Optical and molecular indices of dissolved organic matter along a land-to-ocean continuum

Résumé fourni par la source

. Dissolved organic matter (DOM) is a critical component of the global carbon cycle and various ecosystem functions. The complexity of its chemical structures and environmental processes requires an extensive analytical workflow for comprehensive characterization, whereas DOM implication in various ecological functions necessitates large-scale environmental investigation. Here, we measured the optical properties of chromophoric DOM (CDOM) and the molecular indices of solid-phase extractable DOM (SPE-DOM) in the vast land-to-ocean continuum of the St. Lawrence system, which encompasses diverse geological watersheds, land uses, vegetation, hydrology, water chemistry and ecosystem functions. The continental waters of Lake Ontario’s watersheds and the Saguenay Fjord are characterized by allochthonous DOM of high aromaticity, large molecular size, and low molecular lability. The molecular indices of SPE-DOM are in agreement, as indicated by the double-bond equivalent and aromaticity index. On the other hand, the DOM of larger water bodies and the marine endmember is smaller, less aromatic, more saturated, has lower O/C ratios, and contains more sulphur and phosphorus. While the results of the two approaches are consistent in fresh and marine waters, discrepancies occur in estuarine waters: SPE-DOM molecular indices varied in a conservative manner with increasing practical salinity, whereas CDOM optical properties remained quasi-constant. Such divergence warrants further investigation to calibrate better these two unique sets of techniques for the highly dynamic estuarine ecosystems. The interpretation of DOM dynamics using optical and molecular indices requires caution in integrating the results into a holistic perspective that encompasses the origins of DOM, its transport and transformation processes, and ultimately, CO 2 emissions from continents to the global ocean.