Impacts of Environmental Change

Mangroves are arguably an excellent indicator of how ecosystems will respond to the manifold impacts of global environmental change and land-use disturbance. Given present patterns, the combined effects of climate and land-use change will be noticeably evident in reduced goods and services of mangroves to human systems throughout the tropics in the twenty-first century. For example, accelerated rates in sea-level rise have been speculated as the most critical environmental change affecting the continued existence of mangrove ecosystems. Numerous processes contribute to vertical accretion of mangroves at a rate that balances the increase in regional sea-level rise. Critical rates in sea-level rise have been estimated above which there is a projected collapse of mangrove ecosystems. While some speculation suggests that mangroves cannot sustain existence at sea-level rise >1.2-2.3 mm yr-1, there is evidence that mangroves located in particular environmental settings existed through periods of accelerated sea level rise. Mangroves in Australia can keep pace with changes in sea-level rise with rates ranging from 0.2 to 6 mm yr~ in the south Alligator tidal river. Also, mangrove forests in many estuaries in northern Australia tolerated sea-level rise of 8-10 mm yr-1 in the early Holocene. Many of these mangroves receive terrigenous sediments and exist in macrotidal environments, with critical rates that are much different than for mangroves in microtidal and carbonate environments. In addition, mangrove areas can be sustained along the coastline by migrating inland under conditions of increased sea-level rise. But this inland migration will depend on whether suitable inshore landscapes are available. The most significant recent restriction to mangrove colonization is human land use of available landscapes.

Mangroves in many coastal regions such as Gulf of Mexico and Caribbean are distributed in latitudes where the frequency of hurricanes and cyclones is high, resulting in strong effect on mangrove forest structure and community dynamics. Several patterns have been observed in Florida, Puerto Rico, Mauritius, and British Honduras. Species attributes and availability of propagules are important factors along with the severity of storm and sediment disturbance in projecting recovery patterns. Frequent storm disturbance tends to favor species capable of constant or timely flowering, abundant seedling or sprouting, fast growth in open conditions, and early reproductive maturity. Woody debris resulting from these disturbances have an important role in biogeochemical properties of disturbed mangrove forests. Although mangrove trees show these 'traits', it is important to consider the cumulative impact of human activities on these ecosystems in conjunction with the complex natural cycle of regeneration and growth of mangrove forests. Cyclonic disturbance in areas with higher rates of sea-level rise has been demonstrated to cause sediment collapse (drop in surface elevation) that reduces the ability of mangroves to recolonize disturbed areas. Yet this potential impact may vary across ecogeo-morphic types of mangroves.

River (and surface runoff) diversions that deprive tropical coastal deltas of freshwater and silt result in losses of mangrove species diversity and organic production, and alter the terrestrial and aquatic food webs that mangrove ecosystems support. Freshwater diversion of the Indus River to agriculture in Sind Province over the last several hundred years has reduced the once species-rich Indus River delta to a sparse community dominated by Avicennia marina. It is also responsible for causing significant erosion of the seafront due to sediment starvation and the silting-in of the abandoned spill rivers. A similar phenomenon has been observed in southwestern Bangladesh following natural changes in river channels of the Ganges and the construction of the Farakka barrage that reduced the dry season flow of freshwater into the mangrove-dominated western Sundarbans. Freshwater starvation, both natural and human-induced, has had negative impacts on the biodiversity of mangroves in the Ganges River delta as well along the dry coastal life zone of Colombia (the Cienaga Grande de Santa Marta lagoon).

Deforestation of mangrove wetlands is associated with many uses of coastal environments, including urban, agriculture, and aquaculture reclamation, as well as the use of forest timber for furniture, energy, chip wood, and construction materials. Two reclamation activities that have contributed to examples of massive mangrove deforestation are agriculture and aquaculture enterprises. Agriculture impacts on mangroves are most noted in West Africa and parts of Indonesia. Many of the large agricultural uses are found in humid coastal areas or deltas where freshwater is abundant and intertidal lands are seasonally available for crop production. Mariculture use of the tropical intertidal zones, in the construction and operation of shrimp ponds, has become one of the most significant environmental changes of mangrove wetlands and water quality of tropical estuaries in the last several decades.

Oil spills represent contaminants to mangroves that can alter the succession, productivity, and nutrient cycling of these coastal forested wetlands. These impacts have been well documented in ecological studies in Puerto Rico, Panama, and Gulf of Mexico. An oil slick in a mangrove wetland will cause a certain mortality of trees depending on the concentration of hydrocarbons and species of trees, as well as the edaphic stress levels already existing at the site. Thus, those mangroves in dry coastal environments may be more vulnerable to oil spills than those in more humid environments.

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