Salinity is the saltiness or dissolved inorganic salt content of a body of water. Substances that are dissolved in water are usually called solutes. The typical seawater has a salinity of 35 ppt or 35‰. The average density of seawater at the surface is 1.025g/ml. Seawater is denser than freshwater because of the added weight of the salts. Natrium and sodium make up the majority of the salts dissolved in seawater. The other major ions are magnesium, sulfate, calcium and potassium.
All organisms have to maintain a balance between water and solutes in their bodies to keep their cells alive. This is called homeostasis. To regulate this, they can import and export solutes through their membranes. These membranes are permeable to the substances that are dissolved or suspended in water. The random movement of dissolved substances across a permeable membrane is called diffusion. Diffusion occurs until the concentrations are equal on both sides of the membrane. When a solute cannot move through the membranes, the balance on both sides is maintained by water movement. The movement of water through a membrane in response to differences in solute concentration is called osmosis. This is vital to the survival of cells. If cells lose too much water, it will become dehydrated and die. If cells take in too much water, the cells start to swell and possibly burst. For this reason, organisms invest time and energy in maintaining the homeostasis. If the salt content differs on both sides of the membrane, pure water will move across the membrane in the direction of higher salt content.
Originally, salinity was calculated from the Cl- content (chlorinity). The assumption for this method is that the major ions remain in constant proportions to each other. The chlorinity is measured as the mass in g of halides that can be precipitated from 1,000 g of seawater by Ag+ using a standard $ AgNO_3 $ solution.
The reaction is:
$ 3 Ag^++Cl^-+Br^-+I^- \rightarrow AgCl + AgBr + AgI $
Salinity can be calculated by the following eaquation:
Sal = 1.80655 x chlorinity
Since 1979 the Practical Salinity Scale is used. This is based on the relationship between conductivity, chlorinity, salinity and density of seawater. It is base on standard seawater at 15°C and atmospheric pressure. This standard comes from the North Atlantic and is known as Copenhagen water. Its chlorinity equals 19.374 x 10-3 and it has a practical salinity of 35‰. The electrical conductivity equals to that of a standard KCl solution with exactly 32.4357g of KCl per kg solution.
Classification of water bodies based upon salinity
Waters can be divided into several types, depending on salinity  :
- Hyperhaline waters are waters with a very high salinity.
- Metahaline conditions ranges from 45 to 65 ppt.
- Euhaline waters are waters with a salinity between 30 and 35. These are the most marine and oceanic waters.
- Polyhaline conditions ranges from a salinity of 18 to 30 ppt.
- Mesohaline waters are waters with a salinity between 5 and 18 ppt.
- Oligohaline waters are waters with a salinity from 0.5 to 5 ppt.
- Karleskint G. 1998. Introduction to marine biology. Harcourt Brace & Company. p.378
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