Stable Carbon Isotope Values of Bacteria‐Specific Fatty Acids to Assess the Reactivity of Sedimentary Organic Matter in the St. Lawrence Estuary and Gulf

Abstract Coastal systems have seen increasing pressure from anthropogenic activities since the beginning of the century, affecting the health of these ecosystems. This increased pressure takes the form of an increase discharge of terrestrial organic matter (OM) and nutrients, leading to a higher primary productivity in the surface waters and to higher concentrations of dissolved and particulate OM in the water column. The organic carbon cycle can thus be profoundly modified in these systems, making them more sensitive to further increases in anthropogenic forcings. To improve our understanding of the cycling of organic carbon and heterotrophic bacteria processing of terrestrial and marine OM, the biological lability of OM degraded by bacteria was assessed through the cultivation of bacteria naturally present in sediment in a 13 C‐enriched medium, and assessing the relationship between the δ 13 C values of OM food source and bacterial fatty acids in the St. Lawrence Estuary and Gulf (SLEG). Based on the compound‐specific stable isotope analyses (CSIA) of n ‐alkyl fatty acids (C 10:0 , C 12:0 , C 14:0 , C 16:0 , C 22:0 , C 24:0 , C 26:0 , C 28:0 , C 30:0 ) and branched fatty acid biomarkers ( iso and anteiso C 15:0 ) at 10 stations along the terrestrial‐marine continuum of the SLEG as well as in the cultured cells, the stable carbon isotope composition of the OM degraded by heterotrophs was calculated, suggesting a preferential consumption of the more labile fraction of terrestrial and marine OM sources. The relationship between the bacterial fatty acids and their food source allowed to better understand the dynamics of OM degradation and the factors that control organic carbon cycling in this coastal system.