Date of this Version
U.S. government works are not subject to copyright.
What are coastal wetland ecosystems, what are their limits of distribution, and where do they exist in the overall coastal landscape? There are several general definitions for wetlands, but the Ramsar definition is likely the most broadly encompassing (http://www.ramsar. org/), whereas others are more focused definitions tailored to country-specific protection and management policies (Mitsch and Gosselink, 2006). We offer a very general approach rather than a precise definition: coastal wetlands are ecosystems that are found within an elevation gradient that ranges between subtidal depths where light penetrates to support photosynthesis of benthic plants to the landward edge where the sea passes its hydrologic influence to groundwater and atmospheric processes. At the seaward margin, biofilms, benthic algae, and seagrasses are representative biotic components. At the landward margin, vegetation boundaries range from those located on groundwater seeps or fens in humid climates to relatively barren salt flats in arid climates.
Tidal wetlands are a critical component of the coastal ocean landscape, which consists of a continuum of landscape elements or ecosystems stretching from where rivers enter the coastal zone, through the estuary, and onto the continental shelf (Fig. 1.1). In addition to tidal wetlands, the coastal ecosystems include seagrass meadows, rivers, tidal creeks, estuarine waters and unvegetated subtidal bottoms, tidal flats, coral reefs, and continental shelf waters and bottoms. Quite often there are extremely sharp transitions between landscape elements in the coastal zone, for example, between open water and marsh, marsh and uplands, and seagrass and mangroves (Fig. 1.2A and B). Groundwater (freshwater draining from uplands) is another source of upland-derived waters and materials. The exchange and mixing of water and materials entering from rivers and the ocean defines the overall structure and distribution of landscape elements. The distribution and deposition of sediments from land and the ocean establishes the overall bathymetry. Bathymetry in combination with tidal range and the spatial gradient in salinity are the primary determinants of ecosystem distribution within the coastal ocean landscape. Emergent and submerged vegetation, once established, exert an ecogeomorphic feedback on the fate of river and ocean sources of various materials. For example, vegetation slows the movement of water, promotes the settling of sediment particles, and accelerates estuarine infilling and tidal wetland expansion and accretion.