Marine CO<sub>2</sub> system variability along the northeast Pacific Inside Passage determined from an Alaskan ferry

Abstract

Abstract. Information on marine CO2 system variability has been limited along the northeast Pacific Inside Passage 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. Marine CO2 system patterns were evaluated using measurements made over a 2-year period, which revealed the seasonal cycle as the dominant mode of temporal variability. The amplitude of this signal varied spatially and was modulated by the relative influences of tidal mixing, net community production, and the magnitude and character of freshwater input. Surface water pHT (total hydrogen ion scale) and aragonite saturation state (Ωarag) were determined using carbon dioxide partial pressure (pCO2) data with alkalinity derived from a regional salinity-based relationship, which was evaluated using intervals of discrete seawater samples and underway pH measurements. High-pCO2, low-pHT, and corrosive Ωarag conditions (Ωarag<1) were seen during winter and within persistent tidal mixing zones, and corrosive Ωarag values were also seen in areas that receive significant glacial melt in summer. Biophysical drivers are shown to dominate pCO2 variability over most of the Inside Passage except in areas highly impacted by glacial melt. pHT and Ωarag extremes were also characterized based on degrees of variability and severity, and regional differences were evident. Computations of the time of detection identified tidal mixing zones as strategic observing sites with relatively short time spans required to capture secular trends in seawater pCO2 equivalent to the contemporary rise in atmospheric CO2. Finally, estimates of anthropogenic CO2 showed notable spatiotemporal variability. Changes in total hydrogen ion content ([H+]T), pHT, and Ωarag over the industrial era and to an atmospheric pCO2 level consistent with a 1.5 ∘C warmer climate were theoretically evaluated. These calculations revealed greater absolute changes in [H+]T and pHT in winter as opposed to larger Ωarag change in summer. The contemporary acidification signal everywhere along the Inside Passage exceeded the global average, with specific areas, namely Johnstone Strait and the Salish Sea, standing out as potential bellwethers for the emergence of biological ocean acidification (OA) impacts. Nearly half of the contemporary acidification signal is expected over the coming 15 years, with an atmospheric CO2 trajectory that continues to be shaped by fossil–fuel development.