Sedimentary fatty acids and sterols in the Potomac River

The molecular compositions of fatty acids and sterols in sediments collected along a 320 km longitudinal transect of the Potomac River (mid-Atlantic United States) were investigated to identify the primary sources of lipid organic matter along the hydrologic continuum of the physiographically diverse Potomac River basin. Total fatty acid concentrations in sediments ranged from ∼10 to 400 mug g-1 dry weight, while total sterol concentrations in ranged from ∼3 to 230 mug g-1 dry weight. In all samples, the short-chain C16 fatty acid dominated although longer-chain, terrestrially derived moieties were also present. In most samples, cholesterol dominated although there were also significant amounts of beta-sitosterol and brassicasterol sterols, which are terrestrially and algal derived moieties, respectively. Ratios based on the fatty acid chain structure and sterol structures were used to determine the relative contribution of terrestrial, bacterial, algal, or sewage inputs. Factor analysis of the fatty acids identified two suites of compounds that account for the majority of variability. These fatty acid suites represent allochthonous and autochthonous derived organic matter. Factor analysis of the sterols identified three suites of compounds that account for the majority of variability. These sterol suites represent allochthonous terrestrial plants, allochthonous sewage, and autochthonous marine algae derived organic matter. Samples collected from the upland freshwater sites had prominent terrestrial fatty acid and sterol signatures while samples collected from the tidal region showed significant levels of bacterial fatty acids and algal fatty acid and sterol contributions. Sewage sterol signatures were identified downstream of the point where the South Branch of the Potomac River meets the Potomac located in the upland Ridge and Valley Province and around the urbanized areas of Washington D.C. The fatty acid and sterol profiles revealed changes in the molecular compositions along a downstream gradient ranging from the upland Allegheny Plateau to the Coastal Plain of the Potomac River basin.