Weak, Low-Oxygen Exchange Flow Drives Hypoxia in Puget Sound's Inlets

Puget Sound is a highly productive estuary, and many of its inlets experience seasonal hypoxia (dissolved oxygen concentration < 2 mg L−1), posing a significant threat to ecosystems. Models predict that hypoxia is most prevalent near the terminus of branching channels (i.e., terminal inlets) throughout the region. However, not all of Puget Sound’s terminal inlets become hypoxic, and many remain oxygenated throughout the summer and fall. We use a realistic numerical model of the region and surrounding coastal waters to investigate the characteristics that differentiate oxygenated inlets from hypoxic ones, including three leading factors hypothesized to influence hypoxia: high biological consumption, low inflowing oxygen concentrations, and long flushing times. Two-layer dissolved oxygen budgets are calculated with the Total Exchange Flow method in thirteen terminal inlets across Puget Sound for the year 2017. The net rate of dissolved oxygen decline is no higher in hypoxic inlets than in oxygenated inlets during the summer. Instead, hypoxia in terminal inlets is driven by low inflowing oxygen concentrations and long flushing times, the latter of which is attributed to the strength of the inlets’ exchange flow. These findings imply that management strategies that treat Puget Sound as an interconnected system may be more effective than strategies that target isolated hotspots of hypoxia.