The response of the carbonate system to a green algal bloom during the post-bloom period in the southern Yellow Sea

•We investigate the carbonate system during green algal post-bloom period.•Alkalinity measured by Gran titration is abnormal.•The increase of DIC and decrease of pH are significant.•pCO2 in seawater increases dramatically.

Since 2007, the green algal bloom occurred along the coast of Qingdao city every summer. In this study, we focused on how the carbonate system responded to the green algal bloom. Dissolved inorganic carbon (DIC), total alkalinity (TA) and pH were measured in two cruises during the green algal post-bloom period in summer 2008 in the southern Yellow Sea. Data showed that the average values of alkalinity calculated from DIC and pH were 2217 and 2241 μmol kg−1 during the first and second cruises, respectively. The alkalinity measured by Gran titration, however, was abnormal, with average values of 3333 and 3280 μmol kg−1. The abnormal measured TA values were probably due to both direct contribution of organic matter and its side reaction in determining TA using Gran titration. Another significant phenomenon was an increase of DIC and a decrease of pH after the bloom. The changes of DIC and pH during the two cruises were +87 μmol kg−1 and −0.18, respectively. Normalized DIC and TA showed that DIC was not conserved after the bloom while TA could be still conserved. In contrast to green algal bloom period, during the post-bloom period, pCO2 values were significantly raised in seawater, with average values of 596 and 981 μatm, resulting in CO2 outgassing from seawater. These results showed that carbonate system in seawater could be greatly altered by green algal bloom.