Dissolved Oxygen Variability on the Canadian Pacific Shelf: Trends, Drivers, and Projections in the Context of Emerging Hypoxia in Queen Charlotte Sound. Stevens, S. W., Hannah, C., Evans, W., Klymak, J., Waterman, S., & Ross, T. Global Biogeochemical Cycles, 39(8):e2025GB008608, 2025.
doi  abstract   bibtex   
Abstract Hypoxia is an increasing concern along the Northeast Pacific continental margin, driven by dissolved oxygen () declines related to a warming climate. Although the North Pacific inventory has declined for decades, hypoxia was rarely observed on the central coast of British Columbia, Canada, before 2020. Recent observations from Queen Charlotte Sound (QCS)—the largest shelf sea in the Canadian Pacific—indicate that hypoxia is now an emerging issue in this region. This study synthesizes measurements from numerous platforms to describe regional distributions during 2022 and 2023. These observations reveal persistent shelf-wide hypoxia during summer months, including periods of statistically defined extreme hypoxia. Comparisons with the hydrographic record from 2003 to 2021 show that concentrations were lower and hypoxia more common in 2022 and 2023 than in previous years. This recent hypoxia is caused by long-term deoxygenation trends of approximately 5–10 mol per decade at isopycnals representing QCS deep waters, combined with seasonal and interannual variability of similar or larger magnitude. Drivers of regional variability are identified over a broad range of scales. On sub-decadal time scales, upwelling strength, upwelling timing, and utilization drive variability. On decadal time scales, variability is modulated by the offshore supply of , which appears to propagate from the northwestern Pacific via North Pacific Gyre circulation with an 8-year lag. Data-based projections indicate that bottom water hypoxia will occur with increasing frequency in the coming decades, while larger portions of the regional deep layer could experience consistent summertime hypoxia by 2050. , Plain Language Summary Hypoxia, or low-oxygen conditions detrimental to the health of marine organisms, is becoming more common in the Northeast Pacific. In Queen Charlotte Sound (QCS), a coastal region off Canada's west coast, hypoxic waters were rare before 2020 but were widespread in 2022 and 2023. This study finds that oxygen levels in the region have been declining for decades, likely due to changes in the supply of oxygen-rich water to the coastline from the open Pacific. Local processes, such as the upwelling of deep waters onto the shelf and respiration, also contribute to seasonal and year-to-year changes. If these trends continue as expected, deep waters in QCS will regularly experience hypoxia in coming decades and become largely hypoxic by 2050, with detrimental consequences for marine life and fisheries. , Key Points Observations of deep water hypoxia have become more common in Queen Charlotte Sound during recent years Local shelf processes drive high frequency oxygen variability while offshore oxygen supply drives low frequency variability Regional projections predict more frequent seafloor hypoxia hereafter with larger portions of the deep layer becoming hypoxic by 2050
@Article{	  stevensetal25a,
  Title		= {Dissolved {{Oxygen Variability}} on the {{Canadian Pacific
		  Shelf}}: {{Trends}}, {{Drivers}}, and {{Projections}} in
		  the {{Context}} of {{Emerging Hypoxia}} in {{Queen
		  Charlotte Sound}}},
  ShortTitle	= {Dissolved {{Oxygen Variability}} on the {{Canadian Pacific
		  Shelf}}},
  Author	= {Stevens, S. W. and Hannah, C. and Evans, W. and Klymak, J.
		  and Waterman, S. and Ross, T.},
  Year		= 2025,
  Journal	= {Global Biogeochemical Cycles},
  Volume	= {39},
  Number	= {8},
  Pages		= {e2025GB008608},
  DOI		= {10.1029/2025GB008608},
  URLDate	= {2026-02-02},
  Abstract	= {Abstract Hypoxia is an increasing concern along the
		  Northeast Pacific continental margin, driven by dissolved
		  oxygen () declines related to a warming climate. Although
		  the North Pacific inventory has declined for decades,
		  hypoxia was rarely observed on the central coast of British
		  Columbia, Canada, before 2020. Recent observations from
		  Queen Charlotte Sound (QCS)---the largest shelf sea in the
		  Canadian Pacific---indicate that hypoxia is now an emerging
		  issue in this region. This study synthesizes measurements
		  from numerous platforms to describe regional distributions
		  during 2022 and 2023. These observations reveal persistent
		  shelf-wide hypoxia during summer months, including periods
		  of statistically defined extreme hypoxia. Comparisons with
		  the hydrographic record from 2003 to 2021 show that
		  concentrations were lower and hypoxia more common in 2022
		  and 2023 than in previous years. This recent hypoxia is
		  caused by long-term deoxygenation trends of approximately
		  5--10 mol per decade at isopycnals representing QCS deep
		  waters, combined with seasonal and interannual variability
		  of similar or larger magnitude. Drivers of regional
		  variability are identified over a broad range of scales. On
		  sub-decadal time scales, upwelling strength, upwelling
		  timing, and utilization drive variability. On decadal time
		  scales, variability is modulated by the offshore supply of
		  , which appears to propagate from the northwestern Pacific
		  via North Pacific Gyre circulation with an 8-year lag.
		  Data-based projections indicate that bottom water hypoxia
		  will occur with increasing frequency in the coming decades,
		  while larger portions of the regional deep layer could
		  experience consistent summertime hypoxia by 2050. , Plain
		  Language Summary Hypoxia, or low-oxygen conditions
		  detrimental to the health of marine organisms, is becoming
		  more common in the Northeast Pacific. In Queen Charlotte
		  Sound (QCS), a coastal region off Canada's west coast,
		  hypoxic waters were rare before 2020 but were widespread in
		  2022 and 2023. This study finds that oxygen levels in the
		  region have been declining for decades, likely due to
		  changes in the supply of oxygen-rich water to the coastline
		  from the open Pacific. Local processes, such as the
		  upwelling of deep waters onto the shelf and respiration,
		  also contribute to seasonal and year-to-year changes. If
		  these trends continue as expected, deep waters in QCS will
		  regularly experience hypoxia in coming decades and become
		  largely hypoxic by 2050, with detrimental consequences for
		  marine life and fisheries. , Key Points Observations of
		  deep water hypoxia have become more common in Queen
		  Charlotte Sound during recent years Local shelf processes
		  drive high frequency oxygen variability while offshore
		  oxygen supply drives low frequency variability Regional
		  projections predict more frequent seafloor hypoxia
		  hereafter with larger portions of the deep layer becoming
		  hypoxic by 2050},
  langid	= {english},
  Keywords	= {cproofrefereed,jmkrefereed},
  File		= {/Users/jklymak/Zotero/storage/S3J2GK6P/Stevens et al. -
		  2025 - Dissolved Oxygen Variability on the Canadian Pacific
		  Shelf Trends, Drivers, and Projections in the.pdf}
}
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