Abstract
The contribution of organic alkalinity ( A DOM ), derived from dissolved organic matter, to coastal acid–base chemistry remains poorly understood, particularly regarding its spatial, seasonal, and compositional variability. This study examines A DOM dynamics across distinct Korean coastal environments, focusing on regional distributions in lagoons, estuaries, and salt marshes with tidal flats, and on year‐round temporal variability in a macroalgal‐dominated habitat. A DOM concentrations ranged from μ mol kg −1 , accounting for less than 2% of total alkalinity ( A T ), while DOC ranged from 51 to 359 μ M. The A DOM –DOC correlation was strong in lagoons and river‐influenced systems ( r > 0.8), but weaker in bays, salt marshes, and macroalgal habitat ( r ~ 0.5–0.7), likely reflecting variability in DOM sources and composition. Back‐titration analysis further revealed consistent acid dissociation constants across regions and seasons, primarily associated with carboxyl (pKa 1 ~ 4.6) and phenolic or amine (pKa 2 ~ 7.0) groups. These two functional groups contributed nearly equally to DOM acid–base behavior across all habitats and seasons, suggesting a shared chemical foundation underlying A DOM despite diverse environmental settings in Korea. These findings provide a mechanistic understanding of how DOM functional group composition governs A DOM across diverse coastal environments.