Southeast Asia is a hotspot of riverine export of terrigenous organic carbon to the ocean, accounting for ~10% of the global land-to-ocean riverine flux of terrigenous dissolved organic carbon (tDOC). While anthropogenic disturbance is thought to have increased the tDOC loss from peatlands in Southeast Asia, the fate of this tDOC in the marine environment and the potential impacts of its remineralization on coastal ecosystems remain poorly understood. We collected a multi-year biogeochemical time series in the central Sunda Shelf (Singapore Strait), where the seasonal reversal of ocean currents delivers water masses from the South China Sea first before (during Northeast Monsoon) and then after (during Southwest Monsoon) they have mixed with run-off from peatlands on Sumatra. The concentration and stable isotope composition of dissolved organic carbon, and colored dissolved organic matter spectra, reveal a large input of tDOC to our site during Southwest Monsoon. Using isotope mass balance calculations, we show that 60–70% of the original tDOC input is remineralized in the coastal waters of the Sunda Shelf, causing seasonal acidification by up to 0.10 pH units. The persistent CO2 oversaturation drives a CO2 efflux of 4.1 – 8.2 mol C m-2 yr-1 from the Singapore Strait, suggesting that a large proportion of the remineralized peatland tDOC is ultimately emitted to the atmosphere. However, incubation experiments show that the remaining 30–40% tDOC exhibits surprisingly low lability to microbial and photochemical degradation, suggesting that up to 20–30% of peatland tDOC might be relatively refractory and exported to the open ocean.