Stratification occurs as a result of a density differential between two water layers and can arise as a result of the differences in salinity, temperature or a combination of both. Stratification is more likely when the mixing forces of wind and wave action are minimal and this occurs more often in Summer.

In estuaries, the prime reason for stratification is restricted mixing of freshwater river flows and saltwater tidal incursions. In summer, the effects of temperature can reinforce the salinity stratification. In some estuaries, this can divide the water into two distinct layers which do not mix and are kept separate by a sharp change in density.

In stratified estuaries, the lower, saline water is more rapidly replaced, since less dense freshwater inputs (at lower volumes during summer) do not mix but float above the pycnocline. The saline water is more strongly mixed with seawater by tidal flow and so most exchange with the open coast occurs through the bottom layer. In such estuaries, discharge plumes usually rise to the surface and dilution/dispersion occurs primarily in the surface waters which, during stratification, have an increased retention time. This provides an argument for using summer 'upper layer' retention times to assess environmental impact, rather than whole estuary or annual mean retention times.

In sea lochs with sub-surface sills, the surface layer of water is replaced at a rapid rate by tidal action, while the lower water remains essentially stagnant, physically constrained behind the sills, often with a density barrier (pycnocline) forming between the two layers.

In coastal waters around the UK, stratification is a very transient phenomenon but can occur as a result of high temperatures in periods of calm weather or as a result of very high freshwater flow following severe rainfall.

It is important to know the extent to which stratification occurs in European marine sites comprising coastal waters so that due consideration can be made in the calculation of discharge consent conditions.

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