Organic Alkalinity as an Important Constituent of Total Alkalinity and the Buffering System in River-To-Coast Transition Zones

Organic acid-base species in the dissolved organic carbon pool have been shown to make an important contribution (i.e., organic alkalinity; OrgAlk) to the total alkalinity (TA) in many coastal systems. This study documents an intensive investigation of OrgAlk characteristics in the river-to-coast transition zones of six southeast Chinese rivers. OrgAlk, mainly originating from river input, accounted for an important proportion of TA (0.3%–12%) in six estuaries. Carboxylic acid groups were commonly present in all estuaries. Notable differences in the TA values (1–18 μmol kg⁻¹) determined by several established TA measurement approaches were identified in estuaries where organic acids with pK_a <5.2 were abundant. The most widely used open-cell titration method, in comparison with closed-cell titration and single-step titration, is the best approach to incorporate OrgAlk in titrated TA when the pK_a values of organic acids were >5 in the study estuaries. Across our study sites, OrgAlk might modify H⁺ concentrations by 3%–69% (i.e., pH by 0.01–0.78) and aragonite saturation states by 1%–72%, indicating that OrgAlk can play a significant role in the coastal carbonate buffering system. It is essential to improve current TA measurement approaches to more accurately represent OrgAlk in the coastal system and assess impacts of OrgAlk on coastal carbonate chemistry.