Origin and transport of sedimentary organic matter in two subtropical estuaries: a comparative, biomarker-based study

Two sub-tropical estuaries of south Florida, USA, were studied to determine the origin and transport of organic matter between the freshwater and marine end-members of these systems. Sediments, as well as main biomass components (vegetation) were analyzed for lipid composition and content. The molecular distribution of several biomarker compounds, organism-specific biomarkers and bulk sediment characteristics such as %OM, C/N and δ13C were used to assess differences in OM source and transport between a strongly tidally-influenced estuary (Harney River) and one that is only seasonally influenced by estuarine waters (Taylor River). The data show mixing of combined terrestrial and autochthonous freshwater-derived OM, with marine planktonic and seagrass-derived OM in the middle to lower estuary of the Harney River. In contrast, such mixing did not occur to a measurable degree at the Taylor River, where low water discharge and exchange does not allow for any significant mixing of OM from end-member sources. Differences in hydrological conditions and benthic plant biomass and productivity between the two systems results in different OM inputs, which are reflected in the quality and degree of diagenesis of the sedimentary OM. Specific applications and potential limitations of traditional biomarker compounds for OM source assessment are discussed.