Southeast Asia's peatlands are a globally significant source of terrigenous dissolved organic carbon (tDOC) to the ocean, and field observations show that this tDOC is extensively remineralized within the shelf sea. Yet the processes that drive this remineralization remain unclear. Here, we combined incubation experiments and model simulations to quantify the rate and extent of photodegradation of tDOC in the Sunda Shelf Sea. During laboratory photodegradation experiments, 26%-74% of the peatland tDOC was photomineralized, but realistic in situ rates of photodegradation have not yet been estimated in this region. Based on spectrally resolved apparent quantum yields for tDOC remineralization calculated from experiments, modeled in situ solar irradiance, and measured in-water inherent optical properties, we simulated peatland tDOC photomineralization for two coastal regions of the Sunda Shelf Sea. These simulations show that sunlight can directly remineralize 25 +/- 9% of the tDOC input over its maximum 2-year residence time in the Sunda Shelf Sea, accounting for 38% of the total tDOC remineralization. We also found that photobleaching can remove 54 +/- 4% of colored dissolved organic matter over this time-scale. We further derived a simplified photochemical decay constant null ref of 0.016 day-1 for Southeast Asia's peatland-derived tDOC, which can be used to parameterize the recently proposed UniDOM model. We conclude that direct photodegradation may be a greater sink for tDOC in Southeast Asia's coastal ocean compared to higher latitudes, although it is insufficient to account for the total tDOC remineralization observed in the Sunda Shelf Sea. Tropical peatlands in Southeast Asia contribute a large quantity of organic carbon to the coastal ocean. Field observations have clearly demonstrated that this organic carbon is rapidly decomposed to CO2, but the mechanism driving this remineralization is unclear. Organic carbon from peatlands appears to be easily decomposed upon sunlight irradiation, known as photodegradation. Here, we conducted experiments to measure the photochemical decay efficiency of peatland-derived organic carbon and developed model simulations to calculate how much of the organic carbon is decomposed via photodegradation in natural coastal waters. We found that sunlight can directly remineralize 25% of the peatland-derived organic carbon input to CO2 in the coastal ocean of Southeast Asia. This means that photodegradation decomposes more of the terrestrial organic carbon in Southeast Asia than in coastal oceans at higher latitudes, but photodegradation alone is still insufficient to account for the extent of all degradation observed in field data. Our data allowed us to derive region-specific decay rates of photodegradation for coastal Southeast Asia, which can be used in large-scale ocean carbon biogeochemistry models. We combined incubation experiments and model simulation to quantify photodegradation of peatland dissolved organic carbon in coastal watersDirect photochemical remineralization can consume 25% of peatland dissolved organic carbon input in the coastal Sunda Shelf SeaUp to 74% of the Southeast Asian peatland-derived dissolved organic carbon is photo-remineralizable
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