Abstract

Suspended particulate matter (SPM) in aquatic systems is comprised of many inorganic and organic components with much of the organic matter not characterized. This study characterized and investigated the spatial variability of particulate organic carbon (POC) in the surface waters of the St. Lawrence Estuary and Gulf to discern the contributions of terrestrial and marine organic matter (OM). Using bulk elemental and isotopic analyses alongside lipid molecular biomarkers, specifically hydrocarbons and fatty acids, we characterized POC from surface water samples collected at 19 stations spanning eight scientific missions between 2003 and 2023. This comprehensive dataset is the first to incorporate both molecular and bulk analyses of POC in the surface waters of this system providing a baseline understanding of OM composition and source contributions in this dynamic system. Our findings reveal distinct spatial patterns in OM composition, with terrestrially derived OM dominating the Upper St. Lawrence Estuary (ULSE) and a gradual shift towards marine-derived OM as distance from Quebec City increases i.e., downriver. High molecular weight (HMW) n-alkanes (C27, C29, and C31) are prevalent in terrestrially influenced stations while low molecular weight (LMW) n-alkanes (C15, C17, and C19) dominate marine stations. Stable carbon isotopes (δ13C) and C/N ratios also reflect the transition from depleted to more enriched δ13C values along the gradient.
A multivariate approach was used to identify spatial variability using principal component analysis (PCA), broken stick analysis, and SIMPER. Using these techniques, we attempted to identify the primary drivers of OM composition across the continuum. Salinity, n-alkane proxies, and distance from Quebec emerged as key factors influencing OM distribution. Our results suggest that OM composition in the SLEG (St. Lawrence Estuary and Gulf) is controlled by both hydrodynamic processes and terrestrial-marine interactions. This study establishes a critical baseline for understanding the sources and spatial variability of OM in the SLEG which contribute valuable information into the biogeochemical processes that shape this important system. These findings emphasize the need for continued monitoring to evaluate future changes driven by natural and anthropogenic changes as well as climate change.