Changes in seagrass communities along the runoff gradient of the Trang river, Thailand

Seagrasses are widely distributed from coastal estuaries through to deep waters. We examined the spatial pattern in seagrass species composition, biomass, and their relationships with various physical factors: salinity, mean sea level, light extinction coefficient, temperature, grain size (gravel, coarse sand, fine sand, clay), and soil organic matter between dry and rainy seasons along the gradient of Trang river mouth around Talibong Island, Trang province. Ten species were found in an area of 61 km² in the dry season, whereas only eight species were found in 24 km² in the rainy season. Halophila decipiens Ostenfeld was the dominant species covering 29 km² in the dry season. Enhalus acoroides (Linnaeus f. Royle) and H. ovalis (R. Brown) J.D. Hooker extended over a large area. Density, area covered, and biomass decreased in the rainy season for most species. A clear seasonal variation was observed both in physical and chemical parameters as well as seagrass species composition, biomass, and reproduction. Water depth and light intensity were the limiting factors that influenced species composition, biomass, and reproduction.