In some places the boundary between land and sea is in the form of abrupt and often spectacular cliffs but, elsewhere, the boundary can take the form of a complex environment of intertidal sediments. These environments include shingle banks, sandy beaches, mud flats, saltmarsh and mangrove (or mangal) communities. In some cases one or other of these environments will occur, in others they will be associated with one another. For example, on many North Sea and North American East Coast shorelines, mud flats grade into saltmarshes behind the shelter of shingle spits and sand dunes. In general, saltmarshes and mangroves occupy similar ecological niches with mangroves at lower latitudes (winter temperatures greater than 10°C) and saltmarshes at high latitudes, though in some locations both communities coexist (Chapman, 1977).

The considerable global significance of these intertidal systems is evident from Figure 1.1. Around the coast of Britain alone there are 44 370 hectares (ha) of saltmarsh (Allen & Pye, 1992) and 589 429 ha on the US East Coast (Reimold, 1977) with a total global area of 3.8 × 107ha (Steudler & Peterson, 1984 and references therein). There are 365 500 ha of mangal forest around the Indian subcontinent and another 250 000 ha in the Mekong delta (Blasco, 1977). The total global area of mangal is about 2.4 × 107ha (Twilley, Chen & Hargis, 1992).