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 an estuary, which may be influenced by both
changing ocean conditions and anthropogenic activity. We analyze nearly
100 years of water column profile data in Puget Sound, a temperate,
fjordal, urbanized estuary in Washington State, USA and the southernmost
region of the Salish Sea. 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 section of
Puget Sound. Changes in DO solubility associated with the observed
increase in water temperature can account for approximately 50% of this 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.