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The capacity of mangrove forests to cope with sea-level rise is higher where the rate of sedimentation approximates or exceeds the rate of local sea-level rise. Indeed, Hendry and Digerfeldt (1989) have shown that mangrove communities in western Jamaica were able to keep pace with mid-Holocene sea-level rise (ca. 3.8 mm/yr). However, the adaptive capacity of mangroves and other coastal wetlands to sea-level rise (usually by landward migration) is now severely limited in many localities by increasing human activities. It has been suggested, for instance, that a 1-m rise in sea level in Cuba will drastically affect the viability of 333,000 ha of these wetland communities (approximately 93% of Cuba's mangroves) (Perez et al., 1996). Additionally, adaptive capacity will vary among species; some species of mangroves appear to be more robust and resilient than others to the effects of climate change and sea-level rise (Ellison and Stoddart, 1991; Aksornkaoe and Paphavasit, 1993).
Some ecologists believe that mangrove communities are more likely to survive the effects of sea-level rise in macrotidal, sediment-rich environments-such as northern Australia, where strong tidal currents redistribute sediment (Semeniuk, 1994; Woodroffe, 1995)-than in microtidal, sediment-starved environments like those in many small islands (e.g., in the Caribbean) (Parkinson et al., 1994). Most small islands fall within the latter classification; therefore, they are expected to suffer reductions in the geographical distribution of mangroves. Furthermore, where the rate of shoreline recession increases, mangrove stands are expected to become compressed and suffer reductions in species diversity in the face of rising sea levels.
On the other hand, Snedaker (1993) argues that mangroves in the Caribbean are more likely to be affected by changes in precipitation than by higher temperatures and rising sea levels because they require large amounts of fresh water to reach full growth potential. He hypothesizes that a decrease in rainfall in the Caribbean would reduce mangroves' productive potential and increase their exposure to full-strength seawater. Thus, peat substrates would subside as a result of anaerobic decomposition by sulfate-reducing microorganisms, leading to the elimination of mangroves in affected areas (Snedaker, 1993).
It has been postulated that seagrass meadows-which exist in shallow, intertidal coastal environments-are the ecosystems most likely to be negatively affected by climate change effects, particularly sustained elevation of sea-surface temperature or increases in freshwater runoff from land (Edwards, 1995). There is a growing consensus, however, that the main threats to seagrasses in the future are likely to come not from the effects of climate change but from anthropogenic disturbances-such as dredging, overfishing, water pollution, and land reclamation.
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