18.104.22.168 Changes in coastal wetlands
In the USA, losses in coastal wetlands have been observed in Louisiana (Boesch et al., 1994), the mid-Atlantic region (Kearney et al., 2002), and in parts of New England and New York (Hartig et al., 2002; Hartig and Gornitz, 2004), in spite of recent protective environmental regulations (Kennish, 2001). Many of these marshes have had a long history of anthropogenic modification, including dredging and filling, bulkheading and channelisation, which in turn could have contributed to sediment starvation, eutrophication and ultimately marsh submergence (Donnelly and Bertness, 2001; Bertness et al., 2002). In Europe, losses have been documented in south-east England between 1973 and 1998, although the rate of loss has slowed since 1988 (Cooper et al., 2001); elsewhere there is evidence that not all coastal wetlands are retreating, for example in Normandy, France (Haslett et al., 2003).
Although natural accretion rates of mangroves generally compensate for current rates of sea-level rise, of greater concern at present are the impacts of clearance for agriculture, aquaculture (particularly shrimp), forestry and urbanisation. At least 35% of the world’s mangrove forests have been removed in the last two decades but possible sea-level rise effects were not considered (Valiela et al., 2001). In south-eastern Australia, mangrove encroachment inland into salt-marsh environments is probably related to anthropogenic causes and climate variability, rather than sea-level rise (Saintilan and Williams, 1999). Landward replacement of grassy freshwater marshes by more salt-tolerant mangroves in the south-eastern Florida Everglades since the 1940s has been attributed to the combined effects of sea-level rise and water management, resulting in lowered watertables (Ross et al., 2000).
Sea-level rise can have a larger impact on wetland ecosystems when the human land-use pressure in the coastal area is large, e.g., coasts defended by dykes and urbanisation. Wetlands disappear or become smaller when human land use makes inward movement of the ecosystem impossible (Wolters et al., 2005).