Ocean Acidification Variability in the California Current

Since the industrial revolution, anthropogenic activities have increased the amount of carbon dioxide (CO₂) in the atmosphere. Nearly half of this excess CO₂ has dissolved into the ocean, resulting in changes to the ocean's carbonate chemistry known as ocean acidification. The California Current (CC) ecosystem off the west coast of the United States is an important fishery region that is particularly vulnerable to the effects of ocean acidification. This is because naturally corrosive waters sit just below the surface and are seasonally upwelled. Modeling studies indicate that severe acidification in the CC region will occur in the coming century. Such acidification threaten biodiversity and place economic stress on commercial fisheries in this region, because calcifying organisms at the base of the food web tend to respond negatively to ocean acidification. The observational record of ocean acidification from open ocean monitoring sites suggests that the aragonite saturation state (Ω_arag) in the surface ocean have significantly declined over the past few decades, a key indication of ongoing ocean acidification. Until recently, however, the oceanographic community lacked an observational record for ocean acidification in vulnerable regions such as the CC. In a partnership between Scripps Institution of Oceanography and the National Oceanic and Atmospheric Administration (NOAA), two moorings in the CC, named CCE-1 and CCE-2, were established in 2008 and 2010, respectively. These moorings measure pH and the partial pressure of carbon dioxide (pCO₂) in the surface ocean every three hours, from which other carbonate chemistry parameters may be calculated. Here, we quantify the temporal variability in the surface ocean carbonate chemistry of the CC using the observational mooring data streams. The offshore buoy, CCE-1, had a standard deviation of 0.06 for a mean Ω of 2.5 from 2013 to 2014. Comparably, the near-shore buoy, CCE-2, had a mean Ω_arag of 2.5 from 2013 to 2014. However, CCE-2 showed more variability of Ω evident by a standard deviation of 0.43. Increases in surface ocean carbonate chemistry variability near-shore is explained by oceanic upwelling of naturally corrosive waters, driven by climatic patterns. The El Nino-Southern Oscillation (ENSO) plays a key role in the intensity of these upwelling events.