Origins of sediment organic matter and their contributions at three contrasting wetlands in a coastal semi-enclosed ecosystem

The origins of sediment organic matter (SOM) and their contributions were studied in three contrasting wetlands (mudflat, estuarine and mangrove) of Daya Bay, South China Sea. Lower sediment delta C-13 but higher delta N-15 values were observed in coastal wetland than in offshore water of the bay. Greater terrigenous organic matter (TOM) contribution to SOM was observed in lower tidal area in mudflat and estuarine wetland. Higher concentrations of total organic carbon and total nitrogen in the three wetlands, as well as lower sediment delta C-13, were found in the wet season. Extremely lower sediment delta N-15 with higher seawater ammonia were observed in estuarine wetland than in mudflat and mangrove, which was caused by the input of N-15-depleted ammonia from petrochemical industrial wastewater. Mangrove contributed substantially to SOM, with a larger contribution in mangrove area than in non-mangrove area. The mean contribution of TOM to SOM was lower in mangrove than in mudflat.