Sources and distributions of terrigenous organic matter in a mangrove fringed small tropical estuary in South China.

The sources and distributions of terrigenous organic matter (OM) were investigated in a small tropical estuary in the Hainan Island, South China. Plants, suspended particulate matter (SPM), and surface sediments samples in the estuary and coast were collected. Bulk properties [organic carbon (OC%), total nitrogen (TN%), stable carbon isotope ( δC) and grain size] and lignin phenol concentrations were measured. OC% of mangrove plants was (43.4±2.1)%, which is similar to the values reported form an grove plants in other regions. OC% of sediment samples ranged from 0.07% to 1.42%, and they were related to the sediment texture. Lignin phenols in the sediment ranged from 5.16 mg/100 mg OC in the uppermost station to 0.51 mg/100 mgOC in the coast. The molar ratio of organic carbon to total nitrogen(C/N) (∼7) and δC (∼−31.1×10) of riverine SPM revealed that the major OM sources of riverine SPM were aquatic OM (phytoplankton and/or bacteria). Moreover, the lower lignin concentration (Λ8) and higher (Ad/Al)v of lignin phenols suggest that terrestrial OM in riverine SPM were mainly from soil. Further more, C/N ratio, δC and lignin phenols reveal that mangrove plants were the predominant OM sources of mangrove surface sediment. Based on the δC and lignin phenols, it can be concluded that the major OM sources in estuarine and coastal surface sediments were marine phytoplankton, riverine SPM and mangrove surface sediment. In addition, the higher (Ad/Al)v of lignin phenols in those coastal sediments indicate that sea grass might be a potential OM source in coastal sediments, however, the lower (Ad/Al)v in the estuarine sediments in turn suggests that sea grass could not be transported to the mangrove fringed region. A three-end-member model which is based on lignin concentrations and δC was applied to evaluate the contribution of mangroves to the organic matter preserved in the surface sediments. Around the mangrove fringed region, mangrove could contribute more than 50% to the sedimentary OM, and this value is much higher than riverine OM. Nevertheless, mangrove OM could not be efficiently transported to the coastal region. Our study suggests that mangrove forest is an important OM source in this small estuary. [ABSTRACT FROM AUTHOR]