Variability in the distribution of lipid biomarkers and their molecular isotopic composition in Altamaha estuarine sediments: implications for the relative contribution of organic matter from various sources

The estuarine mixing zone is an effective trap for particulate and dissolved organic matter from many sources, and thus greatly affects transport and deposition of organic matter between the land and ocean. This study examined sedimentary distributions of various fatty acids and their stable isotope compositions at three sites which represent different levels of mixing in the Altamaha River estuary, Georgia, USA. Distributions of other neutral lipids (phytol, C14–C32 fatty alcohols, and two sterols) were also determined as supplementary biomarkers from various sources. Results suggest that POM from terrestrial plant material was dominant at an initial, upper estuary mixing zone (salinity in the water column is near zero but porewater in the sediment is saline) while POM from marine sources was mainly deposited at a more intensive mixing zone (wherein salinity in surface and bottom waters was identical) in the Altamaha. Much less particulate lipid was deposited in Altamaha River mouth sediments even during the high discharge season, suggesting that terrestrial plant-derived POM may not be significantly transported across the mixing zone. Analyses of fatty acid isotopic composition further verify the differential deposition pattern of POM from different sources.