Abstract

The bottom waters of the St. Lawrence Estuary and Gulf are currently characterized by low dissolved oxygen levels (hypoxia), which influence the health of this ecosystem. The progression of hypoxia since the beginning of the century is due in part to an increase in the flux of organic matter (OM) and inorganic nutrients discharged in this aquatic system by the St. Lawrence River. The increase in nutrient abundance leads to higher primary productivity and concentrations of dissolved and particulate OM in the water column, which in turn results in a higher consumption rate of oxygen during the degradation of OM by microorganisms. To further our understanding of the carbon cycle in the St. Lawrence system, the biological lability of terrestrially-derived and marine OM was indirectly estimated through the analysis of the 13C/12C ratio of bacteria-specific fatty acid (iso C15:0 and anteiso C15:0) using gas chromatography coupled to an isotope ratio mass-spectrometer (GC-IRMS). Ubiquitous bacteria strains responsible for the degradation of OM were cultivated in marine broth enriched in 13C with 13C-sodium acetate to assess the relationship and isotopic fractionation between the 13C signature of the food source and that of the bacteria. Using this calibration and the isotopic signature of the terrestrial and marine OM end-members, it was possible to determine the proportions of each type of OM being degraded at the different sampling stations along the St. Lawrence Estuary and Gulf continuum. Better constraining of the role of bacteria in terrestrial and marine OM degradation within the St. Lawrence Estuary and Gulf allows for better understanding of the causes driving deep water hypoxia and, eventually, will allow better remediation efforts to improve the health of this important ecosystem. Finally, as part of a small side project, the method used to decarbonate samples with HCl vapour in preparation for elemental analysis (EA) was investigated using a 13C-labelled short-chain organic acid to assess the extent to which the acid is lost through volatilization upon its protonation.