Salinity is the concentration of dissolved salts in water. It is low in fresh water. It is low in fresh water and high in sea water. In brackish water (at river mouths and swamps), the salinity changes rapidly with the tides.
Aquatic organisms have to maintain the osmotic balance between their body fluids and their aquatic surroundings. Those living in fresh water have adaptations for getting rid of the excess water that enters their bodies; those living in sea water have no problem as the salt concentration of sea water and the body fluids or organisms are nearly the same; and those living in brackish water have body tissues that can tolerate large changes in the concentration of salts in their body fluids.
