Microtidal subterranean estuaries as a source of fresh terrestrial dissolved organic matter to the coastal ocean

This paper reports on the distribution of DOM along the subterranean estuary (STE) of a microtidal beach located in the Gulf of St-Lawrence (Îles-de-la-Madeleine, Quebec, Canada). We combined porewater DOC concentrations as well as CDOM absorbance- and fluorescence-derived indices (SR, SUVA254, FI and BIX) to explore the groundwater-borne DOM transformations along the groundwater flow path to the outflow face of the beach. Based on these optical indices and multivariate analysis, results reveal a highly complex environment where biogeochemical processes and water-rock interaction alter molecular composition of DOM. Marine and groundwater end-members exhibited significantly different optical derived indices. Along the STE, however, DOC and CDOM showed non-conservative behaviour with the production of both “new” high molecular weight (MW) and lignin-derived compounds as well as microbially-derived CDOM. There is no significant relationship between DOC and CDOM optical properties because of the complex and dynamic system. A redundancy analysis (RDA) revealed a relationship between redox conditions and DOM pool signature. The RDA results highlighted a negative correlation between DOC and dissolved Fe, particularly in the suboxic zone of the STE where Fe-oxides are reduced. The RDA showed that the increase of SR and SUVA254, (i.e., from high MW CDOM compounds with low aromaticity to low MW CDOM compounds with high aromaticity) is inversely correlated to dissolved metals, particularly total dissolved Mn. Whatever the processes involved in the carbon behaviour, our results showed the dominance of terrestrial DOM pool all along the STE. Despite the fact that marine-derived particulate solutes are considered as the primary source of DOC in tidal sands, our study illustrated how groundwater-borne solutes may affect reaction in STE and opens questions about the terrestrial DOM export to a coastal bay.