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Dissolved Methane in the World's Largest Semi‐Enclosed Estuarine System: The Estuary and Gulf of St. Lawrence (Canada).

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Title: Dissolved Methane in the World's Largest Semi‐Enclosed Estuarine System: The Estuary and Gulf of St. Lawrence (Canada).
Authors: Li, Y.1 (AUTHOR), Xie, H.1 (AUTHOR) huixiang_xie@uqar.ca, Scarratt, M. G.2 (AUTHOR), Damm, E.3 (AUTHOR), Galbraith, P. S.2 (AUTHOR), Lambert, N.4 (AUTHOR), Geng, L.1,5 (AUTHOR), Wallace, D. W. R.6 (AUTHOR)
Source: Journal of Geophysical Research. Oceans. Aug2022, Vol. 127 Issue 8, p1-27. 27p.
Subject Terms: *Atmospheric methane, *Estuaries, *Bottom water (Oceanography), *Methane, *Water masses, *Carbon dioxide
Geographic Terms: Canada
Abstract: We report the first water‐column dissolved methane data set from the Estuary and Gulf of St. Lawrence (EGSL). Per surface‐water methane concentration and sea‐to‐air flux, the upper estuary behaved like a typical shallow macrotidal estuary, while the lower estuary and the gulf resembled outer shelf seas and ocean slopes, respectively. The EGSL emitted 166.3 (71.5–214.4) × 106 mol CH4 year−1 to the atmosphere, representing 0.3% (0.1%–0.4%) of the total emission from global estuarine environments. A net production of 11.7 × 107 mol CH4 year−1 was required to sustain this emission. Methane distributions in the upper estuary were dominated by physical mixing, while those in the lower estuary and the gulf bore characteristic subsurface maxima and deep minima shedding light on the methane consumption and production pathways. Elevated but highly variable near‐bottom methane concentrations (10.4–695.3 nmol L−1) transpired over pockmarks on the seabed of the lower estuary, inferring an upward diffusive flux of up to ∼700 mmol CH4 m2 d−1. Hypoxia in the lower estuary bottom water had little influence on methane concentrations. Lab incubations yielded methane cycling rates from a net production of 0.0068 nmol L−1 d−1 to net consumption with turnover times of 33.3–263 days. Methane in the EGSL was isotopically enriched with 13C (δ13CCH4: −40.9‰ to −27.4‰ relative to Peedee Belemnite). This study reveals that the EGSL is a smaller proportional contributor to methane emission from estuarine environments and that complex physical‐biogeochemical interactions control methane cycling and isotopic composition in this vast estuarine system. Plain Language Summary: Methane is the second most important greenhouse gas after carbon dioxide. The ocean is a significant source of methane to the atmosphere, with a disproportionally higher share from estuarine and coastal environments. The Estuary and Gulf of St. Lawrence (EGSL) is the largest semi‐enclosed estuarine system in the world. However, methane concentrations and air‐sea fluxes in the EGSL are unknown to date and are thus absent in global‐ocean methane data compilations. We collected a multi‐season and multi‐year data set of methane from the water column of the EGSL to assess methane's distributions, air‐sea fluxes, sources, and cycling in this vast system. Surface‐water methane concentration decreases from the upper estuary to the lower estuary to the gulf. The distribution of methane in the upper is mainly controlled by the mixing of river water with seawater, while the distribution of methane in the lower estuary and the gulf, which are much deeper and wider than the upper estuary, is dictated by oceanic water mass composition and its interaction with different biogeochemical processes. The EGSL accounts for 0.3% of the total methane emission from global estuarine environments, which is a smaller proportional contribution on a per‐surface‐area basis. Key Points: Contrasting methane distributions and dynamics in different subregions of the Estuary and Gulf of St. LawrenceElevated, highly variable methane concentrations in bottom waters near pockmarks but no escape of pockmark methane to the atmosphereBoth methane concentrations and emission rates are at the lower bounds of global estuarine environments [ABSTRACT FROM AUTHOR]
Copyright of Journal of Geophysical Research. Oceans is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Dissolved Methane in the World's Largest Semi‐Enclosed Estuarine System: The Estuary and Gulf of St. Lawrence (Canada).
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  Data: <searchLink fieldCode="AR" term="%22Li%2C+Y%2E%22">Li, Y.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Xie%2C+H%2E%22">Xie, H.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> huixiang_xie@uqar.ca</i><br /><searchLink fieldCode="AR" term="%22Scarratt%2C+M%2E+G%2E%22">Scarratt, M. G.</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Damm%2C+E%2E%22">Damm, E.</searchLink><relatesTo>3</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Galbraith%2C+P%2E+S%2E%22">Galbraith, P. S.</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Lambert%2C+N%2E%22">Lambert, N.</searchLink><relatesTo>4</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Geng%2C+L%2E%22">Geng, L.</searchLink><relatesTo>1,5</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Wallace%2C+D%2E+W%2E+R%2E%22">Wallace, D. W. R.</searchLink><relatesTo>6</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Journal+of+Geophysical+Research%2E+Oceans%22">Journal of Geophysical Research. Oceans</searchLink>. Aug2022, Vol. 127 Issue 8, p1-27. 27p.
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  Data: We report the first water‐column dissolved methane data set from the Estuary and Gulf of St. Lawrence (EGSL). Per surface‐water methane concentration and sea‐to‐air flux, the upper estuary behaved like a typical shallow macrotidal estuary, while the lower estuary and the gulf resembled outer shelf seas and ocean slopes, respectively. The EGSL emitted 166.3 (71.5–214.4) × 106 mol CH4 year−1 to the atmosphere, representing 0.3% (0.1%–0.4%) of the total emission from global estuarine environments. A net production of 11.7 × 107 mol CH4 year−1 was required to sustain this emission. Methane distributions in the upper estuary were dominated by physical mixing, while those in the lower estuary and the gulf bore characteristic subsurface maxima and deep minima shedding light on the methane consumption and production pathways. Elevated but highly variable near‐bottom methane concentrations (10.4–695.3 nmol L−1) transpired over pockmarks on the seabed of the lower estuary, inferring an upward diffusive flux of up to ∼700 mmol CH4 m2 d−1. Hypoxia in the lower estuary bottom water had little influence on methane concentrations. Lab incubations yielded methane cycling rates from a net production of 0.0068 nmol L−1 d−1 to net consumption with turnover times of 33.3–263 days. Methane in the EGSL was isotopically enriched with 13C (δ13CCH4: −40.9‰ to −27.4‰ relative to Peedee Belemnite). This study reveals that the EGSL is a smaller proportional contributor to methane emission from estuarine environments and that complex physical‐biogeochemical interactions control methane cycling and isotopic composition in this vast estuarine system. Plain Language Summary: Methane is the second most important greenhouse gas after carbon dioxide. The ocean is a significant source of methane to the atmosphere, with a disproportionally higher share from estuarine and coastal environments. The Estuary and Gulf of St. Lawrence (EGSL) is the largest semi‐enclosed estuarine system in the world. However, methane concentrations and air‐sea fluxes in the EGSL are unknown to date and are thus absent in global‐ocean methane data compilations. We collected a multi‐season and multi‐year data set of methane from the water column of the EGSL to assess methane's distributions, air‐sea fluxes, sources, and cycling in this vast system. Surface‐water methane concentration decreases from the upper estuary to the lower estuary to the gulf. The distribution of methane in the upper is mainly controlled by the mixing of river water with seawater, while the distribution of methane in the lower estuary and the gulf, which are much deeper and wider than the upper estuary, is dictated by oceanic water mass composition and its interaction with different biogeochemical processes. The EGSL accounts for 0.3% of the total methane emission from global estuarine environments, which is a smaller proportional contribution on a per‐surface‐area basis. Key Points: Contrasting methane distributions and dynamics in different subregions of the Estuary and Gulf of St. LawrenceElevated, highly variable methane concentrations in bottom waters near pockmarks but no escape of pockmark methane to the atmosphereBoth methane concentrations and emission rates are at the lower bounds of global estuarine environments [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
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  Data: <i>Copyright of Journal of Geophysical Research. Oceans is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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      – Type: doi
        Value: 10.1029/2022JC018850
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        Text: English
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        PageCount: 27
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    Subjects:
      – SubjectFull: Atmospheric methane
        Type: general
      – SubjectFull: Estuaries
        Type: general
      – SubjectFull: Bottom water (Oceanography)
        Type: general
      – SubjectFull: Methane
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      – SubjectFull: Water masses
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      – SubjectFull: Carbon dioxide
        Type: general
      – SubjectFull: Canada
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      – TitleFull: Dissolved Methane in the World's Largest Semi‐Enclosed Estuarine System: The Estuary and Gulf of St. Lawrence (Canada).
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              Text: Aug2022
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