Quantitative estimation of the distribution and biomass of seagrasses at Haad Chao Mai National Park, Trang Province, Thailand

Abstract

           A seagrass bed at HaadChaoMaiNational Park, TrangProvince, is one of the largest seagrass beds with the highest species diversity in Thailand. To elucidate distribution and abundance of seagrasses over the entire bed, we carried out a large-scale quantitative census at subtidal and intertidal bottoms covering ca. 18 km² area. On December 1999, a total of 69 stations were set at subtidal and lower intertidal area, and 183 stations along 10 transect lines at upper intertidal area. At each station, coverage of each seagrass species was recorded and aboveground biomass was estimated by the rapid visual technique developed by Mellers (1991). A total of 8 species (Enhalus acoroides, Halophila ovalis, Thalassia hemprichii, Cymodocea rotundata, C. serrulata, Halodule uninervis, H. pinifolia and Syringodium isoetifolium) were recorded by the census. At subtidal and lower intertidal areas, biomass was highest at shallow stations (<2 m) along the coastlines, while seagrass was rarely observed at the deeper stations and at some shallow stations on sand dunes developed at a river mouth. E. acoroides was the most abundant species, followed by H. ovalis and T. hemprichii. Six seagrass species were found at the upper intertidal area. H. ovalis and T. hemprichii were dominant, and they formed monospecific patches at sand dunes and tide pools, respectively. Average aboveground biomass was 1.5 times  higher  at  the  intertidal area  (15.37 gDW m^-2 ) than the subtidal (10.64 gDW m^-2 ). Therefore, the contribution of intertidal seagrass meadow to total standing stock of the study site (23%) was higher than expected from the proportion of area it occupied (17%). Distribution, species composition and biomass of the seagrass bed are affected by at least 2 factors; water depth (possibly related to light availability)  and physical disturbance such as sand movement.