Material Dispersion Mechanism Maintaining Tropical Mangrove Ecosystems

热带/亚热带沿海地区的红树林生态系统在全球尺度上对保护自然环境、食物资源和海岸保护具有重要作用。然而，从水域研究的角度来看，其量化机制却鲜有积累。红树林在潮间带定居。它们的生态系统受到潮汐和波浪引起的水分运动的强烈影响。本研究旨在通过野外调查和计算机数值模拟实验两种方法，量化红树林生态系统中水团和颗粒随水运动的物理扩散过程，积累与红树林生态系统相关的物理信息。结果如下：(1)野外调查1)对于河林型红树林(R型)红树林，来自小溪的洪水淹没红树林沼泽造成了独特的物质弥散，即：2)红树林沼泽的氧气收支受潮汐作用下流入沼泽的近岸水、通过水面的光合作用和曝气产生的水以及沼泽内底层泥浆的消耗强烈控制。3)对于边缘型(F型)红树林，近海波浪受到红树林植被的阻力的很大抑制。消波作用取决于随潮汐变化的水深，因为红树林在底部附近有曼尼拖动体(支柱根和气浮体)。相应地，物质弥散的大小不仅取决于近海的波浪条件，还取决于潮汐、植被密度及其垂直分布。(2)数值试验1)对于R型，颗粒弥散过程与通常没有红树林沼泽的河流(即洪泛区)有很大的不同。2)R型弥散系数的大小取决于红树林植被的密度。

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.