Organic matter sources in an enclosed coastal inlet assessed using lipid biomarkers and stable isotopes

The sources of organic matter (OM) in surface sediments from Wilson Inlet, an enclosed and seasonally barred inlet in Western Australia, have been determined using a combination of lipid biomarkers (including fatty acids, sterols, phytol, long chain alcohols, alkyl diols, tetrahymanol and C 32 hopanol) and stable carbon and nitrogen isotope data for bulk OM and δ 13C data for n-alkanols. The organic composition of sediments from the inflowing rivers was used as a proxy for terrestrial inputs of higher plant OM, since these sediments contained only small amounts of carbon from microalgae (mostly diatoms), bacteria and cyanobacteria. The relative proportions of “marine” and “terrestrial” OM in the sediments could be estimated using the stable carbon isotope signature of the bulk organic carbon. The sediments from Wilson Inlet showed similar lipid compositions and isotope values, and the sterol and fatty acid distributions showed the complexity typical of marine ecosystems. Relative contributions from algal (phytoplankton and microphytobenthos), bacterial, terrestrial plants and seagrass sources were estimated using biomarker/organic carbon ratios in the different source terms. Microalgae and bacteria are the most important source of OM within the inlet (over 80% in most sediments) and most of the algal material originates from the water column, except in sandy shallow sediments at the eastern end of the inlet where a contribution from microphytobenthos can be discerned. The distributions, abundances and δ 13C isotope values of long chain n-alkanols proved to be useful in quantifying the contributions from terrestrial plants and the seagrass Ruppia megacarpa. The latter is a relatively small contributor (< 10%) to the OM in the sediments, despite the presence of large seagrass beds around the edges of the inlet and the presumed importance of seagrass as a source of carbon in the foodwebs of the inlet. Nonetheless, seagrass is a significant contributor of the “higher plant” marker 24-ethylcholesterol (sitosterol) in the sediments.