Marine CO<sub>2</sub> system variability along the Inside Passage of the Pacific Northwest coast of North America determined from an Alaskan ferry

Abstract. Information on marine CO2 system variability has been limited along the Inside Passage of the Pacific Northwest coast of North America despite the region’s rich biodiversity, abundant fisheries, and developing aquaculture industry. Beginning in 2017, the Alaska Marine Highway System M/V Columbia has served as a platform for surface underway data collection while conducting twice weekly ~1600-km transits between Bellingham, Washington and Skagway, Alaska. This dataset allowed for the assessment of marine CO2 system patterns along the Inside Passage, including quantification of the relative importance of key drivers in shaping pCO2 variability. Surface water pH and aragonite saturation state (Ωarag) were determined using the pCO2 data with alkalinity from a regional salinity-based relationship, which was evaluated with discrete seawater samples and underway pH measurements. Low pH and corrosive (Ωarag < 1) Ωarag conditions were seen during winter and in persistent tidal mixing zones, and corrosive Ωarag values were also seen in areas that receive significant glacial melt in summer. The time-of-detection was computed and revealed that tidal mixing zones may be sentinel observing sites with relatively short time spans of observation needed to capture secular trends in seawater pCO2 equivalent to the contemporary atmospheric CO2 increase. Finally, anthropogenic CO2 was estimated and showed notable time and space variability. We theoretically considered the change in hydrogen ion concentration ([H+]), pH, and Ωarag over the industrial era and to an atmospheric pCO2 level consistent with a 1.5 °C warmer climate and revealed greater changes in [H+] and pH in winter as opposed to larger Ωarag change in summer. In addition, the contemporary acidification signal everywhere along the Inside Passage exceeded the global average, with Johnstone Strait and the Salish Sea standing out as potential bellwethers for biological OA impacts. In theory, roughly half the acidification signal experienced thus far over the industrial era may be expected over the coming 15 years with an atmospheric CO2 trajectory that continues to be shaped by fossil-fuel development.