Mixing processes in estuaries

Salinity is defined as the weight in grams of inorganic ions dissolved in 1 kg of water. Seven ions constitute more than 99% of the ions in seawater and the ratios of these ions are constant throughout the world oceans. Consequently, the analysis of one ion can, by proportion, give the concentration of all the others and the salinity. The density of seawater and light and sound transmission all vary with salinity. 

Salinity is measured by the conductance of electrical currents through the water 

Open-ocean waters have a narrow range of salinities (32-37) and most are near 35. In estuaries, values fall to less than 1 approaching the freshwater end-member. In hypersaline environments salinities can exceed those of seawater, reaching values greater than 300. 

If the concentration of the measured component is, like salinity, controlled by simple physical mixing, the relationship will be linear (Fig. 6.3). This is called conservative behaviour and may occur with riverine concentrations higher than, or lower than, those in seawater (Fig. 6.3). By contrast, if there is addition of the component, unrelated to salinity change, the data will plot above the conservative mixing line (Fig. 6.3). Similarly, if there is removal of the component, the data will plot below the conservative mixing line (Fig. 6.3). In most cases, removal or input of a component will occur at low salinities and the data will approach the conservative line at higher salinity (Fig. 6.4). Extrapolation of such a quasiconservative line back to zero salinity can provide, by comparison with the measured zero salinity concentration, an estimate of the extent of removal (Fig. 6.4a) or release (Fig. 6.4b) of the component. 

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