Seasonal marine inorganic carbon dynamics on the Northern Gulf of Alaska continental shelf

Abstract

The Northern Gulf of Alaska supports many socioeconomic and ecosystem services but is subject to increasing ocean temperatures, marine heatwaves, and freshwater runoff, harmful algal blooms, and ocean acidification. In addition, large natural variability in biological and physical drivers complicates characterization of the progression of ocean acidification and variability in seawater inorganic carbon conditions. Here, we present two years (2019-2021) of in situ moored upper-ocean partial pressure of carbon dioxide (pCO2) and pH data on the outer continental shelf of the Northern Gulf of Alaska. The observations show that subsurface pCO2, pH and aragonite mineral saturation state (Ωarag) were highly seasonal, although generally remained at a moderate level with pH varying between 7.9 and 8.2, and Ωarag > 2 throughout the year. The influence of biogeochemistry, either from in situ or vertical exchange changes, strongly drove pCO2, pH and Ωarag anomaly variations throughout the year, while temperature also exerted a strong influence on pCO2 and pH. The thermal and biogeochemical drivers generally compensated each other, lessening the amplitude of seasonal variations; exceptions generated the highest and lowest pH and pCO2 conditions of the year. In spring, primary production and seasonally cold temperatures led to the highest pH and lowest pCO2 of the year. The lowest pH and highest pCO2 of the year occurred in fall as high-frequency events when subseasonal mixing events entrained deeper, CO2-enriched water, coincident with seasonally warm temperatures. Air-sea CO2 flux calculations suggest a greater wintertime atmospheric CO2 source than previously measured. This work complements other ongoing hydrographic and nearshore monitoring, modeling, and experimental work needed to understand the regional progression and impact of ocean acidification on this variable and changing ecosystem.