4 - Tidal wetlands

Abstract

Tidal wetlands are habitats dominated by a major type of vegetation, usually with underlying mud owing to their sheltered nature, and usually tolerant of brackish conditions. Hence, the term includes the saltmarshes (such as Spartina marshes), mangrove areas, upper estuarine alder and willow areas, reedbeds (such as Phragmites beds), as well as intertidal mudflats that will be colonised by the vegetation. While the name of the dominant species in the wetland habitat will vary globally, the organisms still fulfil the same roles as ecosystem engineers. The main roles of the wetlands are as sediment traps and as support for the dominant primary producers (the saltmarsh plants, mangrove trees, etc.), which then fulfil important roles such as providing a habitat for colonisation by organisms and epiphytes, major ecosystem engineers, and the major source of detritus. That detritus in turn supports the biota, not only within the system (the autochthonous material), but that which is exported to the remainder of the estuarine system and further afield (the allochthonous material). Wetland systems are predominantly accreting systems by trapping suspended sediment: thus, their topography rises and, when allowed by the prevailing hydrography (i.e., as long as the erosive forces are not too great), they extend into the estuary. Hence, they provide the central function of protecting the areas behind them from erosion, storm surges, wave action, etc., by absorbing energy and countering sea-level rise. This chapter explains those dynamics. Despite their importance, estuarine tidal wetlands are under threat in most countries from urbanization, agriculturisation including aquaculture, port developments, and industrialisation. This is countered in other countries by habitat restoration.