Abstract
Abstract. The Saguenay Fjord is a major tributary of the St. Lawrence Estuary and is strongly stratified. A 6–8 m wedge of brackish water typically overlies up to 270 m of seawater. Relative to the St. Lawrence River, the surface waters of the Saguenay Fjord are less alkaline and host higher dissolved organic carbon (DOC) concentrations. In view of the latter, surface waters of the fjord are expected to be a net source of CO2 to the atmosphere, as they partly originate from the flushing of organic-rich soil porewaters. Nonetheless, the CO2 dynamics in the fjord are modulated with the rising tide by the intrusion, at the surface, of brackish water from the Upper St. Lawrence Estuary, as well as an overflow of mixed seawater over the shallow sill from the Lower St. Lawrence Estuary. Using geochemical and isotopic tracers, in combination with an optimization multiparameter algorithm (OMP), we determined the relative contribution of known source waters to the water column in the Saguenay Fjord, including waters that originate from the Lower St. Lawrence Estuary and replenish the fjord's deep basins. These results, when included in a conservative mixing model and compared to field measurements, serve to identify the dominant factors, other than physical mixing, such as biological activity (photosynthesis, respiration) and gas exchange at the air–water interface, that impact the water properties (e.g., pH, pCO2) of the fjord. Results indicate that the fjord's surface waters are a net source of CO2 to the atmosphere during periods of high freshwater discharge (e.g., spring freshet), whereas they serve as a net sink of atmospheric CO2 when their practical salinity exceeds ∼5–10.
Highlights
Anthropogenic emissions of carbon dioxide (CO2) have recently propelled atmospheric CO2 concentrations above the 410 ppm mark, the highest concentration recorded in the past 3 million years (Willeit et al, 2019)
The salinity-dependence of the phenol red indicator dissociation constant and molar absorptivities was extended to encompass the range of salinities encountered in this study, but computed pHT values from the revised fit were not significantly different from those obtained with the relationship provided by Robert-Baldo et al (1985)
The SRW and cold intermediate layer (CIL) are dominant contributors, with the SRW forming a brackish surface layer (f = 1 in surface waters) and the CIL replenishing the bottom waters of the fjord (0.7 < f < 1)
Summary
Anthropogenic emissions of carbon dioxide (CO2) have recently propelled atmospheric CO2 concentrations above the 410 ppm mark, the highest concentration recorded in the past 3 million years (Willeit et al, 2019). This study presents (1) the relative contribution of known source waters to the water column in the Saguenay Fjord, estimated from the solution of an optimization multiparameter algorithm (OMP) using geochemical and isotopic tracers, and (2) results of a conservative mixing model, based on results of the OMP analysis, and from which theoretical surface water pCO2 values are derived and compared to field measurements The latter comparison serves to identify the dominant factors, other than physical mixing (i.e., biological activity, gas exchange), that impact the CO2 fluxes at the air–sea interface and modulate their direction and intensity throughout the fjord (i.e., whether it is a source or a sink of CO2 to the atmosphere)
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