The terrigenous dissolved organic carbon (tDOC) exported from the peatlands in Southeast Asia appears to be extensively remineralized in the shelf sea, but 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. Laboratory photodegradation experiments indicate that up to 74% of the peatland tDOC is potentially labile to photochemical remineralization. Based on our estimated apparent quantum yield for tDOC remineralization, modeled in-situ solar irradiance, and measured inherent optical properties of the water column, we simulated peatland tDOC photoremineralization for two coastal regions of the Sunda Shelf Sea. These simulation results show that natural solar radiation can directly remineralize 20±11% of tDOC over 2 years, which corresponds to the approximate residence time of water in the Sunda Shelf Sea, and that significant photobleaching of tDOC can occur in coastal waters over shorter time-scales. We further derived a simplified photochemical decay constant of 0.008–0.017day-1 for Southeast Asia’s peatland-derived tDOC, which can be used to parameterize the recently proposed UniDOM model framework. 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.

A document by Yuan Chen. Click on the document to view its contents.

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.

Coastal tropical waters are experiencing rapid increases in anthropogenic pressures, yet coastal biogeochemical dynamics in the tropics are poorly studied. We present a multi-year biogeochemical time series from the Singapore Strait in Southeast Asia’s Sunda Shelf Sea. Despite being highly urbanised and a major shipping port, the strait harbours numerous biologically diverse habitats, and is a valuable system for understanding how tropical marine ecosystems respond to anthropogenic pressures. Our results show strong seasonality driven by the semi-annual reversal of ocean currents: dissolved inorganic nitrogen (DIN) and phosphorus varied from ≤0.05 µmol l-1 during the intermonsoons to ≥4 µmol l-1 and ≥0.25 µmol l-1, respectively, during the southwest monsoon. Si(OH)4 exceeded DIN year-round. Based on nutrient concentrations, their relationships to salinity and coloured dissolved organic matter, and the isotopic composition of NOx-, we infer that terrestrial input from peatlands is the main nutrient source. This input delivered dissolved organic carbon (DOC) and nitrogen, but was notably depleted in dissolved organic phosphorus. In contrast, particulate organic matter showed little seasonality, and the δ13C of particulate organic carbon (-21.0 ± 1.5‰) is consistent with a primarily autochthonous origin. Diel changes in dissolved O2 varied seasonally with a pattern that suggests that light availability controls primary productivity more than nutrient concentrations. However, diel changes in pH were greater during the southwest monsoon, when remineralisation of terrestrial DOC lowers the seawater buffer capacity. We conclude that terrestrial input results in mesotrophic conditions, and that the strait might be vulnerable to further eutrophication if nutrient inputs increase during seasons when light availability is high. Moreover, the seasonality of diel pH variation suggests that coral reefs exposed to terrestrial organic matter in the Sunda Shelf may be at significant risk from future ocean acidification.