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.

近一个世纪以来，全球诸多沿海海域的溶解氧（dissolved oxygen, DO）浓度均呈现下降趋势。本研究聚焦一处河口湾的溶解氧长期变化，该变化可能受到海洋条件改变与人类活动的共同影响。我们对美国华盛顿州普吉特湾（Puget Sound）——萨利什海（Salish Sea）最南端区域，一处温带峡湾型城市化河口湾——的近百年水柱剖面数据展开分析。研究观测到海水以每世纪1.5℃的速率升温，这一结果与整个萨利什海区域的升温趋势相符，也与邻近沿海海域及当地大气的升温幅度相近。同时观测到底层盐度以缓慢速率上升。最后，我们发现普吉特湾最大组成部分——主海盆（Main basin）的底层溶解氧以每世纪约0.6 mg/L的速率下降。由观测到的水温升高所引发的溶解氧溶解度变化，可解释约50%的溶解氧流失量。在普吉特湾更易出现低氧环境的远端末端湾汊区域，溶解氧的变化趋势通常较为微弱，且溶解氧的波动幅度较大，掩盖了当地的变化趋势。记录河口海水属性的长期变化，可为生态系统健康相关的管理决策提供必要的科学依据。