FY 2022

Dissimilar sensitivities of ocean acidification metrics to anthropogenic carbon accumulation in the Central North Pacific Ocean and California Current Large Marine Ecosystem

Arroyo, M.C., A.J. Fassbender, B.R. Carter, C.A. Edwards, J. Fiechter, A. Norgaard, and R.A. Feely

Geophys. Res. Lett., 49(15), e2022GL097835, doi: 10.1029/2022GL097835, View online (open access) (2022)

We analyze and compare changes in ocean acidification metrics caused by anthropogenic carbon (C_anth) accumulation in the North Pacific Ocean and California Current Large Marine Ecosystem (CCLME). The greatest declines in pH and carbonate mineral saturation state occur near the surface, coincident with the highest C_anth concentrations. However, maximal increases in the partial pressure of carbon dioxide (pCO₂) and hydrogen ion concentration occur subsurface where C_anth values are lower. We attribute dissimilar sensitivities of these metrics to background ocean chemistry, which has naturally high pCO₂ and low buffering capacity in subsurface waters due to accumulated byproducts of organic matter respiration, which interacts with C_anth. In the CCLME, rising subsurface pCO₂ has increased the frequency, duration, and intensity of hypercapnia (pCO₂ ≥ 1,000 μatm) on the continental shelf. Our findings suggest that hypercapnia induced by C_anth accumulation can co-occur with hypoxia in the CCLME and is an additional modern stressor for marine organisms.

Plain Language Summary. The ocean mitigates the extent of global warming by absorbing a portion of the carbon dioxide gas (CO₂) released into the atmosphere by human activities. However, this comes at a cost to ocean health because the uptake of this anthropogenic CO₂ causes changes in ocean chemistry, called ocean acidification (OA), that can be detrimental to marine ecosystems. This study explores how OA metrics have changed in the upper waters of the open North Pacific Ocean and coastal California Current Large Marine Ecosystem (CCLME). We focus on the CCLME due to its global importance and economically important fisheries. We find that different OA metrics exhibit different patterns of change with depth in the water column due to the natural, background ocean chemistry. One such metric shows that there is now more subsurface water containing CO₂ levels elevated enough to threaten the health of marine organisms than there was before the anthropogenic CO₂ addition. Our finding of expanded volumes of water with high-CO₂ levels near the coast is important to consider as a source of stress for marine organisms living both on the seafloor and in the water column.