Spatial patterns in aragonite saturation horizon over the northern California shelf

Ocean acidification along the California Shelf is controlled by the upwelling of deep CO₂-rich waters. This coastal upwelling is driven by along-shore winds, which vary seasonally. We report aragonite undersaturation based on proxy values derived from temperature, salinity, and dissolved oxygen data and show the relationship between the spatial extent of low aragonite saturation state values and upwelling-favorable along-shore winds. Our data were collected on seasonal hydrographic surveys off northern California that span several years. Transect data show a persistent spatial pattern with shoaling of the aragonite saturation horizon over the shelf (during May and July surveys, mean depth of horizon is less than 20 m within ∼20 km of the coast), and surfacing of the saturation horizon during times of strong upwelling events. Our model shows that a higher proportion of the water column is undersaturated in aragonite when northerly winds are stronger. In addition, we found that pteropod abundance was positively correlated with aragonite saturation state, with lower densities associated with lower saturation values. If upwelling-favorable winds increase with global warming, as anticipated, more shelf water habitat will become undersaturated in aragonite, negatively impacting populations of calcifying organisms like pteropods.