Degradation of Terrigenous Organic Matter from Tropical Peatlands and Coastal Ocean Acidification in Southeast Asia

Southeast Asian peatlands are a large source of terrigenous organic matter (tDOM), accounting for up to 10% of the global land-to-ocean DOC flux. Prior studies have mostly reported conservative mixing of tDOM across peat-draining estuaries in Southeast Asia, but the fate of this tDOM in the ocean remains unclear. We will present data from a coastal, biogeochemical time series site in Singapore that show strong seasonal variation in tDOM concentration, and large, correlated variability in the carbonate system. During and after the northeast monsoon (December to March), currents deliver oligotrophic seawater with low DOC (65–75 µM) from the South China Sea to Singapore. tDOM at this time is low, as indicated by low chromophoric dissolved organic matter (CDOM, a₃₅₀ 0.2 m^-1) and high CDOM spectral slope (S_275-295 0.03 nm^-1). pH is >7.95 and pCO₂ is <500 µatm. During the southwest monsoon (June to September), the currents reverse and advect tDOM-rich water influenced by run-off from Sumatran peatlands to Singapore. During this period, CDOM increases 5–10-fold (a₃₅₀ 1–2 m^-1) and CDOM spectral slope S_275-295 drops to ≤0.018 nm^-1, while DOC reaches its annual peak (80–100 μM), coinciding with the lowest pH (~7.85) and the highest pCO₂ (~600 μatm). Mixing calculations between realistic peatland river and coastal seawater endmembers imply an additional source of dissolved inorganic carbon at this time, suggesting that the observed acidification may be driven by tDOM remineralisation. This is consistent with an observed seasonal shift in δ¹³C-DIC from +0.1‰ to -1.2‰, and with incubation experiments that revealed a high degree of photo-remineralization, albeit limited microbial remineralization, of Southeast Asian peatland DOC. Our results suggest that coastal waters in parts of Southeast Asia may be subject to seasonal acidification from the remineralization of tropical peatland tDOM, which may impact coral reefs in this biodiverse region.