Decadal Time‐Series Depletion of Dissolved Oxygen at Abyssal Depths in the Northeast Pacific

Dissolved oxygen depletion in the global ocean is well documented over several decades from the surface ocean to abyssal depths. This decline is especially prevalent in the Northeast Pacific. A significant decline in dissolved oxygen has been measured over 30 years at 4,000–4,100 m depth (Station M) beneath the California Current off central California. Three principal hypotheses examined the relationship of declining oxygen with biological and physical factors over the 30‐year time series. Annual resolution revealed Ekman pumping, coastal upwelling, particulate matter flux, and sediment community oxygen consumption having significant correlations with bottom water dissolved oxygen concentration. Coastal upwelling accounted for 65% of the annual variation in bottom water oxygen concentration. Stepwise regression yielded descriptive models of bottom water dissolved oxygen using coastal upwelling, wind stress and primary production variables. Is continued oxygen depletion in the Northeast Pacific indicative of abyssal regions in the world ocean? Plain Language Summary The dissolved oxygen content of the global ocean has been declining over many decades. This decrease is well documented in the upper ocean particularly in the Northeast Pacific Ocean. To examine the change in dissolved oxygen at greater depths, we measured the bottom water oxygen concentration at 4,000–4,100 m depth at a single station in the Northeast Pacific over a 30‐year period from 1989 through 2019. We recorded a significant decline in dissolved oxygen at this abyssal station over three decades. We then examined possible biological and physical causes of this declining dissolved oxygen. Surface ocean Ekman pumping, coastal upwelling, particulate matter flux, and sediment community oxygen consumption on the seafloor revealed significant relationships with bottom water dissolved oxygen. Coastal upwelling overlying this station accounted for 65% of the annual variation in bottom water oxygen concentration. The question we are now poised to address is whether there is a similar long‐term decline in dissolved oxygen concentration at abyssal depths in other regions of the world ocean. Key Points Oxygen concentration has significantly declined in abyssal waters of the NE Pacific over 30 years Oxygen concentration in the NE Pacific correlates with physical and biological conditions from the surface ocean to the seafloor