TDA-IME Project Final Report June, 2013 Land elevation is important because the physical development of mangrove systems depends on the accumulation and deposition of organic muds around mangrove roots; transport of terrestrial materials from eroding continents by rivers; and the net up-estuary transport of sand-sized materials from nearshore waters. As in-filling and progradation continue and coastal landforms progress through different stages of geological development, mangrove succession occurs, with one plant and animal community preparing the habitat for another, until a climax community is reached. Thus, mangroves can be considered both a pioneer and a climax biological community (Tomlinson, 1986). Mean sea level also varies during the year, which causes more extensive inundation of mangroves after the peak hot season and after the season of most heavy rainfall. The annual cycle in mean sea level along coasts is explained by changes in specific volume (steric changes) as a result of summer heating of shallow waters with depths less than 100 m (Pattullo et al., 1955), and periods of local high rainfall/runoff. The effects are significant, even along tropical coasts, with a seasonal variation in mean sea level of 10-30 cm. In the tropics, the temperature variability is less pronounced; thus seasonal mean sea level variability is more a function of changes in synoptic wind and atmospheric pressure patterns; changes in the coastal ocean circulation, and seasonal pulses in freshwater runoff, including monsoon effects. For example, extreme seasonal mean sea level changes of 1.65 m occur in the upper Gulf of Bengal due to monsoon runoffs (Pattullo et al., 1955). Mangrove ecosystem processes, and hydrological forcing functions, are influenced by the adjacent oceanic conditions, including coastal currents, tide and wave characteristics, sediment dispersal, salinity, meteorologically induced sea level variability (setup and set down), and cyclones/typhoons, and tsunamis, which all affect and control the extent, structure, and productivity of mangrove ecosystems. The most extensive mangrove forests occur on coasts with low wave energy, but these locations may still experience an occasional typhoon or cyclone, or a tsunami-generated wave, that can impact on, or even destroy mangrove forests. Threats and Transboundary Issues Mangrove wetlands in Indochina increasingly are being affected on the seaward side by relative sea level rise along most sedimentary coasts; and by human impacts from the landward side. Overall, hydrological processes in mangroves dominate the resultant environmental setting more than any other process besides conversion of mangroves for alternative economic use. With respect to mangroves, hydrology is the science of the degree and regularity of marine and fresh surface runoff and flooding; of subsurface/groundwater flow, drainage, and storage; and of the cycles and variability in precipitation and evaporation/transpiration. These variables are linked to local weather and climate regimes, and affect the geomorphologic landform characteristics, sedimentation and erosion, soil geochemistry and salinity, and all biotic processes, including mangrove species succession. Perhaps the greatest indirect threat to mangrove wetlands is the modification of coastal hydrological flow and drainage characteristics by construction work: drainage ditches and canals; artificial levees, dikes and roads; and by diversion of water flow for agriculture or aquaculture use. Mangrove ecosystems are not only national in scope; they also extend along coasts independent of political boundaries. The hydrological processes that mangroves depend on may also be transboundary in nature. Mangroves occupy a small area (relative to the size of the entire watershed) in the lower reaches of hydrological basins, which in the case of large river systems can be located in more than one country. Structures such as dams, diversions schemes, other major construction projects, and non-point source agricultural and sewage runoff throughout the watershed can affect the mangrove ecosystem. In this sense, large 28
Transboundary Diagnostic Analysis of Indochina Mangrove Ecosystems
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