The dynamics of absorption coefficients of CDOM and particles in the St. Lawrence estuarine system: Biogeochemical and physical implications

Absorption spectra of chromophoric dissolved organic matter (CDOM) and particles were obtained in May 2007 in the St. Lawrence estuary (SLE, Canada), the northwestern Gulf of St. Lawrence (NWG), and the Saguenay Fjord (CDOM only), the main tributary of the SLE. CDOM absorption generally decreased downstream and with depth and showed an inverse relationship to tidal cycles. Phytoplankton absorption in surface water of the SLE increased seaward while non-algal particle absorption trended oppositely; both variables declined with depth. Surface water CDOM absorption surpassed particle absorption in the SLE while phytoplankton absorption dominated in the NWG. Elevated non-algal and CDOM absorption were found in the turbidity maximum zone near the head of the SLE. Enriched CDOM absorption also occurred in the bottom water of the lower SLE and NWG. The spectral slope ratio of CDOM absorption, defined as the ratio of the spectral slope between 275 and 295 nm to that between 350 and 400 nm, was confirmed to be an indicator of the source and molecular weight of CDOM. This surrogate functionality, however, failed for absorption spectra exhibiting shoulders in short ultraviolet wavelengths observed in deep waters of the SLE and NWG. CDOM absorption mainly displayed conservative mixing behavior in both the SLE and the Saguenay Fjord. CDOM was employed to trace the source identity of the Fjord's deepwater. It was found that the marine end member of the Fjord's deepwater possessed a salinity of 32.92 and a temperature of ca. 1 °C and originated from the intermediate cold layer of the lower SLE. The marine end member contributed 94% of the deepwater by volume while freshwater mainly flown from the Saguenay River supplied the remaining 6%. Implications of our results for remote sensing-based assessments of primary productivity, surface water circulation, and water column photochemistry in the SLE are also discussed.

► CDOM absorption behaves conservatively during estuarine mixing. ► CDOM as a tracer for deepwater formation of the Saguenay Fjord. ► Spectral slope ratio as indicator of CDOM sources and molecular weight. ► Particulate absorption behaves non-conservatively. ► Applications of satellite-based ocean color imaging to the St. Lawrence estuary.