Abstract: To examine the temporal and spatial variation of dissolved organic matter (DOM) and its controlling factors in seagrass bed, seawater DOM was investigated in winter and summer seasons in Xincun Bay, Hainan Island. Seawater salinity showed an increasing trend from the northern estuary to the southern seagrass area, while seawater pH presented a decreasing trend from southwest to northeast region in both winter and summer. The content ranges of dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) were 1.93-8.89, 0.29-2.44 and 0.002-0.45 mg·L^-1, respectively, with the average contents of 4.73, 0.64 and 0.08 mg·L^-1, respectively. The highest content of DOC occurred in seagrass area in winter, and that occurred in estuary in summer. The DON in the two seasons showed high content in fish cagefarming and the adjacent seagrass area. DON content in seagrass area was lower in summer than in winter. The highest content of DOP was found in the estuary or fish cage-farming area. Higher DOC/DON was observed in summer than in winter, while DOC/DOP and DON/DOP showed an inverse trend. Our study revealed that the main sources of DOM were seagrass release, inputs from river runoff and fish cage-farming in Xincun Bay. This study also indicated that the DOM in winter had relatively high bioavailability, which can enhance the cycle of microbial food loop. Seasonal variation of seagrass growth, river runoff and light intensity may alter the distribution and composition of DOM, and influence the biogeochemical cycle of carbon, nitrogen and phosphorus in seagrass bed ecosystems.

Key words: ecosystem assets, ecosystem services, ecosystem accounting.