The cycles of C and N in the St. Lawrence Estuary: a molecular and isotopic approach

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Human activities have affected the global environment in ways that have profoundly impacted the global carbon cycle and, in particular, the atmospheric CO2 budget.  While industrialization is responsible for the major fraction of anthropogenic CO2 emissions, deforestation and agriculture also lead to significant changes in CO2 fluxes between the terrestrial and atmospheric carbon pools and dramatically modify terrestrial organic matter (TOM) fluxes to aquatic ecosystems.  TOM represents a huge source of reduced carbon to estuaries and deltas where it is so extensively removed by sedimentation and mineralization that very little trace of it can be found either in the dissolved organic carbon (DOC) pool of the ocean or in marine sediments.  Environments such as the Lower St. Lawrence Estuary (LSLE) constitute the preferred sites of removal for TOM, where the rapid increase in salinity results in the coagulation and sedimentation of small and colloidal particles.  As deforestation and agriculture dramatically increase TOM fluxes to and TOM metabolism within rivers, their impact on the geochemistry of aquatic systems is substantial.  The detailed mechanisms by which TOM is so rapidly oxidized or removed following its discharge to estuaries are still largely unknown.  Consequently, it is currently impossible (i) to decipher the fine (molecular) controls on CO2 fluxes between rivers/estuaries and the atmosphere, (ii) to comprehensively assess the impact of direct (pollution, nutrient fertilization, land use change) and indirect (climate forcing) anthropogenic disturbances on the biogeochemical environment of estuaries (including the recent development of hypoxic water columns as in the LSLE), and (iii) to efficiently link the terrestrial and marine domains of the global carbon and nitrogen cycles.  To fill these gaps in our knowledge, we will use novel sampling techniques, coupled to state-of-the-art bulk and molecular isotopic methods (including compound-specific isotope analysis), to study the biogeochemical cycles of carbon and nitrogen, and decipher fate of TOM in the LSLE.