Spatial Distribution and Temporal Variability of Dissolved O ₂ in the Santa Barbara Channel and Basin, California

Dissolved oxygen (O 2 ) has been declining in coastal and open‐ocean regions, including the Santa Barbara Channel (SBC) in the Southern California Borderland. While previous studies have investigated the biogeochemical consequences of declining O 2 in the SBC, its spatial distribution and temporal variability in the region remain poorly characterized. Because O 2 dynamics shape the cycles of redox‐sensitive elements such as iron (Fe), improved characterization is essential for understanding regional biogeochemistry. We integrate observations of dissolved O 2 spanning 1985–2023 using Autonomous Underwater Vehicle Sentry and the Remotely Operated Vehicle Jason . This new data set provides the first spatially resolved characterization of dissolved O 2 across the SBC and Santa Barbara Basin. Above the Basin's sill, O 2 exhibits a vigorous seasonal cycle, with low concentrations during springtime upwelling superimposed on a long‐term declining trend linked to decadal variability. This contrasts with the seasonal cycle below the sill, where higher O 2 concentrations are observed in spring, reflecting reoxygenation by intermittent flushing events, which have weakened and become less frequent over time. Severe hypoxia and anoxia are widespread in the deep Basin, punctuated by coherent pockets of oxygenated waters, likely the remnants of flushing events. By combining reconstructions of the O 2 distribution with a benthic Fe flux parameterization, we estimate the release of ∼24,000 tons of Fe annually from the sediment—far exceeding the Fe requirements of net primary production in the region. Our study provides new insights into O 2 dynamics and their implications for biogeochemistry in the SBC, with relevance to global coastal deoxygenation.