Extensive Remineralization of Peatland-Derived Dissolved Organic Carbon and Ocean Acidification in the Sunda Shelf Sea, Southeast Asia

Southeast Asia is a hotspot of riverine export of terrigenous organic carbon to the ocean, accounting for similar to 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 DOC, 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. The persistent CO2 oversaturation drives a CO2 efflux of 2.4-4.9 mol 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.

Automated summary

Southeast Asia is a hotspot of terrigenous organic carbon export to the ocean, with around 10% of global land-to-ocean riverine flux. Research in the central Sunda Shelf reveals a large input of terrigenous dissolved organic carbon (tDOC) during the Southwest Monsoon, leading to seasonal acidification. Isotope mass balance calculations show that 60%-70% of the original tDOC input is remineralized in coastal waters, contributing to CO2 efflux. Additionally, incubation experiments suggest that 30%-40% of tDOC is relatively refractory and may be exported to the open ocean.