Spatiotemporal variations of organic matter sources in two mangrove-fringed estuaries in Hainan, China

Abstract Mangrove systems represent important long-term sinks for carbon since they have much higher carbon burial rates than terrestrial forests or typical coastal ecosystems. However, quantifying the sources of organic matter (OM) in estuarine and coastal sediments, where mangroves occur but are not the only source of OM, is challenging due to the variety of OM sources and diverse transport processes in these dynamic environments. The sources of OM in subtidal surface sediments of two mangrove-fringed estuaries in Hainan Province, China, were investigated using the mangrove-specific biomarker taraxerol and other lipid biomarkers, as well as stable carbon isotopes. Mixing models based on the concentration of taraxerol, plant wax n-alkanes and δ13COM indicate that terrestrial non-mangrove plant OM accounted for 40–57% of the OM in the two estuaries, phytoplankton OM accounted for 24–45%, and OM from mangroves comprised 15–19% of the total. Terrestrial plants contributed 10–21% more of the OM to subtidal sediments of Bamen Bay, which is on the wetter, eastern side of Hainan Island, than to Danzhou Bay, but the phytoplankton OM fraction was 16–24% lower than that in Danzhou Bay subtidal sediments. In both estuaries, mangrove and phytoplankton OM fractions increased seaward while the terrestrial OM fraction decreased. On a seasonal basis, lipid biomarker concentrations indicated a small change of OM sources. The biomarker and carbon isotope approach used here can be applied to semi-quantitatively estimate spatial and temporal variations of the sources of organic carbon in tropical estuarine and coastal sediments, a major sink for carbon in the ocean.