Seasonal dynamics of the carbonate system and acidification sensitivity in a river-dominated coastal system: the Northern Yellow Sea, China.

This study investigated the carbonate dynamics in the northern coastal zone of the Northern Yellow Sea adjacent to the Yalu River estuary during two distinct hydrological periods in 2024. During the dry season, seawater dissolved inorganic carbon (DIC) ranged narrowly from 1862 μmol kg to 2141 μmol kg, while pH ranged from 7.70 to 8.02. In contrast, during the flood season, seawater DIC decreased to 1332-2111 μmol kg, with pH exhibiting greater variability from 7.63 to 8.48. End-member mixing modeling revealed that physical mixing was the dominant controlling factor during the dry season. However, during the flood season, the surface water exhibited a DIC depletion of up to -316 μmol kg and an increase in pH by 0.54 relative to conservative mixing. Conversely, the bottom water showed an inverse pattern, with DIC accumulation reaching up to 210 μmol kg accompanied by a decrease in pH of 0.17. Combined with stable carbon isotopic analysis, both carbon dioxide outgassing and biological activities demonstrated considerable influence despite strengthened physical mixing due to intensified river input. Furthermore, the critical role of acid-base buffering capacity in regulating pH was presented. Seawater with higher buffering capacity during the flood season showed improved ability to maintain pH stability. Primary production may enhance seawater buffering capacity, while organic matter degradation could weaken it. These mechanistic insights provide a foundation for developing targeted strategies aimed at enhancing buffering capacity in anthropogenically impacted coastal ecosystems to promote CO sinking and build resilience to ocean acidification.