Century-Scale Changes in Dissolved Oxygen, Temperature, and Salinity in Puget Sound

Over the last century, many coastal ocean regions have experienced a decrease in dissolved oxygen (DO) concentrations. In this work we consider long-term changes in DO in Puget Sound, a temperate, fjordal, urbanized estuary in Washington State, USA based on nearly 100 years of water column profile data. We observe warming of 1.5°C/century, consistent with warming found throughout the Salish Sea region and similar to coastal ocean and local atmospheric warming. We observe that bottom salinity is increasing at a low rate. Finally, we find that bottom DO is declining at a rate of about 0.6 mg/L/century in Main basin, the largest, central section of Puget Sound. Changes in DO solubility associated with the observed increase in water temperature can account for approximately 50% of the observed DO loss. In Puget Sound’s distal terminal inlets where hypoxic conditions more commonly occur, trends in DO are generally small and the variability in DO is high, obscuring trends there. Documenting long-term changes in estuary water properties is imperative to inform management decisions for ecosystem health.