[AOS16-P01] Seasonal change in the relationship between pH and AOU in the Ariake Sound and the Amakusa-nada
Keywords:pH, Oean acidification, AOU
Vertical seawater samples were collected on 3 cruises at Amakusa-nada in June, August and October 2016, and 2 cruises along a transect from inner part of the Ariake Sound to the Amakusa-nada in May and September 2017. The pH of seawater was determined using a glass/reference electrode cell on the total hydrogen ion concentration pH scale. Temperature, salinity and dissolved oxygen (DO) data obtained by CTD sensors were used for calculation of AOU.
In the Ariake Sound, low pH (<7.9) waters were observed near the bottom in the inner area both in May and September. There was the correspondence in distribution between low pH and low DO waters, as well as high pH and high chlorophyll waters. In the Amakusa-nada, the low pH values, as low as <7.6, were measured in the deep water throughout the observation period. An inverse relationship between pH and AOU was obtained from the field data collected in the Ariake Sound and the Amakusa-nada, suggesting that the release of CO2 by biological decomposition of organic matter in the water column is playing a major role in lowering the pH of seawater. The observed decrease in pH for each unit increase in AOU (ΔpH/AOU) was different between the inner part of the Ariake Sound and the offshore area in September. When AOU values of the intermediate and deep waters were below ca. 100 μmol/L, the ΔpH/AOU showed increasing trend from spring to autumn in the Amakusa-nada. These results suggest that seasonal change in the quality of decomposing organic matters could have influence on the relationship between pH and AOU in the Amakusa-nada.