The Priming Effect in Sediments of a Cold Temperate Estuarine System: An Assessment Using Compound Specific Stable Carbon Isotope Ratios Measurements on Bacterial Fatty Acids

As the largest semi-enclosed estuarine system in the world, the St. Lawrence Estuary and Gulf is an ecosystem rich in natural resources and very important in terms of biodiversity, as well as economic, transportation and recreational activities. Since the beginning of the industrialization of the St. Lawrence Valley, this aquatic system has been threatened by human activities resulting in increased industrial and agricultural pollution, eutrophication (nutrient enrichment), biodiversity loss, and landscape deterioration, culminating in the depletion of dissolved oxygen in its bottom waters (hypoxia). Deep water hypoxia conditions have been steadily worsening in the past 80 years, reaching dissolved O₂concentrations has low as 35 µM in the fall of 2021. Hypoxia in this system is fueled by changes in oceanic circulation in the North Atlantic as well as by an increase in the water column flux of organic matter (OM) either discharged by the St. Lawrence River and other tributaries (terrestrial OM), or produced in the surface waters from discharged and upwelled nutrients (marine OM). The consumption of the more labile OM components of this sedimenting OM by aerobic heterotrophic bacteria results in sustained pressure on dissolved O₂ concentrations and the accumulation of the more recalcitrant fraction of this OM. As cold temperate estuarine systems such as the St. Lawrence are characterized by large seasonal variations in riverine discharge rates and in situ primary production, mineralization of the more recalcitrant sedimentary OM components should be strongly modulated by the priming effect resulting from sudden influxes of fresh and more labile OM. In this study, we will attempt to quantify the priming effect in this system using elemental (organic carbon and total nitrogen) and isotopic (∂¹³C and ∂¹⁵N) mass balances, as well as compound specific stable carbon isotope analysis of the bacterial fatty acids iso- and anteiso-C15:0. We will use a batch incubation approach in which natural sediments from the St. Lawrence Estuary and Gulf will be amended with fresh terrestrial or marine OM characterized by a very different ¹³C/¹²C ratio (difference of between 10 and 14 ‰ depending on the sampling station). Quenching of the incubations followed by the extraction, quantification and isotopic characterization of the bacterial fatty acids will allow determining the effect of labile OM on the mineralization of recalcitrant OM in this system. Acquisition of this knowledge, combined with results from other studies carried out in our lab, will provide a better understanding of the relative importance of terrestrial and marine OM processing in the onset of hypoxia and will be exploited as a guide for remedial efforts aiming to improve the health of such an important ecosystem.