Understanding how climatic variability impacts coastal water quality is increasingly urgent, but in many regions we lack multi-year, coastal biogeochemical time series that can provide these insights. We analyzed a 7–9-year time series of salinity, dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) from a site in the centre of Southeast Asia’s Sunda Shelf Sea, the Singapore Strait, to understand the biogeochemical variability and corresponding climatic drivers. Our site receives substantial terrestrial inputs via rivers, especially from regional peatlands, but is also subject to seasonal reversal in ocean circulation due to the Asian monsoon system, delivering different water masses in different seasons. We therefore additionally performed realistic hindcast simulations with a regional physical circulation model to understand the impact of physical oceanography. We show that salinity, DOC, and CDOM at our site are significantly correlated with regional precipitation, seawater volume transport, and the climate phenomena El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), both at seasonal and interannual time scales. The impacts of ENSO and IOD appear to result partly from their influence over regional precipitation, but also from direct effects on regional physical circulation. Our results illustrate how physical oceanographic variability can interact with climatic variability to drive coastal biogeochemistry. This highlights the importance of accurately representing such drivers in models for future projections of coastal water quality.

The flux of dissolved organic carbon (DOC) from land to sea is an important net transfer within the global carbon cycle. The biogeochemical fate of this terrestrial DOC (tDOC) remains poorly understood and is usually neglected in ocean models. Southeast Asia accounts for around 10% of global tDOC flux, mostly from tropical peatland-draining rivers discharging onto the Sunda Shelf. We developed a new light-driven parameterization of tDOC remineralization that accounts for photochemical, microbial, and interactive photochemical–microbial degradation. Using this, we simulated the transport and remineralization of tDOC through the Sunda Shelf seas using the regional 3D hydrodynamical–biogeochemical models HAMSOM–ECOHAM. Our realistic hindcast simulations for 1958–2022 show that about 50% of riverine tDOC is remineralized before leaving the shelf. This lowers seawater pH across the entire inner Sunda Shelf by an average of 0.005 units (by up to 0.05 units in the Malacca Strait). Correspondingly, seawater pCO2 is raised, increasing CO2 outgassing from the shelf by 3.1 Tg C yr−1 (0.14 mol m−2 yr−1 ) during 2013-2022. Even regional ocean acidification trends increase, because river discharge and tDOC flux increase. Our model reveals large spatial variability with greatest inputs and remineralization of tDOC close to major peatlands, especially off Sumatra and Borneo. The interannual variability in tDOC input and the monsoonal current reversal lead to strong temporal variability in carbonate system parameters in these areas. Our results highlight the importance of representing tDOC in ocean models, and reveal the fate of tropical peatland tDOC.

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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.