Lipids as biomarkers for carbon cycling on the Northwest Shelf of Australia: results from a sediment trap study

Sediment traps were deployed on the Northwest Shelf (NWS) of Australia in November 1996, to determine fluxes of organic matter and inorganic elements from the photic zone to deeper waters in a transect extending from Exmouth Shelf to Exmouth Plateau. Infiltrex II water samplers collected particulate and dissolved organics from the water column near the trap sites. Surface sediments and sediment cores were also collected over the study region. Lipid biomarkers were used to determine the sources of organic carbon and its cycling processes on the NWS. Dry weight fluxes from the traps ranged from 124 to 616 mg m−2 day−1and particulate organic carbon (POC) fluxes ranged from 22 to 42 mg m−2 day−1. The biogenic lipids consisted of biomarkers indicative of marine zooplankton, phytoplankton and bacteria, plus traces of land plant markers. A large contribution of unresolved complex material (UCM), which is indicative of petroleum, was detected at four times the biogenic hydrocarbon flux at shallow stations, and up to seven times the biogenic hydrocarbon flux at the most offshore station. There is essentially no river input, and only trace aeolian-derived material to contribute to primary production on the NWS of Australia. Most of the organic matter produced are rapidly recycled in the water column and the small fraction of lipids that settle to the sediments is already partially degraded and undergoes further rapid degradation in the surface sediments. Natural oil seeps also provide utilisable organic carbon to the system. The production and vertical flux rates of organics determined in this study are comparable to those reported in studies of shallow traps in oceanic areas from long-term studies in the Arabian Sea, and other coastal margins such as the Bay of Biscay (France) and California (USA). In offshore areas, most “living” lipid materials passed through the GFF filters thus invalidating POC estimates based on high volume sampling. To adequately assess living (particulate) carbon, gentle filtration of low volume seawater samples is more accurate, as shown by this solid phase absorption study of individual lipid biomarkers.