Seagrass beds are among the most important components in coastal ecosystem, but are susceptible to human-induced changes in terrestrial ecosystem. As a first step to understand the effects of terrestrial input through river discharge on tropical seagrass ecosystems, we carried out quantitative mapping of seagrass beds at Pang-gna and Trang Provinces, southwestern Thailand to examine spatial variation in species composition, species diversity and biomass of seagrasses. Five seagrass beds were chosen as study sites that locate at different distances from the major river mouths, and thus subjected to different impacts of river discharge. These seagrass beds differed greatly in environmental conditions, seagrass diversity and abundance. Seagrass beds located near large river mouths showed higher silt-clay content in the sediments and light attenuation of water, possibly due to transportation of sediment from rivers and its resuspension. A total of nine seagrass species occurred. Enhalus acoroides and Halophila ovalis occurred in all the stations, whereas distribution of other species were mostly confined to seagrass beds with small or moderate degree of siltation and light attenuation. Seagrass species diversity measured by species richness and Simpson\u27s diversity index was lower in seagrass beds near the river mouth. These findings highlight negative impacts of terrestrial ecosystems through river discharge on seagrass biodiversity and its functioning along the coastal areas of southwestern Thailand
