Aragonite saturation state dynamics in a coastal upwelling zone

Coastal upwelling zones may be at enhanced risk from ocean acidification as upwelling brings low aragonite saturation state (Ω_Ar) waters to the surface that are further suppressed by anthropogenic CO₂. Ω_Ar was calculated with pH, pCO₂, and salinity-derived alkalinity time series data from autonomous pH and pCO ₂ instruments moored on the Oregon shelf and shelf break during different seasons from 2007 to 2011. Surface Ω_Ar values ranged between 0.66 ± 0.04 and 3.9 ± 0.04 compared to an estimated pre-industrial range of 1.0 ± 0.1 to 4.7 ± 0.1. Upwelling of high-CO₂ water and subsequent removal of CO₂ by phytoplankton imparts a dynamic range to Ω_Ar from ∼1.0 to ∼4.0 between spring and autumn. Freshwater input also suppresses saturation states during the spring. Winter Ω_Ar is less variable than during other seasons and is controlled primarily by mixing of the water column.