17.2.4.2. Mangroves

Mangroves provide important functions as protection against storms, tides, cyclones, and storm surges and are used as "filters" against the introduction of pests and exotic insects (Menendez and Priego, 1994; Suman, 1994). Mangroves have important ecological and socioeconomic functions as well, particularly in relation to animal and plant productivity, as nutrient sinks, for substrate stabilization, and as a source of wood products. These functions sometimes may be in conflict and differ in importance between riverine, basin, and coastal fringe mangroves; the latter are important primarily for shoreline protection.

Many mangrove forests are under stress from excessive exploitation, reducing resilience in the face of sea-level rise. The importance of sediment flux in determining mangrove response to rising sea levels is well established in the literature. Ellison and Stoddart (1991), Ellison (1993), and Parkinson and Delaune (1994) have suggested that mangrove accretion in low- and high-island settings with low sediment supply may not be able to keep up with future rates of sea-level rise, but Snedaker and Meeder (1994) have suggested that low-island mangroves may be able to accommodate much higher rates. These observations may not necessarily represent conflicting views because the resilience of mangroves to sea-level rise also is conditioned by the composition and status of the stands and other factors such as tidal range and sediment supply (Woodroffe, 1995; Ewel et al., 1998; Farnsworth, 1998). In some protected coastal settings, inundation of low-lying coastal land actually may promote progressive expansion of mangrove forest with rising sea level (Richmond et al., 1997), provided vertical accretion keeps pace.

Notwithstanding the foregoing, studies have shown that mangrove forests in some small islands will be lost as a result of elevated sea levels. For example, it is projected that with a 1-m sea-level rise in Cuba, more than 300 ha of mangroves, representing approximately 3% of that country's forests, would be at risk (Perez et al., 1999). Under similar conditions, Alleng (1998) projects a complete collapse of the Port Royal mangrove wetland in Jamaica, which has shown little capacity to migrate in the past 300 years; Suman (1994) envisages that accelerated sea-level rise would adversely affect mangroves in Puerto Rico, where 62% already has been eliminated by direct human activity.

17.2.4.3. Seagrasses

Seagrass communities provide useful habitat for many marine fish, particularly in the shallow, intertidal environments of many islands. It is postulated that an increase in SST will adversely affect seagrass communities because these ecosystems already are sensitive to land-based pollution and runoff in coastal environments (Edwards, 1995). It is argued further that the distribution of seagrasses will shift as a result of temperature stress, which in turn can cause changes in sexual reproduction patterns (Short and Neckles, 1999). In addition, an increase in temperature will alter seagrasses' growth rates and other physiological functions. Sea-level rise would mean increasing water depth and reduction of the amount of light reaching the seagrass beds, which would reduce plant productivity.

The effect of increased CO₂ in the water column will vary according to species and environmental circumstances but will likely alter the competition between species, as well as between seagrasses and algal populations (Beer and Koch, 1996). Laboratory experiments suggest that some seagrasses, such as Zostera marina, are able to respond positively to increased CO₂ levels by increasing their rate of photosynthesis (Zimmerman et al., 1997), although earlier research on field-collected samples of Thallassia testudinum suggests that maximum photosynthesis is lower with elevated CO₂ (Durako, 1993).

As with other submerged aquatic plants, seagrasses are sensitive to ultraviolet-B (UV-B) radiation because such radiation can penetrate depths of up to 10 m (Larkum and Wood, 1993). Laboratory experiments have demonstrated that the response can vary from strong photosynthetic tolerance, in the case of Halophila wrightii; to moderate tolerance, as with Syringodium filiforme; to little photosynthetic tolerance, as exhibited by Halophila engelmanni (Hader, 1993; Short and Neckles, 1999).

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