Abstract

The St. Lawrence River is one of the most important in the world in terms of water discharge to the global ocean. Several studies have been made to describe the environment, from its water dynamics to the activity of its phytoplankton communities. Despite this, a thorough characterization of the St. Lawrence Estuary (SLE) with respect to its organic carbon (OC) sources and sinks has not been done, and shortcomings pertaining to information needed for a comprehensive OC budget for the SLE have not been identified. Using samples collected over several sampling missions since 2003, quantitation and characterization of organic matter (OM) has been performed on dissolved, particulate and sedimentary samples. Measurements, using a DOC-analyzer coupled to an isotope mass spectrometer (IRMS), and elemental analyzer (EA) coupled to an IRMS, have shown that dissolved OC (DOC) and particulate OC (POC) concentrations decreased closer to the St. Lawrence Gulf. In addition, DOC, POC and sedimentary OC (SOC) samples closer to the gulf showed 13C enrichment in, and a decrease in C/N atomic ratios in particulate samples. Parallel to these trends, an increase in Fe-OC association was observed. In this thesis, major sources and sinks have been identified and a gradual shift from terrestrial to marine OM characteristics have been observed in carbon stable isotope signatures, C/N atomic ratios and degree of Fe-OC association. Furthermore, a simple budget was constructed to help direct future research efforts towards a more complete understanding of the SLE.