Spatial-temporal variation of estuarine acidification in the southeastern Yellow Sea, Korea

Geographically, an estuary is a transition zone where river water and seawater mix. In estuaries, where river water and sea water meet, acidification can occur due to carbon dioxide (CO₂) changes due to strong horizontal stratification, long residence time, eutrophication, and weak acid-base buffering capacity.

Despite the potential consequences, studies on acidification in Korean estuaries are notably scarce. This research focuses on the seasonal variations in aragonite saturation in the Han River estuary (an open estuary), and the Geum River and Yeongsan River estuaries (constrained by estuary dams) to assess the status of estuary acidification.

Seasonal changes in aragonite saturation (Ωarg) recorded values of 1.5±0.5, 1.8±0.8, and 2.1±0.4 at the mouths of the Han River (HRE), Geum River (GRE), and Yeongsan River estuaries (YRE), respectively. Acidification was weak at the YRE, where dissolved inorganic carbon and total alkalinity were high. Conversely, acidification was pronounced at the HRE, where dissolved inorganic carbon (DIC) and total alkalinity (ALK) were low. Remarkably, downstream areas of the estuary, particularly those near large cities like Seoul, exhibited heightened vulnerability to acidification.

In all three estuaries, aragonite saturation was lower in the upper reaches, influenced by river water with weaker acid-base buffering capacity than in the lower reaches. This underscores the potential for estuarine acidification to either worsen or alleviate based on future changes in the river's carbonate system, nutrient supply rates, and biological communities.

Should estuary acidification intensify, the buffering capacity of estuaries will be compromised, potentially leading to the transfer of acidification to the ocean. This research sheds light on the intricate dynamics of estuarine acidification and emphasizes the need for continued monitoring and understanding of these crucial ecosystems.