Material Dispersion Mechanism Maintaining Tropical Mangrove Ecosystems

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Mangrove ecosystems in tropical/sub-tropical coastal areas play much inportant roles for preservation of natural environment, food resources and coastal protection in grobal scale. However, their quantitative mechanisms are few accumulated, especially from the view of the study of water area. Mangroves colonize in the intertidal area. Their ecosystems are strongly affected by the movement of water due to tide and waves.In this study, we aim mainly to quantify the processes of physical dispersion of water-mass and particles moving with water, and to accumulate the physical information connecting the mangrove ecosystems, according to two methods, the field investigation and the numerical model experiment with computer.The results are as follows ;(1) Field investigation1) For the riverine forest type mangroves (R-type), the flooding over the mangrove swamp from the creek makes the unique material dispersion, ie. "tidal trapping".2) The oxygen budget in the mangrove swamps is strongly controlled by the offshore water flowing into the swamp with tidal motion, the production by photosynthesis and the aeration through the water surface and the consumption at the bottom mud inside the swamps.3) For the fringing type mangroves (F-type), the offshore waves are considerably damped by the drag force of mangrove vegetation. The wave reduction depends on the water depth which varies with tide, because the mangrove trees have manny drag bodies (prop roots and pneumatophore) near the bottom. Accordingly, the magnitude of the material dispersion depends on the conditions of not only waves in offshore but also tide, the density of vegetation and its vertical distribution.(2) Numerical experiment1) For the R-type, the process of particle dispersion is very different from the usual river which has not mangrove swamp (ie.flooding area).2) The magnitude of the dispersion coefficient in the R-type varies depending on the density of mangrove vegetation.