Situated at the heart of the Coral Triangle, the Indonesian maritime continent (IMC) is an ecologically crucial area yet a vulnerable ecosystem in the face of ocean warming and acidification. Nevertheless, a comprehensive evaluation of ocean acidification in the IMC remains absent. Here, we investigate ocean acidification (OA) signals and their major controls in the region based on a dataset collected from two cruises to the eastern Indonesian Seas. Other historical cruise-based observational data are also integrated. We show that pH and aragonite saturation state at 25°C (pH_-25 and Ω_arag-25) were slightly lower in the western Indonesian Seas-Sunda Shelf across all seasons compared to the eastern Indonesian Seas and adjacent ocean. Lower pH_-25 and Ω_arag-25 values were observed during the second monsoon transition in the Malacca Strait and southern Java, with ranges from 7.96 to 8.00 for pH_-25 and 2.2 to 2.9 for Ωarag_-25. The Revelle Factor and the ratio of dissolved inorganic carbon to total alkalinity, both indicators of buffering capacity, were slightly elevated during the southeast monsoon (8.5-11.3) and the second monsoon transition (0.84-0.90) in the southeastern tropical Indian Ocean and the western Indonesian Seas-Sunda Shelf. Additionally, carbonate ion concentration decreased during the same season of different years, indicating strong signals of OA in the region. It is suggestive that physical processes and monsoonal systems are additional key drivers of OA, primarily through upwelling of CO₂-enriched waters from depths. Long-term trends show significant declines in pH and Ω_arag at rates of 0.011±0.003 year^-1 and 0.0016±0.0004 year^-1, respectively during 1978-2018. This trend is further corroborated by model simulations of two future scenarios.
 

Indonesian maritime continent, ocean acidification, marine carbonate system, buffer capacity, seasonal monsoon