Ocean Acidification

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 pH_T (total hydrogen ion scale) and aragonite saturation state (Ω_arag) 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-pH_T, and corrosive Ω_arag conditions (Ω_arag<1) were seen during winter and within persistent tidal mixing zones, and corrosive Ω_arag 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. pH_T and Ω_arag 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), pH_T, and Ω_arag 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 pH_T in winter as opposed to larger Ω_arag 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.

Evans W., Lebon G. T., Harrington C. D., Takeshita Y. & Bidlack A., 2022. Marine CO₂ system variability along the northeast Pacific Inside Passage determined from an Alaskan ferry. Biogeosciences 19(4): 1277–1301. Article.