Spatial variability in surface-water ip/iCOsub2/sub and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

摘要

The incomplete spatial coverage of COsub2/sub partial pressure (ip/iCOsub2/sub) measurements across estuary types represents a significant knowledge gap in current regional- and global-scale estimates of estuarine COsub2/sub emissions. Given the limited research on COsub2/sub dynamics in large estuaries and bay systems, as well as the sources of error in the calculation of ip/iCOsub2/sub (carbonic acid dissociation constants, organic alkalinity), estimates of air–sea COsub2/sub 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 ip/iCOsub2/sub 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 ip/iCOsub2/sub 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 COsub2/sub to the atmosphere, with an area-averaged COsub2/sub degassing flux of 0.98 to 2.02?mmol?C?msup−2/sup?dsup−1/sup (0.36 to 0.74?mol?C?msup−2/sup?yrsup−1/sup). A preliminary analysis revealed that respiration (upper estuary), photosynthesis (lower estuary), and temperature (Gulf of St. Lawrence) controlled the spatial variability in surface-water ip/iCOsub2/sub. Whereas we used the dissociation constants of Cai and Wang (1998) to calculate estuarine ip/iCOsub2/sub, formulations recommended for best practices in open ocean environments may underestimate ip/iCOsub2/sub at low salinities, while those of Millero (2010) may result in overestimates.