Abstract
Abstract. The incomplete spatial coverage of CO2 partial pressure (pCO2) measurements across estuary types represents a significant knowledge gap in current regional- and global-scale estimates of estuarine CO2 emissions. Given the limited research on CO2 dynamics in large estuaries and bay systems, as well as the sources of error in the calculation of pCO2 (carbonic acid dissociation constants, organic alkalinity), estimates of air–sea CO2 fluxes in estuaries are subject to large uncertainties. The Estuary and Gulf of St. Lawrence (EGSL) at the lower limit of the subarctic region in eastern Canada is the world's largest estuarine system, and is characterized by an exceptional richness in environmental diversity. It is among the world's most intensively studied estuaries, yet there are no published data on its surface-water pCO2 distribution. To fill this data gap, a comprehensive dataset was compiled from direct and indirect measurements of carbonate system parameters in the surface waters of the EGSL during the spring or summer of 2003–2016. The calculated surface-water pCO2 ranged from 435 to 765 µatm in the shallow partially mixed upper estuary, 139–578 µatm in the deep stratified lower estuary, and 207–478 µatm along the Laurentian Channel in the Gulf of St. Lawrence. Overall, at the time of sampling, the St. Lawrence Estuary served as a very weak source of CO2 to the atmosphere, with an area-averaged CO2 degassing flux of 0.98 to 2.02 mmol C m−2 d−1 (0.36 to 0.74 mol C m−2 yr−1). A preliminary analysis revealed that respiration (upper estuary), photosynthesis (lower estuary), and temperature (Gulf of St. Lawrence) controlled the spatial variability in surface-water pCO2. Whereas we used the dissociation constants of Cai and Wang (1998) to calculate estuarine pCO2, formulations recommended for best practices in open ocean environments may underestimate pCO2 at low salinities, while those of Millero (2010) may result in overestimates.
Highlights
Estuaries occupy a very small fraction (0.2 %) of the global ocean surface area, their CO2 emissions are disproportionately large compared with CO2 exchanges between the open ocean and the atmosphere (Bauer et al, 2013)
No systematic study of the CO2 dynamics in the St. Lawrence Estuary (SLE) has been published and, the present study provides the first comprehensive description of its mixed-layer carbonate chemistry, including (1) a multi-year compilation of springtime and summertime pressure of CO2 (pCO2) calculated from direct measurements of pH and alkalinity; (2) an area-averaged estimate of the air–sea CO2 gas flux; and (3) an analysis of the relative importance of thermodynamic and biological processes in controlling the spatial variability in surface-water pCO2
Data were compiled from all 10 cruises to describe the inorganic carbon chemistry in the mixed-layer waters of the St
Summary
Estuaries occupy a very small fraction (0.2 %) of the global ocean surface area, their CO2 emissions are disproportionately large compared with CO2 exchanges between the open ocean and the atmosphere (Bauer et al, 2013). In strongly tidal (macrotidal) systems, long water and particle residence times (on the order of weeks to months; Middelburg and Herman, 2007) allow for the extensive modification and degradation of particulate organic carbon during estuarine transport (Borges et al, 2006; Chen and Borges, 2009). No systematic study of the CO2 dynamics in the SLE has been published and, the present study provides the first comprehensive description of its mixed-layer carbonate chemistry, including (1) a multi-year compilation of springtime and summertime pCO2 calculated from direct measurements of pH and alkalinity; (2) an area-averaged estimate of the air–sea CO2 gas flux; and (3) an analysis of the relative importance of thermodynamic (temperature) and biological (photosynthesis and respiration) processes in controlling the spatial variability in surface-water pCO2
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
