Estuaries: Classification, Ecology and Human Impacts Chapter 3 TAGUS ESTUARY SA STRUCTURE AND DYNAMICS: A HISTORICAL PERSPECTIVE Isabel Caçador and Bernardo Duarte Center of Oceanography, Faculty of Sciences of the University of Lisbon, Campo Grande 1749-016, Lisbon, Portugal A T Salt marshes are a very important part of the estuarine ecosystem, with an important role within the biogeochemical cycles, being areas of high primary production, which also contribute importantly as shoreline stabilizers. Besides, periodical tidal flooding of salt marshes also causes the transport of significant quantities of pollutants, which tend to accumulate in the marsh ecosystem. Therefore, salt marshes are considered to be important sinks namely of heavy metals. Their important role has been recently admitted by the inclusion of these ecosystems in the European Water Framework Directive (WFD). Multiple services of wetlands and its value are already well known. The Tagus estuary is the largest estuary in Western Europe, covering an area of approximately 320 km 2 . The estuary has a vast inner bay with extensive intertidal areas from which about 15% of these areas are salt marshes. The Tagus estuary is a mesotidal system with a mean tidal range of 2.4 m, varying from about 1