Intertidal sand and mudflats are sensitive to increased rainfall and thus an increased freshwater input. This may cause scouring of intertidal areas, changes in intertidal creeks and possibly a reduction in salinity in localised areas. Salinity is an important variable which influences the populations of intertidal and subtidal areas, especially in estuaries where it is the dominant factor (McLusky, 1989). On open coasts it is less important but it may have a significant local influence.

The physiological affects of salinity change are well described (e.g. McLusky, 1989) and species in intertidal areas are adapted to tolerate changes in salinity by osmoregulation, reducing oxygen consumption and reducing metabolic activity to conserve energy (Brown, 1983) or by moving seaward if they are mobile. Thus salinity gradients over intertidal mud and sandflats will produce zonation in the fauna. In extreme cases of run-off and freshwater dilution, euryhaline species such as Macoma balthica, Corophium volutator, Nereis diversicolor, Hydrobia ulvae and brackish oligochaete species may become permanently established at the top of beaches.

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