The morphology and development of coastal wetlands in the tropics
Tropical coastal wetlands, comprising mangrove forests and the associated freshwater wetlands, are most extensive where there are low-lying coastal plains extending across the intertidal zone. The relatively species-poor communities of the Atlantic-East Pacific province contrast with the diverse forests associated with more complex habitats in the Indo-West Pacific province, centered on southeast Asia and northern Australia. Sedimentation in these wetlands is a function of the type of substrate and the extent to which sediment is sourced from the inland catchment (e.g., terrestrial mud) or the nearshore (e.g., marine carbonate). In some situations, the substrate is primarily organic and derived from the wetland vegetation (e.g., fibrous peat). Stratigraphical and chronological studies of mangrove sediments indicate that sea level has been a fundamental control on the way in which the wetlands have developed. In the West Indies, the pattern of sea level rise at a decelerating rate has resulted in a transgressive sequence of peat and sediments, which has only recently changed to progradation of mangrove wetlands in a few locations. In southeast Asia and northern Australia, the change from transgression to regression occurred around 7000 years ago when sea level stabilized around its present level. Since that time, extensive coastal plains have been deposited initially beneath mangrove forests, transitioning into freshwater wetlands once the substrate has built above the limit of tidal inundation. The nature of the wetlands that have developed on the landward margin of mangrove forests is climatically determined; extensive salt flats occur in arid and semiarid areas; seasonally flooded grass and sedgelands dominate the wet-dry tropics; and peat swamp forest occurs in the wettest areas. Understanding the response of these wetlands to past sea level changes can yield important insights into how they may respond to future higher sea levels.