6.4.1.4 Mangroves, saltmarshes and sea grasses

Coastal vegetated wetlands are sensitive to climate change and long-term sea-level change as their location is intimately linked to sea level. Modelling of all coastal wetlands (but excluding sea grasses) by McFadden et al. (2007a) suggests global losses from 2000 to 2080 of 33% and 44% given a 36 cm and 72 cm rise in sea level, respectively. Regionally, losses would be most severe on the Atlantic and Gulf of Mexico coasts of North and Central America, the Caribbean, the Mediterranean, the Baltic and most small island regions due to their low tidal range (Nicholls, 2004). However, wetland processes are complex, and Cahoon et al. (2006) developed a broad regional to global geographical model relating wetland accretion, elevation, and shallow subsidence in different plate tectonic, climatic and geomorphic settings for both temperate saltmarshes and tropical mangrove forests. Changes in storm intensity can also affect vegetated coastal wetlands. Cahoon et al. (2003) analysed the elevation responses from a variety of hurricane-influenced coastal settings and found that a storm can simultaneously influence both surface and subsurface soil processes, but with much variability.

Saltmarshes (halophytic grasses, sedges, rushes and succulents) are common features of temperate depositional coastlines. Hydrology and energy regimes are two key factors that influence the coastal zonation of the plant species which typically grade inland from salt, to brackish, to freshwater species. Climate change will likely have its most pronounced effects on brackish and freshwater marshes in the coastal zone through alteration of hydrological regimes (Burkett and Kusler, 2000; Baldwin et al., 2001; Sun et al., 2002), specifically, the nature and variability of hydroperiod and the number and severity of extreme events. Other variables – altered biogeochemistry, altered amounts and pattern of suspended sediments loading, fire, oxidation of organic sediments, and the physical effects of wave energy – may also play important roles in determining regional and local impacts.

Sea-level rise does not necessarily lead to loss of saltmarsh areas, especially where there are significant tides, because these marshes accrete vertically and maintain their elevation relative to sea level where the supply of sediment is sufficient (Hughes, 2004; Cahoon et al., 2006). The threshold at which wetlands drown varies widely depending upon local morphodynamic processes. Saltmarshes of some mesotidal and high tide range estuaries (e.g., Tagus estuary, Portugal) are susceptible to sea-level rise only in a worst-case scenario. Similarly, wetlands with high sediment inputs in the south-east United States would remain stable relative to sea level unless the rate of sea-level rise accelerates to nearly four times its current rate (Morris et al., 2002). Yet, even sediment inputs from frequently recurring hurricanes cannot compensate for subsidence effects combined with predicted accelerations in sea-level rise in rapidly subsiding marshes of the Mississippi River delta (Rybczyk and Cahoon, 2002).

Mangrove forests dominate intertidal subtropical and tropical coastlines between 25ºN and 25ºS latitude. Mangrove communities are likely to show a blend of positive responses to climate change, such as enhanced growth resulting from higher levels of CO₂ and temperature, as well as negative impacts, such as increased saline intrusion and erosion, largely depending on site-specific factors (Saenger, 2002). The response of coastal forested wetlands to climate change has not received the detailed research and modelling that has been directed towards the saltmarsh coasts of North America (Morris et al., 2002; Reed, 2002; Rybczyk and Cahoon, 2002) and north-west Europe (Allen, 2000, 2003). Nevertheless, it seems highly likely that similar principles are in operation and that the sedimentary response of the shoreline is a function of both the availability of sediment (Walsh and Nittrouer, 2004) and the ability of the organic production by mangroves themselves to fill accommodation space provided by sea-level rise (Simas et al., 2001). Mangroves are able to produce root material that builds up the substrate beneath them (Middleton and McKee, 2001; Jennerjahn and Ittekkot, 2002), but collapse of peat occurs rapidly in the absence of new root growth, as observed after Hurricane Mitch (Cahoon et al., 2003) and after lightning strikes (Sherman et al., 2000). Groundwater levels play an important role in the elevation of mangrove soils by processes affecting soil shrink and swell. Hence, the influence of hydrology should be considered when evaluating the effect of disturbances, sea-level rise and water management decisions on mangrove systems (Whelan et al., 2005). A global assessment of mangrove accretion rates by Saenger (2002) indicates that vertical accretion is variable but commonly approaches 5 mm/yr. However, many mangrove shorelines are subsiding and thus experiencing a more rapid relative sea-level rise (Cahoon et al., 2003).

A landward migration of mangroves into adjacent wetland communities has been recorded in the Florida Everglades during the past 50 years (Ross et al., 2000), apparently responding to sea-level rise over that period. Mangroves have extended landward into saltmarsh over the past five decades throughout south-east Australia, but the influence of sea-level rise in this region is considered minor compared to that of human disturbance (Saintilan and Williams, 1999) and land surface subsidence (Rogers et al., 2005, 2006). Rapid expansion of tidal creeks has been observed in northern Australia (Finlayson and Eliot, 2001; Hughes, 2003). Sea-level rise and salt water intrusion have been identified as a causal factor in the decline of coastal bald cypress (Taxodium disticum) forests in Louisiana (Krauss et al., 2000; Melillo et al., 2000) and die off of cabbage palm (Sabal palmetto) forests in coastal Florida (Williams et al., 1999, 2003).

On balance, coastal wetlands will decline with rising sea levels and other climate and human pressures (reduced sediment inputs, coastal squeeze constraints on landward migration, etc.) will tend to exacerbate these losses. However, the processes shaping these environments are complex and while our understanding has improved significantly over the last 10 years, it remains far from complete. Continued work on the basic science and its application to future prognosis at local, regional and global scales remains a priority (Cahoon et al., 2006; McFadden et al., 2007a).

Sea grasses appear to be declining around many coasts due to human impacts, and this is expected to accelerate if climate change alters environmental conditions in coastal waters (Duarte, 2002). Changes in salinity and temperature and increased sea level, atmospheric CO₂, storm activity and ultraviolet irradiance alter sea grass distribution, productivity and community composition (Short and Neckles, 1999). Increases in the amount of dissolved CO₂ and, for some species, HCO₃ present in aquatic environments, will lead to higher rates of photosynthesis in submerged aquatic vegetation, similar to the effects of CO₂ enrichment on most terrestrial plants, if nutrient availability or other limiting factors do not offset the potential for enhanced productivity. Increases in growth and biomass with elevated CO₂ have been observed for the sea grass Z. marina (Zimmerman et al., 1997). Algae growth in lagoons and estuaries may also respond positively to elevated dissolved inorganic carbon (DIC), though marine macroalgae do not appear to be limited by DIC levels (Beer and Koch, 1996). An increase in epiphytic or suspended algae would decrease light available to submerged aquatic vegetation in estuarine and lagoonal systems.