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.
Spatial variations in CO<sub>2</sub> fluxes in the Saguenay Fjord (Quebec, Canada) and results of a water mixing model
