Abstract: To examine the spatial distribution of key enzyme activities influencing sediment organic carbon (SOC) transformation in surface sediment and their response to nutrient enrichment in seagrass bed in Xincun bay,Hainan Island,three transects were carried out according to different nutrient loading.The content ranges of polyphenol oxidase,peroxidase,invertase and cellulas were 0.10~0.24 mg/g/2 h,0.14~0.73 mg/g/2 h,0.24~0.69 mg/g/24 h and 0.03~0.36 mg/g/72 h,respectively,with the average content as 0.17 mg/g/2 h、0.36 mg/g/2 h、0.43 mg/g/24 h and 0.19 mg/g/72 h,respectively.Enzyme activities in surface sediment beneath Thalassia hemprichii were generally higher than that of Enhalus acoroides in the same transect.In general,enzyme activities in our investigating locations were lower than other coastal wetlands.Moreover,these enzyme activities showed an increasing trend with increasing nutrient loading.Therefore,increasing nutrient load was probably favorable for stimulating seagrass surface sediment enzyme activities and accelerating SOC transformation rate,which might change carbon biogeochemical cycle in particular seagrass bed ecosystem.Further eutrophication might weaken seagrass beds sediment carbon storage,and thus influence the "blue carbon" function.