Salinity is the total content of dissolved inorganic salts in seawater. It is usually expressed as parts per thousand (o/oo) or ppt.), or g.l-1 (same as o/oo). The salinity for normal open ocean sea water ranges between 33 o/oo and 37 o/oo. In the Red Sea it reaches 41 o/oo. Where there is a lot of fresh water inflow, such as in estuaries and the Arctic, plants can experience salinity fluctuations ranging from 0 to 35 o/oo seasonally or over a tidal cycle. In rock pools, where evaporation is high, salinity  may be as high as 70 o/oo. The flagellate Dunaliella salina is particularly adapted to these high salinity conditions.

Diagrammatic representation of a salinity pycnocline (A), and a corresponding graphical representation (B)

The slight salinity fluctuations that occur in the open ocean are of little consequence to marine plants, although they are important in helping oceanographers identify the source of a water mass. Salinity fluctuations in coastal waters may be important, however, especially in areas with seasonal runoff, such as estuaries. Because of its lower density, lower salinity water will rest above the more dense, higher salinity water. When there is a layer of low salinity water overlying a layer of higher salinity water, the boundary between them is known as a pycnocline. A pycnocline is any steep density gradient, so a thermocline is also a type of pycnocline.

When a density gradient exists, be it due to salinity or temperature (or both), we can think of it in terms of the amount of energy required to break it down. This energy will be a function of the difference in density between the two layers, and the thickness of the surface layer. In natural bodies of water, wind is the usual source of energy that breaks down pycnoclines. A lower salinity layer will spread out, and become more easily mixed as it moves away from the coast.

Light filtration


Thermal stratification

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