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

Biogeosciences Discussions

article

Delaigue, Louise¹  Thomas, Helmuth²  Mucci, Alfonso¹ 

[1] GEOTOP and Department of Earth and Planetary Sciences, McGill University, 3450 University Street;Department of Oceanography, Dalhousie University;Center for Materials and Coastal Research

DOI  :  10.5194/bg-17-547-2020

学科分类：生物科学（综合）

来源: Copernicus Publications

【 摘 要 】

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 CO 2 to the atmosphere, as they partly originate from the flushing of organic-rich soil porewaters. Nonetheless, the CO 2 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, p CO 2 ) of the fjord. Results indicate that the fjord's surface waters are a net source of CO 2 to the atmosphere during periods of high freshwater discharge (e.g., spring freshet), whereas they serve as a net sink of atmospheric CO 2 when their practical salinity exceeds ∼5 –10.

【 授权许可】

CC BY