UH Manoa scientists analyze Project ALOHA data on ocean acidificationUniversity of Hawaiʻi at Mānoa
Despite the global environmental importance of ocean acidification, there are few studies of sufficient duration, accuracy and sampling intensity to document the rate of change of ocean pH and shed light on the factors controlling its variability. In 1988, Dave Karl and Roger Lukas of the School of Ocean, Earth Science and Technology (SOEST) at the University of Hawaiʻi at Mānoa founded the Hawaiʻi Ocean Time-series (HOT) program, in part to establish a long-term record of the oceanic response to rising atmospheric CO2. Monthly research cruises to Station ALOHA, north of Oʻahu, have yielded after 20 years the most detailed record to date on ocean acidification in the Pacific. Reporting in this weekʻs issue of Proceedings of the National Academy of Sciences, lead author and former SOEST researcher John Dore (now at Montana State University) presents an analysis of the changes of pH at Station ALOHA over time and depth. Dore, along with SOEST co-authors Karl, Lukas, Matt Church and Dan Sadler, found that over the two decades of observation, the surface ocean grew more acidic at exactly the rate expected from chemical equilibration with the atmosphere. However, that rate of change varied considerably on seasonal and inter-annual timescales, and even reversed for one period of nearly five years. The year-to-year changes appear to be driven by climate-induced changes in ocean mixing and attendant biological responses to mixing events. The authors also found distinct layers at depth in which pH declines were actually faster than at the surface. Dore and colleagues attribute these strata of elevated acidification rates to increases in biological activity and to the intrusion at Station ALOHA of remotely formed water masses with different chemical histories.