Dissolved Oxygen Variability on the Canadian Pacific Margin: Subarctic Source-Water Variability and Equatorial Water Influence

Subsurface dissolved oxygen (O₂) variability on the Canadian Pacific margin can reflect changes in the offshore waters that supply the shelf and slope, but the relative roles of water mass composition and source-water variability remain uncertain. Here we combine hydrographic observations from the west coast of Vancouver Island and Queen Charlotte Sound with reanalysis fields and a two-endmember Pacific Equatorial Water (PEW)–Pacific Subarctic Upper Water (PSUW) mixing model. In the Canadian Pacific, PEW influence reaches a regional subsurface maximum near σ θ =26.4–26.5 kg m⁻³ and varies coherently between the two study regions. Increasing PEW-like conditions are associated with lower O₂ and lower values of the quasi-conservative tracer NO, together with higher oxidized nitrogen and apparent oxygen utilization. Variations along the PEW–PSUW mixing axis explain approximately one-third of O₂ variability and 45–62% of NO variability. PSUW endmember properties vary more strongly than PEW properties and have remained relatively warm, salty, and PEW-like since the mid-2010s. Sustained positive spice anomalies in the northern California Current System therefore need not represent increased PEW transport alone; they can also reflect more PEW-like properties arising from a warmer, saltier subarctic endmember. After accounting for variability along the PEW–PSUW mixing axis, residual O₂ and nitrogen covary at near-Redfield ratios, indicating that much of the remaining variability is consistent with differences in O₂ utilization. Regional low-O₂ events occur when elevated PEW influence coincides with additional O₂ depletion. Together, these findings highlight the importance of water-mass composition, evolving source-water properties, and oxygen-utilization history in determining regional O₂ supply.