Lipid biomarkers in the Conwy Estuary (North Wales, U.K.): a comparison between fatty alcohols and sterols

Fatty alcohols (FAs) and sterols were quantified in surface sediment samples taken from Conwy Bay, the Conwy Estuary and in the freshwater tributaries of the river. Total FA concentrations ranged from ˜ 230 to ˜ 20,000 μg g−1 dry weight and the total sterols from 2.4 to 125 μg g−1. In the marine samples, the short-chain C16 FAs dominated although longer-chain, terrestrially derived moieties were also present. Samples from the freshwater sites had C22---C26 FAs present in the greatest concentration. A series of Alcohol Source Indices (ASI) based on the ratio between selected terrestrial FAs and marine FAs was developed on this basis. Other such ratios based on the short/long, odd/even and branched/n-alkanol distribution highlighted regions of proposed bacterial diagenesis. Comparison was made to the traditionally used terrestrially derived sterols (β-sitosterol and stigmasterol) although the ergosterol (associated with decaying organic matter)/cholesterol ratio provided the best correlation to the ASI. A new sterol derived from the short-term diagenesis of ergosterol in mildly reducing conditions (ergosta 5,8(14),22-trien-β-ol, proposed trivial name, munsterol) was identified in the lower estuarine site and Principal Components Analysis of the data indicates that a clearer separation of the geochemical sources can be seen with the sterols and suggests that the fatty alcohol provide more information on secondary processes rather than primary origins. On the basis of the biomarker profiles, the estuary was divided into the marine environment, the lower estuary dominated by sewage discharges and marine production, the upper estuary with increasing amounts of terrestrial organic matter, and the freshwater sites with terrestrially derived materials and sewage discharges.