What is Salinity?
Absolute (or ideal) salinity is the mass fraction of salts in seawater [17]. In practical terms, salinity is expressed as PSU (practical salinity units) which are based on water temperature and conductivity measurements [17]. Salinity used to be expressed in parts per thousand (ppt). For oceanic seawater, ppt and PSU are very close. Salinity of estuaries usually increases away from a freshwater source such as a river (Figure 1 and 2), although evaporation sometimes causes the salinity at the head of the estuary to exceed seawater. The vertical salinity structure and the nature of salinity variation along the estuary (i.e. how rapidly salinity varies in the vertical and horizontal) are the defining features of the salinity structure of coastal waterways (Figure 2). Figure 1. Plan view of the salinity distribution of a hypothetical estuary in which salinity (as PSU) increases away from the freshwater source.
