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

Abstract

Puget Sound is a highly productive estuary, and many of its inlets experience seasonal hypoxia (dissolved oxygen concentration < 2 mg/L), posing a significant threat to bottom-dwelling species. 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. Therefore, to fully describe the underlying processes driving Puget Sound hypoxia, it is crucial to identify the characteristics which differentiate oxygenated inlets from hypoxic ones. Leading hypotheses that explain hypoxia include high productivity, low initial oxygen concentrations, and long flushing times.A realistic numerical model of the region and surrounding coastal waters is used to identify the dominant factors contributing to hypoxia. Two-layer dissolved oxygen budgets are calculated with the Total Exchange Flow method in twenty-one terminal inlets across Puget Sound for the year 2017. Biological oxygen drawdown rates in hypoxic inlets are found to be no higher than those in oxygenated inlets. Additionally, the net rate of dissolved oxygen decrease is similar in both hypoxic and oxygenated inlets during the summer. Instead, hypoxia in terminal inlets is driven by low inflowing oxygen concentrations and long flushing times, both of which are attributed to the exchange flow of the inlet. 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.