocean-water. On the other hand, salinity itself will exert some influence on the rate of evaporation.
Salinity of the ocean water exerts considerable influence on the distribution of marine animals which are significantly affected by relatively small changes in their environmental conditions. One of these is the change of salinity or the relative abundance of water to the amount of dissolved materials within it.
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Certain marine organisms living in the open ocean cannot withstand great changes of salinity. These organisms are stenohaline. On the contrary, some marine animals like oysters living at the mouths of rivers can withstand a considerable range of salinity.
Besides, various other organisms living in the coastal regions are capable of withstanding a wide range of salinity conditions. They are euryhaline.
Certain plants and animals living in ocean water make important changes in the amount of dissolved material in the ocean water. Similarly, marine fish are largely affected by the degree of salinity in ocean water. They drink large quantities of water, extract the salts of that water, and secrete them through special cells.
As stated earlier, salinity to a large extent controls the density of seawater. The ocean water with more than 24.70%o salinity goes on increasing its density with increasing salinity till it reaches its freezing point. The refractive index of seawater increases with decreasing temperature and increasing salinity.
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It is because of the presence of various kinds of salts in ocean water that its freezing point is relatively lower i.e. -1.8°C. However, depending on the amount of dissolved substances the freezing point of seawater undergoes a change. Besides, it is only due to the salinity of water that the boiling point of ocean water is relatively higher than that of fresh water.
Salinity also exerts considerable influence on various other physical properties of the ocean water, such as its osmotic pressure, viscosity, diffusion, electrical conductivity, compressibility, scattering and surface tension.