Sources and Exchange of Particulate Organic Matter in an Estuarine Mangrove Ecosystem of Xuan Thuy National Park, Vietnam

The spatio-temporal variations in stable isotope signatures (δ 13 C and δ 15 N) and C/N ratios of particulate organic matter (POM), and physicochemical parameters in a creek water column were examined in an estuarine mangrove ecosystem of Xuan Thuy National Park, Vietnam. The objective was to examine the factors influencing creek water properties, and the sources and exchange of POM in this important mangrove ecosystem. The diel and seasonal variations in water temperature, flow velocity, pH, dissolved oxygen, and salinity demonstrated that tidal level, season, and biological factors affected the creek water properties. Mangroves had relatively low δ 15 N and very low δ 13 C values, with respective average values of 1.5±0.9‰ and −28.1±1.4‰. The low mangrove leaf δ 15 N indicated minor anthropogenic nitrogen loading to the mangrove forests. A significant positive correlation between POM—δ 13 C and salinity along the axis of Ba Lat Estuary, Red River, indicated that marine phytoplankton (δ 13 C value, −21.4±0.5‰) was the predominant source of POM at the estuary mouth. Based on the co-variation of δ 13 C and C/N ratios, marine phytoplankton and mangrove detritus were predominant in POM of major creeks and small creeks, respectively. During the diurnal tidal cycle, the dynamics of POM were affected by sources of organic matter, tidal energy, and seasonal factors. The contribution of mangrove detritus to POM reached a maximum at the low tide and was enhanced during the rainy season, whereas marine phytoplankton contribution was highest at high tide.