| Summary: | The continental shelf region off the Washington coast is seasonally exposed to water with a low aragonite saturation state by coastal upwelling of deep waters. However, the extent of its evolution in late summer has been largely unknown. Along this continental margin, upwelling, biological productivity, and respiration processes in subsurface waters are major contributors to the variability in aragonite saturation state. In the late summers of 2011 and 2012, we conducted large-scale chemical, biological, and hydrographic surveys of the region in order to better understand the interrelationships between these natural and human-induced processes and their effects on calcium carbonate saturation. The uptake of anthropogenic CO2 has caused the aragonite saturation horizon to shoal by approximately 40-50 m since preindustrial times so that it is well within the density layers that are currently being upwelled along the west coast of North America to depths between 10 and 80 m. Although the majority of the corrosive character of these waters is the result of respiration processes at intermediate depths, reducing aragonite saturation state by about 0.2-0.3 and pH by 0.3-0.5, this region continues to accumulate more anthropogenic CO2 and, therefore, the upwelling processes will expose coastal organisms living in the water column or at the sea floor to less saturated waters, exacerbating the biological impacts of ocean acidification. Our research shows this is happening now with some pteropod species.
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