Depth-dependent DOM dynamics in the Saguenay Fjord (Quebec, Canada)

Dissolved organic matter (DOM) is central to carbon cycling in coastal and estuarine systems, yet its dynamics in strongly stratified fjords remain poorly understood. We examined depth-dependent DOM composition in the Saguenay Fjord, a sill-bounded system influenced by freshwater from the Saguenay River and saline intrusions from the St. Lawrence Estuary. Using optical measurements and high-resolution mass spectrometry in both positive and negative ionization modes, we identified distinct molecular and optical signatures across depths and basins. DOM was largely allochthonous, with humic-like fluorescence dominating surface waters (80 ± 7%) and decreasing at depth (50 ± 8%). Molecular profiles revealed highly unsaturated carbon (31–33%), carboxyl-rich alicyclic molecules (19–20%), and condensed aromatics (18–19%), with CRAM enriched upstream and condensed aromatics near the fjord bottom. Nitrogenous compounds and increasing apparent oxygen utilization at depth indicated microbial and sediment-driven transformations. Strong stratification isolates deep waters, while surface mixing promotes transport. These findings highlight how terrestrial inputs, microbial activity, and water mass structure shape DOM composition, influencing carbon cycling and export to the St. Lawrence Estuary.

Presentation Preference: Standard Oral (12 Minutes)

Primary Presenter: Celine Gueguen, Universite de Sherbrooke (celine.gueguen@usherbrooke.ca)

Authors:

Céline Guéguen, Universite de Sherbrooke  (celine.gueguen@usherbrooke.ca)

Mohamed Amri, Universite de Sherbrooke  (mohamed.hedi.amri@usherbrooke.ca)