Lipid biogeochemistry in the Laurentian Trough: I—fatty acids, sterols and aliphatic hydrocarbons in rapidly settling particles

Sinking particles intercepted at 150 m depth in May and July at landward (L) and seaward (S) sites in the Laurentian Trough contained 540–2910 μg g−1 of fatty acids (FA), 54–990 μg g−1 of sterols and 101–184 μg g−1 of aliphatic hydrocarbons. The vertical fluxes averaged 7.2, 3.1 and 0.8 mg m−2 d−1, respectively. FA fluxes (greater at site L) and concentrations (higher at S and during July) reflected the terrestrial-marine gradient and the seasonal patterns of primary production in the estuary. Unresolved anthropogenic hydrocarbons (UCM) displayed an opposite trend. The sources and behaviour of individual lipid biomarkers were evaluated by multivariate techniques. FA 16:0 and 16:1, heneicosahexaene and 24-methylcholest-5-en-3β-ol were the most consistent tracers of an early diatom bloom sampled at the seaward site. FA 14:0 and 15:0, squalene and 24-methylcholesta-5,22E-dien-3β-ol also traced this event but were less specific. The summer increase of primary production was reflected by cholestanol, 24-methylenecholesterol, 24-norcholesta-5,22E-dien-3β-ol, 24-ethylcholesta-5,22E-dien-3β-ol and dinosterol. Zooplanktonic pristane, unsaturated FA, cholesterol and cholesta-5,22E-dien-3β-ol, were abundant in all the traps. At the landward site, cholesterol and cholesta-5,22E-dien-3β-ol covaried with dinosterol, suggesting a dinoflagellate contribution as well. These samples were very heterogeneous and showed variable inputs from higher plants (stigmastanol, FA 24:0, 26:0, long-chain n-alkanes), petrogenic hydrocarbons (n-alkanes, UCM) and plankton (unsaturated FA, cholesterol), reflecting the combined contribution from the Upper St. Lawrence Estuary, Saguenay Fjord and a periodic upwelling.