Accelerated acidification in eastern boundary current systems

Eastern Boundary Upwelling Systems (EBUS), such as the California Current System (CalCS), are particularly sensitive to ocean acidification: the pH of their surface waters is comparatively low and their change in pH for a given uptake of anthropogenic CO2 is particularly high. This effect is particularly strong in the Pacific Ocean, as evidenced by the dramatic shoaling of the aragonite saturation horizon off the U.S. West Coast to depth of less than 100 m (Feely et al., 2008). We investigate this high vulnerability to ocean acidification of Eastern Boundary Current upwelling systems by conducting simulations with the ETH-UCLA Regional Oceanic Modeling System (ROMS) at eddy-resolving resolution for two of the four EBUS, namely the California, and the Canary Current Systems. Our simulations for the CalCS show that between pre-industrial times and present, the mean pH of the surface ocean has decreased by already about 0.1 pH units. As a result, the aragonite saturation horizon has shoaled by ~100 m and now reaches the euphotic zone in a few eddies and in near-shore environments during upwelling, in agreement with recent observations. Projections up to 2050 (IPCC SRES A2-scenario) suggest an additional drop of pH by ~0.2 units and a wide-spread and year-round shoaling of the saturation horizon into the euphotic zone. Transient simulations are currently underway to determine when critical thresholds are passed. We expect that the aragonite saturation horizon will be reaching up into the euphotic zone in nearshore environments throughout the upwelling season already during the next decade. The passing of certain thresholds will not be gradual, however. Due to the high temporal and spatial variability that characterizes EBUS, organisms are exposed to a wide range of pH (variations of up to 0.3 to 0.4 units) and saturation states, an effect that likely will increase in the future due to enhanced upwelling and lowered buffer capcity. As a result, the effect of ocean acidification in EBUS will consist of a simultaneous downward shift and a widening of the probability density function for pH and saturation state. Both effects will cause a rapid increase with time of the frequency, at which given thresholds will be passed. What this means for organisms can only be speculated about at present.

Hauri, C., Gruber, N., Lachkar, Z. & Plattner, G.-K., 2009. Accelerated acidification in eastern boundary current systems. Awards Ceremony Speeches and Abstracts of the 19th Annual V.M. Goldschmidt Conference, V.M. Goldschmidt Conference. Geochimica et Cosmochimica Acta 73(13)-1:A503. Supplement.

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