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 CO 2 . Ω Ar was calculated with pH, p CO 2 , and salinity‐derived alkalinity time series data from autonomous pH and p CO 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‐CO 2 water and subsequent removal of CO 2 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.