Estuaries are some of the most dynamic and complex environments in the world. The places where rivers meet the sea are subject to cycles of constant change – from the waves and currents that move particles of sand and mud every second, to the daily ebb and flow of the tides and the more dramatic effects of periodic floods, droughts and storms. Over thousands of years, estuaries are created, destroyed and recreated by sea-level changes. Few of the earth’s landforms enjoy such a brief and varied existence. The complex interplay between terrestrial, fluvial, coastal and marine realms makes estuaries unique in terms of their landscapes and biodiversity. The mixing of salt and fresh water in estuaries is physiologically challenging to plants and animals, both aquatic and marine. Species inhabiting the transitional zones between land and sea have evolved to deal with these challenges in often surprising ways, adapting their lifecycles and metabolisms to take advantage of the rich resources that estuaries provide. Per square metre, estuaries rank among the most productive habitats worldwide (McLusky & Elliott 2004).