“Ocean acidification” is one of the most important and popular topics today. Our present knowledge from time-series studies has largely focused in the open ocean rather than in the marginal seas. In this study, a total of 43 cruises from September 1999 to September 2012 with measurements of the hydrological and carbon chemical parameters in surface seawater have been conducted to better understand the ocean acidification at SEATS (SouthEast Asian Time-series Study, SEATS) site in the northern South China Sea (SCS). A comparison of results from this station with those from the HOT (Hawaii Ocean Time-series) and BATS (Bermuda Atlantic Time-series Study) time series stations was thoroughly delineated.
Results shows that rate of the pH decrease between September 1999 to September 2012 was 0.0018 yr-1 at the SEATS site, and the normalized dissolved inorganic carbon concentration (nDIC) has increased at a rate of 1.3 ~ 1.4 mol kg-1 yr-1. Such results show that surface seawater at the SEATS station is acidifying in line with other oceanic region.
As compared to HOT (-0.0007 yr-1) and BATS (-0.0036 yr-1), surface seawater pH at SEATS station declines at a rate of -0.0018 yr-1. It has been suggested that the acidification rate of the surface seawater is latitudinally related, as seawater at the high latitude regions absorb more carbon dioxide than those of the lower latitudes. However, our observed results show that the acidification rate at the SEATS station is faster than the HOT station, presumably due to the increasing mixed layer depth at the SEATS became deeper (approximately 1.6m yr-1) during the study period. Comparing our results with those from the HOT and BATS stations, it appears that the SEATS station locates in a subtropical region where the mixed layer is relatively shallow. When the mixed layer becomes deeper, more higher DIC, TA but low pH sub-surface water reaches to the surface, causing surface seawater to be more acidic. From 2004 to 2008, the mixed layer depth was deeper than those of the other years and so is salinity. However, these were not observed at the other two stations. In other words, the deeper the mixed layer at the SEATS site, more acidic of seawater at the SEATS is.
Chang S.-w., 2013. Seawater acidification over the past decade (1999~2012) at the SEATS site. Master thesis, 84 p. Thesis (restricted access).