Spatial variability in surface-water <i>p</i>CO<sub>2</sub> and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)
The incomplete spatial coverage of CO<sub>2</sub> partial pressure (<i>p</i>CO<sub>2</sub>) measurements across estuary types represents a significant knowledge gap in current regional- and global-scale estimates of estuarine CO<sub>2</sub> emissions...
Main Authors: | , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2017-07-01
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Series: | Biogeosciences |
Online Access: | https://www.biogeosciences.net/14/3221/2017/bg-14-3221-2017.pdf |
Summary: | The incomplete spatial coverage of CO<sub>2</sub> partial pressure (<i>p</i>CO<sub>2</sub>)
measurements across estuary types represents a significant knowledge gap in
current regional- and global-scale estimates of estuarine CO<sub>2</sub> emissions.
Given the limited research on CO<sub>2</sub> dynamics in large estuaries and bay
systems, as well as the sources of error in the calculation of <i>p</i>CO<sub>2</sub>
(carbonic acid dissociation constants, organic alkalinity), estimates of
air–sea CO<sub>2</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 <i>p</i>CO<sub>2</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
<i>p</i>CO<sub>2</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 CO<sub>2</sub> to the atmosphere, with an area-averaged
CO<sub>2</sub> degassing flux of 0.98 to 2.02 mmol C m<sup>−2</sup> d<sup>−1</sup> (0.36 to
0.74 mol C m<sup>−2</sup> yr<sup>−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 <i>p</i>CO<sub>2</sub>.
Whereas we used the dissociation constants of Cai and Wang (1998) to calculate estuarine <i>p</i>CO<sub>2</sub>, formulations recommended for best practices in open ocean environments may underestimate <i>p</i>CO<sub>2</sub> at low salinities, while those of Millero (2010) may result in overestimates. |
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ISSN: | 1726-4170 1726-4189 |