Understanding present-day carbonate system variability in coastal waters is critical to assess the possible impacts of future ocean acidification. However, the carbonate system in coastal waters is particularly dynamic and spatially variable, and we therefore need more time series in coastal waters, especially in so far under-studied tropical regions. We will present data from a monthly time series in the Singapore Strait, in the centre of Southeast Asia's Sunda Shelf Sea, where we have measured carbonate system data (from DIC and TA) since 2018. We show that our site experiences strong seasonal and interannual variability, due to an interplay of climatic, hydrographic, and biogeochemical processes. The dominant biogeochemical process impacting the carbonate system is the remineralization of terrestrial organic carbon that enters the coastal ocean from peatland-draining rivers, which lowers pH and raises pCO2. However, because the monsoon system causes the ocean currents to reverse direction seasonally, our site is only exposed to this terrestrially influenced, acidified water during May to September. When the circulation flows in reverse during November to March, unacidified water from the South China Sea is brought to our site. The fact that both the physical circulation and the river input of terrestrial organic carbon vary interannually means that we also observe interannual variability in the carbonate system, and not only seasonal variability. The seasonal and interannual variability greatly complicate the assessment of long-term anthropogenically driven ocean acidification trends. However, because long-term acidification due to atmospheric CO2 uptake and on-shelf production of dissolved CO2 from terrestrial organic carbon remineralization are independent, it is likely that long-term ocean acidification rates will be enhanced in this region, underscoring the importance of high-quality coastal carbonate system time series.

ocean acidification, terrestrial organic carbon, East Asian monsoon, coastal biogeochemistry