Amplified bottom water acidification rates on the Bering Sea shelf from 1970–2022

Abstract. The Bering Sea shelf supports a highly productive marine ecosystem that is vulnerable to ocean acidification (OA) due to the cold, carbon rich waters. Previous observational evidence suggests that bottom waters on the shelf are already seasonally undersaturated with respect to aragonite (i.e. Ω_arag < 1), and that OA will continue to increase the spatial extent, duration, and intensity of these conditions. Here, we use a regional ocean biogeochemical model to simulate changes in ocean carbon chemistry for the Bering Sea shelf from 1970–2022. Over this timeframe, surface Ω_arag decreases by -0.043 decade^-1 and surface pH by -0.014 decade^-1, comparable to observed global rates of OA. However, bottom water pH decreases at twice the rate of surface pH, while bottom [H⁺] decreases at nearly three times the rate of surface [H⁺]. This amplified bottom water acidification emerges over the past 25 years and is likely driven by a combination of anthropogenic carbon accumulation and a trend of increasing primary productivity and increasing subsurface respiration and remineralization. Due to this enhanced bottom water acidification, the spatial extent of bottom waters with Ω_arag < 1 has greatly expanded over the past two decades, along with pH conditions harmful to red king crab. Interannual variability in surface and bottom Ω_arag, pH, and [H⁺] has also increased over the past two decades, resulting in part from the increased physical climate variability. We also find that the Bering Sea shelf is a net annual carbon sink of 1.1–7.9 TgC/year, with the range resulting from the difference in the two different atmospheric forcing reanalysis products used. Seasonally, the shelf is a significant carbon sink from April–October but a somewhat weaker carbon source from November–March.